(Current version containing all changes through 3/15/94) DATA USERS GUIDES 1: Digital Line Graphs from 1:24,000-Scale Maps 2: Digital Line Graphs from 1:100,000-Scale Maps 3: Digital Line Graphs from 1:2,000,000-Scale Maps 4: Land Use and Land Cover Digital Data from 1:250,000- and 1:100,000-Scale Maps 5: Digital Elevation Models 6: Geographic Names Information System 7: Alaska Interim Land Cover Mapping Program Data Users Guides 1-7 generally replace the Geological Survey Circular 895. Questions regarding availability and ordering of US GeoData (all types of digital cartographic and geographic data produced and distributed by the U.S. Geological Survey) should be addressed to: Earth Science Information Center U.S. Geological Survey 507 National Center Reston, Virginia 22092 (703)860-6045 Technical questions and comments should be addressed to: Branch of Technical Management U.S. Geological Survey 510 National Center Reston, Virginia 22092 UNITED STATES DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY DIGITAL LINE GRAPHS FROM 1:24,000-SCALE MAPS _______________________________________________________________ Data Users Guide 1 Reston, Virginia 1990 First printing, 1986 Second printing (revised), 1990 CONTENTS Page Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .1 Data content . . . . . . . . . . . . . . . . . . . . . . . . . .2 Data structure . . . . . . . . . . . . . . . . . . . . . . . . .3 Levels of structuring . . . . . . . . . . . . . . . . . . .3 Topology . . . . . . . . . . . . . . . . . . . . . . . . .4 Topological elements. . . . . . . . . . . . . . . . . . . .4 Attribute codes. . . . . . . . . . . . . . . . . . . . . . . . .4 Major attribute codes . . . . . . . . . . . . . . . . . . .6 Minor attribute codes . . . . . . . . . . . . . . . . . . .6 Sample attribute codes. . . . . . . . . . . . . . . . . . .7 Sample line graph structure. . . . . . . . . . . . . . . . . . .8 Graph theory in DLG data . . . . . . . . . . . . . . . . . . . 10 Area case . . . . . . . . . . . . . . . . . . . . . . . . 10 Network case. . . . . . . . . . . . . . . . . . . . . . . 11 Area-Hybrid case. . . . . . . . . . . . . . . . . . . . . 11 Distribution formats . . . . . . . . . . . . . . . . . . . . . 17 Source materials . . . . . . . . . . . . . . . . . . . . . . . 18 Cell size and file extent. . . . . . . . . . . . . . . . . . . 18 Coordinate systems . . . . . . . . . . . . . . . . . . . . . . 18 Standard distribution format. . . . . . . . . . . . . . . 18 Optional distribution format. . . . . . . . . . . . . . . 19 Data validation. . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix A.Standard DLG distribution format (record contents). 25 B. Optional DLG distribution format (record contents). . 36 C. Map projection parameters . . . . . . . . . . . . . . 49 D. DLG attribute codes . . . . . . . . . . . . . . . . . 50 E. Coordinate conversion . . . . . . . . . . . . . . . . 71 F. Sample DLG data file (standard distribution format) . 72 G. Sample DLG data file (optional distribution format) . 76 H. Pre-1983 Hydrographic attribute codes . . . . . . . . 79 I. Pre-1985 Transportation attribute codes . . . . . . . 85 J. Pre-1985 Other Significant Cultural Feature attribute codes92 K. Origins of the U.S. Rectangular Surveys . . . . . . . 97 L. Named Land Grant codes. . . . . . . . . . . . . . . . 98 ILLUSTRATIONS Page Figure 1. Map elements showing roads, railroads, buildings, streams, and lake and forest areas . . . . . . . . 5 2. Sample line graph . . . . . . . . . . . . . . . . . 8 3. Window from the Oneco, Connecticut-Rhode Island, USGS 1:24,000-scale quadrangle map . . . . . . . .12 4. Window from the line graph of hydrography, Oneco, Connecticut-Rhode Island, quadrangle . . . . . . .13 5. Location of origin of file reference coordinates. .19 TABLES Page Table 1. Major codes used for DLG base categories . . . . . . 6 2. Digital description of sample DLG-3. . . . . . . . . 9 3. Selected sample of standard format DLG-3 records for Oneco, Connecticut-Rhode Island, hydrography . . . .14 4. Standard and optional DLG format . . . . . . . . . .17 DIGITAL LINE GRAPHS FROM 1:24,000-SCALE MAPS _______________________________________________________________ INTRODUCTION The Earth Science Information Centers (ESIC) distribute digital cartographic/geographic data files produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program. Digital cartographic data files are grouped into four basic types. The first of these, called a Digital Line Graph (DLG), is line map information in digital form. These data files include information on planimetric base categories, such as transportation, hydrography, and boundaries. The second type, called a Digital Elevation Model (DEM), consists of a sampled array of elevations for a number of ground positions that are usually at regularly spaced intervals. The third type is Land Use and Land Cover digital data, which provides information on nine major classes of land use such as urban, agricultural, or forest as well as associated map data such as political units and Federal land ownership. The fourth type, the Geographic Names Information System, provides primary information for all known places, features, and areas in the United States identified by a proper name. The digital cartographic data files from selected quadrangles currently available from ESIC include the following: o Digital Line Graphs (DLG ) --1.24,000-scale --1:62,500-scale --1:63,360-scale --1:100,000-scale --1:2,000,000-scale o Digital Elevation Models (DEM) --7.5-minute --15-minute --30-minute --1-degree o Land Use and Land Cover digital data --1:250,000- and 1:100,000-scale Land Use and Land Cover and associated maps --1:250,000-scale Alaska Interim Land Cover o Geographic Names The digital data are useful for the production of cartographic products such as plotting base maps and for various kinds of spatial analysis. A major use of these digital cartographic/geographic data is to combine them with other geographically referenced data enabling scientists to conduct automated analysis in support of various decision making processes. _______________ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Manuscript approved for publication September 4, 1986. This document describes the Digital Line Graphs (DLG's) prepared primarily from the 1:24,000 materials associated with the USGS Topographic Map Series. The series will eventually provide complete national coverage. DATA CONTENT The DLG data files derived from the 1:24,000-scale and other large-scale maps contain selected base categories of cartographic data in digital form; these data categories do not necessarily correspond to the traditional feature separates associated with the maps. The attribute coding scheme for these data has undergone several revisions since the start of the digital program. A major revision of these codes has been printed as Standards for Digital Line Graphs - Part 3, Attribute Coding, which is available for purchase from a USGS ESIC office (see the ordering information inside the front cover). Currently, DLG data entered in the National Digital Cartographic Data Base (NDCDB) are coded in accordance with the Standards for Digital Line Graphs. The implementation of the new coding standards will require the updating of existing files in the NDCDB in order to have a consistent product available for users. Software and procedures are being developed to convert existing data files to these codes during the next several years. Priority will be given to converting files retrieved in response to sales requests. In the meantime, a data base query will provide identification of the coding scheme used for any file in the NDCDB. This information will be supplied to customers when orders are submitted, and upon transmittal of data files. The following categories are included in current large-scale DLG files: o Boundaries -- This category of data consists of (1) political boundaries that identify States, counties, cities, and other municipalities, and (2) administrative boundaries that identify areas such as National and State forests. Political and administrative boundaries are always collected as a single data set. o Hydrography -- This category of data is currently being collected as combined hydrography consisting of all flowing water, standing water, and wetlands. Prior to l983, hydrographic data were differentiated into two components: streams and water bodies. Streams represent flowing water and were digitized as a network intended for hydrologic flow modeling. Streams included the banks of double-line rivers and centerline connectors placed through double-line rivers and lakes. Water bodies include standing water such as lakes and ponds. Wetlands and coastal hydrographic data were not collected. Appendix H contains a list of the attribute codes used in these files. o Public Land Survey System (PLSS) -- This category of data describes the rectangular system of land surveys that is administered by the U.S. Bureau of Land Management. PLSS data are only collected for areas falling solely, or in part, within the States that were formed from the public domain. The PLSS subdivides the public domain and represents property boundaries or references to property boundaries. These DLG data are not intended to be official or authoritative. They are presented as cartographic reference information. The only legal basis for determining land boundaries remains the original survey. o Transportation -- This category of data includes major transportation systems collected in three separate overlays labeled: (1) Roads and Trails, (2) Railroads, and (3) Pipelines, Transmission Lines, and Miscellaneous Transportation Features. In the last quarter of 1985, new transportation attribute codes were implemented. The principal difference between the old and new coding schemes is that under the old transportation subcategory, certain miscellaneous transportation features were not collected and descriptive attribute codes were not used. Appendix I contains a list of the attribute codes formerly used. o Other Significant Manmade Structures -- This category of data includes miscellaneous cultural features not included in the other major data categories. New attribute codes for Other Significant Manmade Structures were implemented in the last quarter of 1985. Very little data from this category currently reside in the NDCDB. Appendix J contains a list of the attribute codes used for these older files. The attribute codes for the following base categories were newly defined in late 1985. Currently, there are very little data available in these categories. o Hypsography -- This category of data consists of information on topographic relief (primarily contour data). o Surface Cover -- This category of data consists of information about vegetative surface cover such as woods, scrub, orchards, and vineyards. Vegetative features associated with wetlands, such as marshes and swamps, are collected under Hydrography. o Non-Vegetative Surface Features -- This category of data consists of information about the natural surface of the Earth as symbolized on the map such as lava, sand, and gravel features. This category is not all-inclusive, as other non-vegetative surface features are found in the category of Hydrography. o Survey Control and Markers -- This category of data consists of information about the points of established position and third-order or better elevations that are used as fixed references in positioning and correlating map features. DATA STRUCTURE Levels of Structuring The term Digital Line Graph (DLG) is used by the USGS to describe a digital map data set in vector form. Originally, three levels of DLG data (DLG-1, DLG-2, and DLG-3) were envisioned; these levels were differentiated by their positional accuracy, level of attribute coding, and relational spatial information. It was found, however, that the widest user-community would be served by producing DLG-3 data, which have the full range of attribute codes and are fully topologically structured. These two properties are required by users whose work includes both graphic and analytic applications. Therefore, all DLG data in the National Digital Cartographic Data Base are level 3. Topology Current data collection from 1:24,000-scale and other large-scale maps is exclusively directed toward producing fully topologically structured level 3 DLG data referred to as DLG-3. The DLG-3 concept is based on graph theory in which a two-dimensional diagram is expressed as a set of nodes (topologically significant points), lines, and areas in a manner that explicitly expresses logical relationships. Applied to a map, this concept is used to encode the digital data with the spatial relationships between map elements which are obvious when the map is examined visually. The spatial relationships include such concepts as adjacency and connectivity between features on the map. The abstraction of the map data according to the rules of graph theory preserves the spatial relationships inherent in the map graphic and creates a logical and consistent data file structure for computer processing. A digital file of cartographic or geographic data that maintains the spatial relationships inherent in the map is called a topologically structured data file. A topologically structured data file can support simple graphic applications, such as plotting streams and roads for base maps, as well as more advanced applications, such as computations and analyses involving areas and lines and their spatial relationships. Topological Elements A DLG-3 file is composed of three separate, but related, elements: nodes, lines, and area identifiers. Nodes define the location of the endpoints of every line, and a single node may mark the start or end of one or more lines. Thus, nodes occur at intersections of linear features and other places on linear features where the feature is subdivided into separate line segments. A line is an ordered set of points that describes the position and shape of a linear feature on the map. Each line starts at a node and ends at a node, and has an area to the left of its direction of travel, and has an area to the right of its direction of travel. The direction of travel is arbitrarily determined at the time of data capture. Lines connect to each other at nodes, and a line does not cross itself or any other line. A line may describe the boundary between two areal map features, such as counties, or may define a map feature by itself, such as a road. A special line, called a degenerate line, is used to define features symbolized as independent points on a map. A degenerate line starts and ends at the same node, has two identical coordinate pairs, has zero length, and has the same area to the left and right of the direction of travel; that is, it is totally enclosed inside one map area. An area is a portion of the map bounded by lines. All portions of the map must be assigned some area point. Each area is identified in a DLG-3 data file by a point chosen to represent the characteristics of the area. Newer versions of the processing software, the DLG Production System or PROSYS, locate a given area point inside the area it represents, although this is not a structural requirement. Every DLG data file will have at least two areas identified: one representing the area covered by the file and the other representing the area outside the coverage of the file. Additional areas will be identified as necessary to subdivide the area covered by the file. Polygons as unique features are not defined explicitly in a DLG file. However, polygons can be constructed using line-area linkages built into the DLG data structure. ATTRIBUTE CODES In addition to locational and topological information, DLG data elements may have explicitly encoded attributes. Attribute codes, also called feature codes or classification attributes, are used to describe the map information represented by a node, area, or line. For example, the attribute code for an area might identify a lake or swamp; the attribute code for a line might identify a road, railroad, stream, or shoreline (fig. l). Figure 1.--Map elements showing roads, railroads, buildings, streams, and lake and forest areas. The codes are based on the cartographic features symbolized on the USGS Topographic Map Series. These maps are the basic source material used to digitize and to encode the data elements, and therefore the map symbology has a strong influence on the overall classification strategy. A listing of all the attribute codes currently assigned and used in 1:24,000- and 1:100,000-scale DLG files is given in Appendix D. Detailed information on how to apply and interpret the attribute coding system is given in Standards for Digital Line Graphs, Part 3: Attribute Coding. (This publication may be purchased from the U.S. Geological Survey. See the ordering information inside the front cover.) Each attribute code identifies the major category to which a data element belongs, as well as the specific nature of the element. Codes also may provide additional descriptive information. Most elements are uniquely described by a single attribute code. Others, however, may require two or more codes for a complete description. If multiple attributes are needed to describe an element, the order is not usually significant. Allowing for a variable number of attribute codes creates an open-ended structure to which information may be added at any time. It is not necessary for each element to have associated attributes; in general, attribute codes are not assigned to an element if the attributes can be derived based on relationships to adjacent elements. For example, a U.S. Public Land Survey section line is not assigned an attribute code because the line record carries a reference to the areas to the left and right, that will be assigned attribute codes identifying the two different section numbers. The fact that the line is a section line is derivable. A DLG attribute code is composed of two distinct numeric fields: a three-digit major code, which identifies the major category to which the element belongs, and a four-digit minor code, which specifically describes the element. In the digital file, the major and minor attributes are encoded in two integer fields of six digits, right justified with leading blanks (FORTRAN 2I6 format). In this document, major codes are presented as three digits, and minor codes are presented as four digits. Leading zeros are shown for clarity; for example: 050 0412. Major Attribute Codes A list of the major codes and the categories that are currently being collected is contained in table 1. The first two digits of the major code uniquely identify the category to which the described element belongs. The third digit of the major code is used to modify the minor code in two ways: o If zero, the minor code represents a description or classification of the element. o If non-zero, the minor code which follows is a parameter requiring special interpretation according to instructions given in the codes for each category (see next section). Transportation systems have been assigned more than one major code so that their components may be readily separated for analytical applications. Table 1.--Major codes used for DLG base categories _______________________________________________________________ Major Code Base Category _______________________________________________________________ 020 Hypsography 0501 Hydrography 070 Surface Cover 080 Non-Vegetative Surface Features 090 Boundaries 150 Survey Control and Markers 1702 Transportation--Roads and Trails 1802 Transportation--Railroads 1902 Transportation Systems--Pipelines, Transmission Lines, Miscellaneous Transportation Features 2003 Other Significant Manmade Structures 300 U.S. Public Land Survey System ________________________________________________________________ 1 Prior to 1983, hydrographic features were digitized as two separate categories, (1) 030-Streams and (2) 040-Water bodies. Hydrographic features are currently digitized as a single category, 050-Hydrography. See Appendix H for a list of hydrographic attribute codes used prior to 1983. 2 In the last quarter of 1985, the transition was made to these new transportation attribute codes, adding codes for certain miscellaneous transportation features. See Appendix I for a list of transportation attribute codes used prior to this transition. 3 In the last quarter of 1985, codes for this category were created to replace an earlier version of codes. See Appendix J for a list of the previous attribute codes. Minor Attribute Codes The first digit of the minor code is normally zero. If non-zero, it is used as a modifier to provide additional information such as road access or railroad status. The remaining three digits are normally used to indicate the cartographic interpretation to be applied to specific elements. The type of element described by a particular code usually can be determined from the range of value of the last three digits: 001 - 099 = nodes 100 - 199 = areas 200 - 299 = lines 300 - 399 = degenerate lines 400 - 499 = codes which may be applied to any element type (nodes, lines, areas, or points) 601 - 699 = general descriptive codes The last three digits (and occasionally all four digits) also may be used as a parameter code. Parameters are used when a minor code can legitimately assume a range of values such as a water elevation or a highway route number. The meaning of a parameter code is indicated by the (non-zero) third digit of the major code. Sample Attribute Codes Four examples using the DLG attribute codes follow and should be interpreted with reference to Appendix D. Example A: 050 04l2 The major code 050 indicates the Hydrography category. The minor code 04l2 identifies the feature as a stream. Example B: 170 0201 The major code 170 indicates the Roads and Trails overlay in the Transportation category. The minor code 0201 identifies the feature as a class 1 highway. l70 0603 The major code l70 indicates the Roads and Trails overlay in the Transportation category. The minor code identifies the feature as a road under construction. This code would be used in addition to the code describing the class of road, and would appear in the same record with the code 170 0201. Example C: 055 0033 The major code 055 indicates a river mile mark for the Hydrography category. Because the last digit of the major code is non-zero, the minor code is a parameter. The minor code 0033 indicates that the value of the river mile mark at that point is 33. Example D: 306 0033 The major code 306 indicates an Origin of Survey code for the U.S. Public Land Survey System category. Because the last digit of the major code is non-zero, the minor code 0033 indicates that the area element is referenced to the Willamette Meridian. SAMPLE LINE GRAPH STRUCTURE Examples of a line graph and its corresponding digital records are given in figure 2 and table 2. These examples are simplified representations of the concepts used in the DLG-3 structure; they are not actual data files. The examples shown are composed of 13 nodes, 5 areas, and 15 lines. The 13 nodes are labeled N1 through N13, the 5 areas are labeled A1 through A5, and the 15 lines are labeled L1 through L15. Each element type is maintained as a separate list in the digital data. Figure 2.--Sample line graph. The map represented by the example is divided into five distinct areas labeled Al through A5. Area A1 represents all the area outside of the map border. There is one outside area for each DLG-3. It is always the first area encountered and has the attribute code 000 0000. In the example given in figure 2, the portion of the map inside the border is divided into four areas, each bounded (closed) by lines. Area A2 is bounded by lines L14, L1, L4, and L5. Area A3 is bounded by lines L3, L13, L4, L6, L7, L8, L15, and L9. Area A4 is bounded by lines L8, L15, and L9. Area A5 is bounded by lines L5, L6, L7, and L10 and L2. In this example, line elements contain the only explicit topological references. Each line contains pointers to its bounding nodes (starting and ending) and the areas that it bounds (left and right of the line). Table 2.--Digital description of the topological elements and relationships of a sample line graph (see fig. 2) _______________________________________________________________ Nodes Areas _______________________________________________________________ Internal Internal Id Id Number X Coordinate Y Coordinate Number X Coordinate Y Coordinate N1 1 28 A1 0 0 N2 13 14 A2 6 24 N3 23 28 A3 3 10 N4 13 1 A4 8 7 N5 13 7 A5 18 14 N6 22 10 N7 6 5 N8 10 4 N9 11 24 N10 23 1 N11 1 17 N12 1 1 N13 9 9 _______________________________________________________________ Lines _______________________________________________________________ Nodes Area Coordinates Number Starting Ending Left Right (first x y last x y) L1 1 3 1 2 1, 28 23, 28 L2 3 10 1 5 23, 28 23, 1 L3 4 12 1 3 13, 1 1, 1 L4 11 2 2 3 1, 17 .... 13, 14 L5 2 3 2 5 13, 14 23, 28 L6 2 5 5 3 13, 14 13, 7 L7 5 4 5 3 13, 7 13, 1 L8 13 7 4 3 9, 9 .... 6, 5 L9 7 8 4 3 6, 5 .... 10, 4 L10 4 10 5 1 13, 1 23, 1 L11 5 6 5 5 13, 7 .... 22, 10 L12 9 9 2 2 11, 24 11, 24 L13 12 11 1 3 1, 1 1, 17 L14 11 1 1 2 1, 17 1, 28 L15 8 13 4 3 10, 4 .... 9, 9 _______________________________________________________________ This format is simi1ar in concept to the standard DLG-3 data structure, which minimizes redundant linkages to achieve efficient data encoding and storage. The lines in figure 2 are labeled L1 through L15. The lines can be identified by their starting node number, ending node number, number of the area to the left of the direction of travel, number of the area to the right of the direction of travel, and string of coordinates describing the alignment of the line. In this example, only two pairs of coordinates are shown; however, in an actual file, an irregular line would have a variable number of coordinate pairs up to a limit of 3,000 coordinate pairs. The direction of travel of the line is arbitrarily determined during the digitizing operation. In this example, L1 is encoded as proceeding clockwise around area A2. Thus line L1 starts at node N1, ends at node N3, has area A1 to the left of the direction of travel, and has area A2 to the right of the direction of travel. The coordinate string describing the alignment of the line will start with the same coordinate values as that of node N1 and will end with the same coordinate values as that of node N3. Because the area to the left of its direction of travel, A1, is different from the area to the right of its direction of travel, A2, the line is known to be a boundary between the two areas. Lines L11 and L12 are examples of lines that lie within one area. In this example, line L11 starts at node N5, ends at node N6, has area A5 to the left of the direction of travel, and again has area A5 to the right of the direction of travel. The coordinate string for the line will start with the same coordinate values as that of node N5 and will end with the same coordinate value as that of node N6. Line L12 is an example of a degenerate line. The line starts at node N9, ends at node N9, and has area A2 as both the area to its left and right. There are two coordinate pairs in the string defining the line, and both points have the same coordinate values as node N9; thus, the two points are the same and the line has zero length. The line graph concept allows all of the points on the map to be described as a member of a line graph element (node, area, or line) with minimal redundancy. The relationships between the various elements are indicated by the structure. Note that in this example the x and y coordinates are numbered from the lower left corner to simplify the drawing. In an actual DLG-3 file, the origin is the center of the map and the internal file coordinates are numbered plus or minus 1 to 32,767 in thousandths of inches. See the section labeled "coordinate systems" for more detail. GRAPH THEORY IN DLG DATA The digital line graph concept is based on graph theory, in which a diagram can be expressed as a set of elements (nodes, areas, and lines) in a manner that shows logical spatial relationships with minimal redundancy. There are three ways to implement the line graph concept in DLG files: the area case, the network case, and the area-hybrid case. All NMD files are collected and processed as area-hybrid case DLG's. Area Case Area line graphs can be used to represent area features such as political entities or the U.S. Public Land Survey System. In the area case, all closed circuits of lines form unique areas. All line elements bound two different area elements. Line elements for area line graphs are not normally assigned primary attributes. The characteristics of lines in these categories can usually be derived by examining the attributes of the area elements on each side of the line. Network Case Network line graphs can be used to represent linear features such as roads, single-line streams, or railroads. The network case differs from the area case in that, irrespective of the number of closed areas forming the graph, only two area elements are encoded: (l) the area outside the graph, termed the outside area; and (2) the area within the graph, termed the background area. All lines except the graph boundary are considered to be contained within the background area. The major topological relationship expressed by network data is that of connectivity. Data encoded in network line graph form are suitable for various forms of network analysis, such as minimum path computations. Area-Hybrid case In the area-hybrid case, network and area type information is gathered in a single DLG file. In this approach, all closed circuits of lines define unique areas. However, some lines may exist which do not form boundaries between two areas. The unique areas which represent features for the overlay are given attribute codes. For example, in the hydrography category there are areal features, such as lakes, reservoirs, and swamps, that are represented by unique, attributed area elements. There are also linear features, such as single-line streams and aqueducts, that are significant in themselves and are also assigned attribute codes. These features may occupy a position in an area of no other hydrographic significance, that is, an unattributed background area. Therefore, in processing area-hybrid data, the background area itself is broken into numerous unattributed area records that distinguish the background areas from hydrographic areas. To further illustrate the area-hybrid case, a detailed description of a representative line graph follows. Figure 3 shows a window taken from the Oneco, Connecticut-Rhode Island, 1:24,000-scale USGS quadrangle map. Figure 4 shows the line graph encoded for the hydrography of the same area. Certain nodes, areas, and lines are labeled. Table 3 contains some of the digital data records, extracted from the standard format DLG file, which describe this portion of the graph. The internal sequence identification numbers shown reflect the order of these features in the original file. (Note: Descriptions of DLG-3 formats are contained in Appendixes A and B, and a list of attribute codes is contained in Appendix D.) In the Oneco example, each node and area element is described by one or two logical records: (1) a type D.l record that describes the element, and (2) an optional type F record that lists the attribute codes associated with the element. The first record (type D.1) for each node and area element contains the following fields: 1. Type of record indicator, N for node or A for area. 2. Internal sequence identification number. 3. X coordinate of node or representative area point. 4. Y coordinate of node or representative area point. 5. Number of attribute codes that describe the element. 6. Number of pairs of characters in the text string that describes the element. Figure 3.--Window from the Oneco, Connecticut-Rhode Island, 1:24,000-scale quadrangle map. The second record (type F) for each node and area element contains n attribute codes (expressed as major and minor code pairs), where n is the number specified in field 5 of the first (type D.l) record. Each line element in the Oneco example is described by two or three logical records: (1) a type D.2 line description record, and (2) a type E record that lists the x,y coordinate pairs that define the shape of the line, and, if appropriate, (3) a type F (attribute code) record. The first record (type D.2) for each line element contains the following fields: 1. Type of record indicator (L). 2. Internal sequence identification number. 3. Internal sequence number of starting node. 4. Internal sequence number of ending node. 5. Internal sequence number of the area to the left of the line. 6. Internal sequence number of the area to the right of the line. 7. Number of x,y coordinate pairs that locate the line on the map. 8. Number of attribute codes that describe the line. 9. Number of pairs of characters in the text string that describes the line. Figure 4.--Window from line graph of hydrography, Oneco, Connecticut Rhode Island, quadrangle. The second logical record (type E) for each line element contains n coordinate pairs, where n is the number specified in field 7 of the first (type D.2) record. The type F record is as described above. The records listed in table 3 describe several hydrographic features in the center of the north edge of figure 3, including a portion of Quanduck Brook, a small pond, a swamp, and two small streams. The records referred to in the following description have been extracted from a complete DLG. Therefore, the internal sequence identification numbers shown reflect the order of these features in the original file. The records are referred to in this description by these internal sequence numbers; for example, node 141, area 13, line 119. Background area 13 has an x,y coordinate of 0,0. This is a result of the processing software automatically determining the numerous background areas within a DLG and assigning these areas an x,y coordinate of 0,0 (which is the origin of a DLG, usually located at the center of a quad). The area outside of the map is represented by area record 1 and is identified by the attribute code 000 0000. Table 3.--Selected sample of standard format DLG-3 records for Oneco, Connecticut-Rhode Island, Hydrography _______________________________________________________________ HYDROGRAPHY 2360 808 1180 368 1572 1021 N 141 1654 8143 0 0 N 142 949 7238 0 0 N 150 1501 8058 0 0 N 151 794 7190 0 0 N 364 1484 9210 1 0 50 1 N 387 -1571 10532 1 0 50 1 N 511 893 7972 0 0 A 1 -9137 146 1 0 0 0 A 13 0 0 0 0 A 50 -3179 6522 1 0 50 412 A 169 -1557 10407 2 0 50 111 50 613 A 221 893 7991 2 0 50 421 50 0 L 119 141 140 13 50 26 1 0 1654 8143 1681 8142 1695 8134 1709 8106 1721 8071 1727 8023 1758 7963 1765 7956 1785 7958 1856 8012 1954 8106 2008 8162 2057 8191 2082 8198 2113 8220 2309 8328 2342 8332 2362 8327 2411 8287 2424 8279 2448 8283 2507 8310 2565 8348 2590 8362 2638 8391 2655 8406 50 605 L 120 142 141 13 50 22 1 0 949 7238 952 7275 926 7357 9297 394 941 7427 972 7490 1004 7542 1095 7665 1121 7691 1176 7754 1277 7852 1306 7883 1324 7921 1372 7980 1382 7988 1407 8027 1462 8072 1526 8083 1558 8100 1620 8137 1641 8144 1654 8143 50 605 Table 3.--Selected sample of standard format DLG-3 records for Oneco, Connecticut-Rhode Island, Hydrography--continued L 121 143 142 13 50 33 1 0 219 6694 211 6712 211 6745 200 6785 206 6809 223 6821 264 6825 288 6844 292 6871 285 6907 276 6928 274 6959 284 6978 300 6990 319 6992 349 6991 363 7001 431 7099 442 7112 536 7275 563 7290 609 7303 665 7307 717 7296 731 7281 739 7262 784 7230 816 7195 844 7182 877 7183 909 7200 937 7225 949 7238 50 605 L 129 150 139 50 13 33 1 0 1501 8058 1572 8070 1601 8086 1626 8105 1651 8114 1671 8109 1685 8081 1707 8020 1718 8001 1731 7950 1743 7938 1765 7927 1788 7928 1820 7948 1844 7966 1954 8074 2011 8123 2034 9151 2085 8179 2104 8192 2114 8195 2180 8239 2253 8280 2298 8304 2321 8313 2345 8312 2359 8305 2400 8269 2424 8263 2452 8266 2476 8277 2500 8286 2512 8294 50 606 L 130 151 150 50 13 25 1 0 794 7190 834 7159 854 7157 903 7169 927 7181 953 7205 973 7239 975 7259 955 7337 947 7351 952 7390 981 7461 1008 7492 1065 7577 1107 7645 1143 7679 1162 7697 1193 7741 1288 7831 1338 7890 1348 7909 1413 7996 1453 8038 1491 8057 1501 8058 50 606 L 131 152 151 50 13 36 1 0 59 6583 109 6597 205 6612 225 6621 239 6637 245 6657 237 6715 237 6748 222 6775 226 6791 237 6802 278 6809 315 6835 325 6859 319 6885 313 6900 311 6914 297 6939 302 6959 313 6967 360 6970 375 6975 394 7004 413 7037 423 7042 453 7082 510 7183 542 7252 568 7275 638 7291 670 7295 709 7283 734 7245 767 7213 786 7201 794 7190 50 606 L 339 364 141 13 13 21 1 0 1484 9210 1490 9148 1531 8966 1550 8852 1560 8820 1566 8780 1594 8677 1599 8629 1626 8580 1636 8559 1648 8540 1661 8529 1673 8510 1675 8450 1698 8376 1701 8350 1690 8318 1643 8261 1641 8219 1653 8159 1654 8143 50 412 L 358 387 21 13 13 23 1 0 -1571 10532 -1578 10608 -1597 10705 -164110804-165710832-168311865 -1689 10877 -1730 10922 -1769 10946 -184110979-194511041-197911049 -2019 11074 -2061 11110 -2080 11131 -209311172-210111212-210911230 -2131 11263 -2135 11273 -2143 11280 -218511369-220111395 50 412 Table 3.--Selected sample of standard format DLG-3 records for Oneco, Connecticut-Rhode Island, Hydrography--continued L 501 511 511 221 13 7 2 0 893 7972 915 7976 912 7997 890 8020 871 7996 864 7974 893 7972 50 0 50 200 L 895 387 387 169 13 20 1 0 -1571 10532 -1586 10532 -1619 10513 -162210504-162210454-163810426 -1655 10415 -1671 10409 -1698 10404 -170710392-169710373-166710365 -1562 10348 -1542 10354 -1526 10373 -152310412-153010462-154310503 -1563 10527 -1571 10532 50 204 _______________________________________________________________ The double-line stream, Quanduck Brook, is represented by area record 50 and identified by the attribute code 050 0412 (stream). Area record 13 is the background area on either side of the stream, and as such has no attribute code assigned. Line records 129, 130, and 131 form the left bank of the river, coded as though one were facing downstream. These lines are identified by the attribute code 050 0606 (left bank) and can be chained by referring to the common nodes. Line record 131 begins outside figure 4 and ends at node 151. Line record 130 starts at node 151 and ends at node 150. Line record 129 starts at node 150 and ends outside figure 4. The right bank of the river is formed in a similar fashion by line records 119, 120, and 121, which are identified by the attribute code 050 0605 (right bank). They are similarly linked through the nodes 141 and 142. Note that the identity of the shoreline as either left or right bank (coded as such to indicate downstream flow) is established by the attribute code, so the given line segments making up the shoreline may be digitized in either direction without altering its identity. Area record 221 describes the small pond just north of Quanduck Brook. It is identified by two attribute codes: 050 0421 (lake or pond) and 050 0000 (photorevised feature). Its shoreline is formed by line record 501 and identified by the attribute codes 050 0200 (shoreline) and 050 0000 (photorevised feature). This line can be identified as bounding area 221 by the reference within the line record to area 221 being located to the left of the line (indicating that line 501 was digitized counterclockwise). Area record 169 describes the marsh in the upper-left corner of figure 4. It is identified by two attribute codes: 050 0111 (marsh, wetland, swamp, bog) and 050 0613 (wooded). The perimeter of the swamp is formed by line record 895 and identified by the attribute code 050 0204 (apparent limit). This line can be identified as bounding area 169 by the reference within the line record to area 169 being located to the left of the line (indicating that line 895 was digitized counter-clockwise). The single-line stream flowing into Quanduck Brook is represented by line record 339 and identified by attribute code 050 0412 (stream). Note that this stream has the same attribute code as the Quanduck Brook (050 0412). This is because both are streams, one of which is digitized as a line and one of which is digitized as an area and its delimiting banks. Line 339 extends from node 364 to node 141. The direction of flow of this stream can be derived from the fact that node 364 is identified with attribute code 050 0001 (upper origin of stream). Background area 13 is located on both sides of the stream. DISTRIBUTION FORMATS The 1:24,000-scale and other large-scale DLG data are available in two distribution formats: (1) standard and (2) optional. The standard distribution format is intended to minimize storage requirements. Explicit topological linkages are contained only in the line elements (starting node, ending node, area to the left of direction of travel, area to the right of direction of travel). A sample DLG in standard format is found in Appendix F. The optional distribution format was designed to facilitate data usage. The topological relationships explicitly encoded include starting node, ending node, area to the left of direction of travel and area to the right of direction of travel for line elements, bounding lines for area elements, and bounding lines for node elements. These files are typically larger than those in the standard format but, for certain applications, can simplify processing requirements. For example, topological linkages are explicitly encoded for all line, node, and area elements, allowing a polygon data structure to be easily created. These linkages facilitate GIS applications of DLG data as well as generation of graphic products. A sample DLG in optional format is found in Appendix G. The characteristics of the standard and optional DLG formats are summarized in table 4. Table 4.--Standard and optional DLG format _______________________________________________________________ Standard Optional _______________________________________________________________ Character set 8-bit ASCII 8-bit ASCII Logical record 144 bytes 80 bytes length Physical record Variable in Variable in length (blocksize) multiples of multiples of 144 bytes. 80 bytes. Coordinate system Internal file Ground (thousandths of planimetric a map inch). (UTM). Topological Contained only Contained in linkages in line elements. node, area, and line elements. _______________________________________________________________ These formats are described in detail in Appendixes A and B. SOURCE MATERIALS The DLG data files described in this document are derived from USGS topographic maps published as 7.5-minute quadrangles at 1:24,000- or 1:25,000-scale. Where 7.5-minute coverage is not available, the following sources are used, in order of preference: 1. Advance manuscripts for 7.5-minute maps, 2. Published 1:62,500 scale 15-minute quadrangle maps (1:63,360 scale in Alaska), or 3. Archival compilation materials for 15-minute quadrangle maps, if available at a larger scale than the published map, such as 1:48,000 scale. The scale of the source materials used to generate a DLG is contained in the file header. The scale is also reflected in the resolution field, which states the ground length in meters of the smallest data collection unit (0.001 inch) for each scale. Source scale Resolution 1:24,000 0.61 meter 1:25,000 0.635 meter 1:48,000 1.22 meters 1:62,500 1.587 meters 1:63,360 1.61 meters CELL SIZE AND FILE EXTENT In general, the DLG's are stored and distributed in standard cells of 7.5 minutes of latitude by 7.5 minutes of longitude. The majority of 1:24,000-scale data collected from l5-minute quadrangles are digitized as four 7.5-minute units. A few older data files collected from 15-minute quadrangles were not digitized in 7.5-minute units, but in 15-minute units. Nonstandard cells are collected in coastal areas where map format is sometimes extended to conform to the shoreline. It is anticipated that these nonstandard files will eventually be partitioned into standard 7.5-minute by 7.5-minute files. COORDINATE SYSTEMS The positional descriptions for DLG data elements are expressed in one of two coordinate systems, dependent upon the distribution format selected. These are described as follows as the standard distribution format and the optional distribution format. Standard Distribution Format The DLG data in the standard distribution format are encoded using an internal file coordinate system to minimize storage requirements. The characteristics of this system are as follows: 1. The coordinate system is Cartesian. 2. The origin (x=0, y=0) is normally at the center of the cell. Some older files will have their origin below and to the left of the lower left corner of the cell (see fig. 5). 3. The x-axis of the coordinate system is parallel to a theoretical straight line connecting the southwest and southeast corners of the cell; y-axis is perpendicular to that line. 4. One unit is equal to 0.00l-inch at map scale. 5. The coordinate domain is limited to the range -32768 to +32767. Figure 5.--Location of origin of file reference coordinates. The file header contains the parameters of a transformation which can be used to convert the internal file coordinates to the ground coordinate system, which is the Universal Transverse Mercator (UTM) for 1:24,000-scale DLG's. An example of this transformation is given in Appendix E. Optional Distribution Format The DLG data in the optional distribution format are expressed in the units of the ground coordinate system; that is, meters in the UTM coordinate system. DATA VALIDATION The DLG data do not currently carry quantified accuracy statements. The following procedures, however, are used to validate the data files before they are released for distribution: 1. File fidelity and completeness -- The data are either manually digitized using equipment with a resolution of 0.00l inch and an absolute accuracy of from 0.003 to 0.005 inch, or are scanned on an automatic device with a resolution of 30 points per millimeter, or 0.0013 inch. The positional accuracy of the data and completeness of the file are checked by visually comparing proof plots with the original stable-base source material. These proof plots are generated using automated drafting machines with a resolution of 0.00l inch and an absolute accuracy of from 0.003 to 0.005 inch. 2. Attribute accuracy -- DLG attribute codes are checked by software against a table of valid codes to ensure that each attribute in a file is valid for the category and element type to which it is assigned. Validating the codes for correct application is currently a manual process involving the correlation of formatted listings with proof plots. 3. Topological fidelity -- The topological structure of each DLG file is fully validated by software. There are no extraneous intersections; that is, a line does not join or cross another line, or itself, except at a node. No line extends through a node. Polygon (area) adjacency is also validated; that is, area left and right topological attributes of lines are consistent throughout the file. The neatline is free of gaps. Validation of DLG data is performed for each category within a file. Additional data validation is being implemented as follows: 1. Edge matching -- Validation software provides for checking the edges of each quadrangle against the edges of the four adjacent quadrangles. Each edge of a DLG-3 is checked for positional and attribute matching along the neatlines of the adjoining DLG-3 cells, provided that the surrounding data cells are available at the time the DLG-3 is entered into the NDCDB. There is currently no attempt, other than the coding of coincident features, to provide fully integrated data categories. 2. Quality Control Flags -- Information in the header of the DLG-3 file indicates the status of the file with respect to the edge matching described above. Twelve bytes at the end of record A.1 in the standard distribution format and at the end of record 3 in the optional distribution format of the ASCII file is set aside for quality control flags (see Appendixes A and B). The first three of these flag positions are for future use. The fourth flag position contains a value encoded in the bit pattern that is used only by the database manager to check the edge status. The remaining either flags indicate to the user the edge status code and the status reason code. The four status flags contain the status of the West, North, East, and South edges of a DLG-3 as compared to the edges of the four adjoining DLG-3 files. Each of the four flags is followed by a status reason code that explains the status of the four edges respectively. The possible status values for a DLG-3 entered into the NDCDB are: o "b" - unchecked ("b" = blank) o "0" - passed edge match test o "1" - alignment discontinuity o "2" - attribute discontinuity o "3" - attribute and alignment discontinuity The possible reason codes are: o "b" - no reason code set ("b" = blank) o "4" - data do not exist o "5" - adjacent data cell not currently available for test o "6" - discontinuity due to temporal differences in source materials o "7" - attribute mismatch valid o "8" - paneling unauthorized A value of "4" indicates that the data cannot be matched because there are no adjacent data. This situation occurs where the quadrangle is on the coast and no adjoining map exists. A reason code with the value of "5" may be reset as the adjoining data cell becomes available for edge match verification. A reason code with the value of "6" means that the adjoining quadrangles were mapped at different times and there are features that do not match in alignment or classification due to the time elapsed between the compilation of the two sets of source materials. A value of "7" indicates that the discrepancy in attribute codes between the two files has been checked and is valid. A reason code with the value of "8" indicates that there was no authorization in place for edge matching at the time the data were archived. When an edge status code is other than "0", the DLG-3 file will be entered into the NDCDB only when the reason code has also been set as a result of examination of the file. The following combinations of status flags and reason flags are currently valid for the processing software. blank, blank blank, 4 blank, 5 blank, 8 0,blank 1,6 1,7 2,6 2,7 3,6 3,7 APPENDIXES APPENDIX A.--Standard DLG Distribution Format (Record Contents) In the standard DLG distribution format, the topological linkages are contained only in the line elements. The files are physically comprised of standard 8-bit ASCII characters organized into fixed-length logical records of l44 characters. Nine distinct record types are defined. Logical record type Content A Header record containing DLG identification information. B Header record containing projection information and registration points. C Header record identifying data categories contained in this DLG and indicating the number of nodes, areas, and lines in each category. D.l A node or an area record. D.2 A line record. E Record containing x,y coordinate string. F Record containing attribute codes. G Record containing text string (not currently used). H Accuracy estimate (not currently used). The actual sequence of records in a standard distribution DLG file is as follows: l. Header records Type A (one record) Type B (one record) Type C (one record) 2. Data records Node records Repeated Node description (D.l) for each Attribute codes (F) node within a Text string (G) data category Area records Area description (D.l) Repeated Repeated Attribute codes (F) for each for each Text string (G) area within a data category data category Line records Line description (D.2) Repeated x,y coordinates (E) for each Attribute codes (F) line within a Text string (G) data category 3. Accuracy estimate Type H (one record) (not currently used) Descriptions of the contents of records A-F are contained in the following tables. The tables also reflect the relationship between these record types and l44-byte logical records. APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued [Integer fields with a value of zero have leading zeros suppressed. Any field with the format of D24.l5 which has a value of zero will be represented as "bbb0.0bbbbbbbbbbbbbbbbbb", the last four positions of the fractional portion being reserved for a decimal exponent. (b=blank)] Logical Record Type A Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment A.l 1 Name of digital ALPHA A40 l 40 When practical, the name of cartographic unit the source map followed by the State two-character designator(s). --- --- Filler --- --- 4l 4l l space A.l 2 Date of original ALPHA Al0 42 5l Year of original source source materials material, followed by latest revision date if applicable. For example, 1956, 1965. A.1 3 Date qualifier ALPHA A1 52 52 Qualifier to discriminate revision date, if present. (P=photorevision, I=photo- inspected, L=limited revision) A.1 4 Scale of original INTEGER*4 I8 53 60 Scale denominator of source source material material; for example, 24000. --- --- Filler --- 6l 63 3 spaces A.1 5 Arbitrary quad ALPHA A3 64 66 Quad number assigned for USGS number internal use. Filler --- --- 67 113 47 spaces APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type A Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment A.1 6 Largest primary ALPHA A4 114 117 Largest primary contour contour interval interval, followed by the interval unit (1=feet, 2=meters). Present only if two or more primary contour intervals exist. (Selected overlays.) A.1 7 Comma ALPHA A1 118 118 Comma separator A.1 8 Largest primary ALPHA A4 119 122 Largest primary bathy- bathymetric contour metric interval, followed interval by the interval unit (1=feet 2=meters). Present only if two or more primary contour intervals exist. --- --- Filler 1 space A.1 9 Smallest primary ALPHA A4 124 127 Smallest or only primary contour interval contour interval, followed by the interval unit (1=feet, 2=meters). (Selected overlays.) A.1 10 Comma ALPHA A1 128 128 Comma separator A.1 11 Smallest primary ALPHA A4 129 132 Smallest or only primary bathymetric contour bathymetric interval, interval followed by the interval unit (1=feet, 2=meters). A.1 12-14 Coded Flags ALPHA A1 133 135 3 flags for future use APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type A--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment A.1 15 Coded Flag ALPHA A1 136 136 Database coded edge flag A.1 16 EDGEWS ALPHA A1 137 137 Status flag for west edge, values are: b = unchecked 0 = passed, 1 = alignment discontinuity, 2 = attribute discontinuity, 3 = attribute and alignment discontinuity. A.1 17 EDGEWR ALPHA A1 138 138 Reason for EDGEWS, values are: b = unchecked, 4 = adjacent data do not exist 5 = adjacent data unavail- able, 6 = temporal discontinuity, 7 = attribute mismatch valid, 8 = paneling unauthorized. A.1 18 EDGENS ALPHA A1 139 139 Status flag for north edge, values = b,0,1,2, or 3 as above. A.1 19 EDGENR ALPHA A1 140 140 Reason for EDGENS, values are b,4,5,6,7, or 8 as above. A.1 20 EDGEES ALPHA A1 141 141 Status flag for east edge. Values are b,0,1,2, or 3 as above. A.1 21 EDGEER ALPHA A1 142 142 Reason for EDGEES. Values are b,4,5,6,7, or 8 as above. A.1 22 EDGESS ALPHA A1 143 143 Status flag for south edge. Values are b,0,1,2, or 3 as above. A.1 23 EDGESR ALPHA A1 144 144 Reason for EDGESS. Values are b,4,5,6,7, or 8 as above. APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type A--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment A.2 1 DLG level code INTEGER*2 I6 l 6 Code=3, DLG-3 A.2 2 Code defining INTEGER*2 I6 7 l2 Code=l, Universal Transverse ground planimetric Mercator (UTM) reference system A.2 3 Code defining zone INTEGER*2 I6 l3 l8 Codes for UTM coordinate in ground plan- zones are given in imetric reference Appendix C. system A.2 4 Map projection REAL*8 5D24.l5 l9 l38 This field contains the parameters first 5 of l5 map projection parameters. Parameters for the UTM projection are given in Appendix C. --- --- Filler --- --- l39 l44 6 spaces A.3 1 Map projection REAL*8 6D24.l5 l l44 This record contains parameters projection parameters 6 thru ll. Parameters for the UTM projection are given in Appendix C. A.4 1 Map projection REAL*8 4D24.l5 l 96 This field contains the parameters last 4 projection parameters. Parameters for the UTM projection are given in Appendix C. A.4 2 Code defining units of INTEGER*2 I6 97 l02 Code=2, meters measure for ground planimetric coordinates throughout the file APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type A--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment A.4 3 Resolution REAL*8 D24.l5 l03 l26 The true ground distance corresponding to one unit (0.001 inch at map scale) in the file internal reference system. Scale Resolution 1:24,000 0.61 M 1:25,000 0.635 M 1:48,000 1.22 M 1:62,500 1.587 M A.4 4 Accuracy code of INTEGER*2 I6 l27 l32 Code=0, unknown accuracy planimetric data A.4 5 Number (n) of sides INTEGER*2 I6 l33 l38 n=4 in the polygon which defines the coverage of the cell --- --- Filler --- --- 139 l44 6 spaces A.5 1 A (4,2) array contain- REAL*8 3(2D24.l5) 1 l44 The four registration points A.6 ing geographic 2D24.l5 l 48 will usually coincide with an coordinates which area defined by one of the constitute the regis- standard map formats of the tration points for the National Mapping Program. DLG. In quadrangle- Coordinates are in geographic based mapping, these longitude and latitude in points form a geographic units of degrees and decimal rectangle/square which degrees and are expressed in contains the domain of the DLG. the order=SW, NW, NE, SE. --- --- Filler --- --- 49 l44 96 spaces APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type B Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment B.l 1 Parameters (Al, A2, REAL*8 4D24.l5 1 96 X,Y coordinates resulting A3, A4) of file- from this transformation to-ground projection will be in ground meters in transformation; the UTM zone defined by data explicit form of the element 3 of record A.2. transformation is: X=Alx+A2y+A3 Y=Aly-A2x+A4 where: x,y are coordinates in file internal reference system X,Y are coor- dinates in map projec- tion reference system B.l 2 Number (m) of regis- INTEGER*2 I6 97 l02 m=4 tration points --- --- Filler --- --- l03 l44 42 spaces B.2 1 A (4,3) array con- ALPHA/ 4(A2, 2I6) l 56 The corners of a four-sided taining identifications INTEGER*2 polygon are used as and coordinates of registration points. The registration points. identification sequence is Coordinates are SW, NW, NE, SE. The array expressed in the file is stored by row. internal reference Coordinates in the file system. internal reference system are expressed in units of thousandths of an inch and fall in the range -32768 to +32767. These coordinates correspond to the geographic coordinates contained in records A.5 and A.6. --- --- Filler --- --- 57 l44 88 spaces APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type C Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment C.l 1 Number (q) of INTEGER*4 I6 1 6 The value of q may be from 1 categories to 32. Up to 32 categories in the DLG file can be represented in a given file. The value will be 1. --- --- Filler --- --- 7 l44 l38 spaces C.21 1 A (q,7) array contain- ALPHA/ q (A20,6I6) 1 56 This array is stored by to C.N ing category names as INTEGER*2 (57 112) row. The first element is well as maximum and the category name consisting actual number of node, of 20 alphanumeric characters area, and line elements the first four of which are in each category unique. Columns 2 and 3 of the array contain maximum and actual number of nodes in the category. Columns 4 and 5 contain maximum and actual number of areas in the category. Columns 6 and 7 are the maximum and actual number of line segments. (Note: the maximum number of nodes or areas within a category is 25, 960. The maximum number of lines is 25,938. This field is used only during initial processing of data). --- --- Filler --- --- --- 144 32 or 88 spaces 1The number of categories "q" is given in record C.1. There will be 56 bytes of data per category, and thus a maximum of two categories can be described on a 144-character record. The space filler will vary in size depending on the value of "q." APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type D Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment D.l 1 Type of element ALPHA A2 1 2 Code ='Nb' for Node element, code 'Ab' for Area element. D.l 2 Element's internal INTEGER*2 I6 3 8 Number is positive and se identification number quential from 1-n within each category and element type. D.l 3 x,y file coordinate INTEGER*2 2I6 9 20 The representative area of node point or repre- point is usually, but not sentative point for the always, contained within the area element area it represents. D.l 4 Number (t) of attri- INTEGER*2 I6 2l 26 Absence of attribute codes bute codes which are is indicated by t=0. attached to the node or area element (t>0) D.l 5 Number (k) of pairs of INTEGER*2 I6 27 32 k=0. Not currently used. text characters which are attached to the node or area element (k>0) --- --- Filler --- --- 33 l44 ll2 spaces D.2 1 Code indicating a ALPHA A2 l 2 Code='Lb' for line segment. line segment graph element D.2 2 Line segment's internal INTEGER*2 I6 3 8 Number is positive and identification number sequential from 1-n within each category and element type. D.2 3 Internal identification INTEGER*2 I6 9 l4 Number refers to data number of starting node element 2 in record D.l. APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type D--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment D.2 4 Internal identification INTEGER*2 I6 l5 20 Number refers to data number of ending node element 2 in record D.l. D.2 5 Internal identification INTEGER*2 I6 21 26 Number refers to data number of left area element 2 in record D.l. D.2 6 Internal identification INTEGER*2 I6 27 32 Number refers to data number of right area element 2 in record D.l. D.2 7 Number (v) of coordinate INTEGER*2 I6 33 38 The value of v is from pairs which define the 2 to 3000. line segment D.2 8 Number (t) of attribute INTEGER*2 I6 39 44 Absence of classification codes which are attached attribute codes is indicated to the line segment (t>0) by t=0. D.2 9 Number (k) of pairs of INTEGER*2 I6 45 50 k=0. Not currently used. text characters which are attached to the line segment (k>0) --- Filler --- --- 5l 144 94 spaces E.l to2 1 A (v,2) array contain- INTEGER*2 v(2I6) 1 Coordinates are expressed E.n ing an ordered sequence in internal file reference of coordinate pairs which system, in units of define the image presen- thousandths of an inch. tation of a line element The array is stored by row. --- --- Filler --- --- --- 144 0 to l32 spaces 2The number of coordinate pairs, "v", is given in record D.2. There will be v(2I6) coordinate pairs of which a maximum of l2 pairs will fit on a l44-character ASCII record. The space filler will vary in size depending on the value of "v." If "v" is an integer multiple of l2, there will be no spaces as filler at the end of the record. APPENDIX A.--Standard DLG Distribution Format (Record Contents)--continued Logical Record Type F Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment F.l3 1 A (t,2) array containing INTEGER* 2 t(2I6) 1 The array is stored by row to F.n major and minor attribute with the first column con- codes for a graph element taining the major attribute code and the second column containing the minor attri- bute code. --- --- Filler --- --- --- 144 0 to l32 spaces 3The number of feature (attribute) codes, "t" is given in the D.l and D.2 records. The F record is an array of t(2I6) codes of which a maximum of l2(2I6) will fit on a l44 character ASCII record. The space filler will vary depending on the value of "t". If "t" is an integer multiple of l2 there will be no spaces as filler at the end of the record. APPENDIX B.--Optional DLG Distribution Format (Record Contents) In the optional DLG distribution format, topological linkages are explicitly encoded for node and area elements as well as for line elements. The files are physically comprised of 8-bit ASCII characters organized into fixed-length logical records of 80 characters (bytes). Bytes 1-72 of each record may contain DLG data, and bytes 73-80 may contain a record sequence number. The 11 distinct record types used in the optional DLG distribution format may be categorized as header and data records. Four types of records are considered header records: o File identification and description records o Accuracy records (not currently used) o Control-point identification records o Data-category identification records Seven types of records are considered data records: o Node and area identification records o Node-to-line linkage records o Area-to-line linkage records o Line identification records (also contains line-to-node and line-to-area linkages) o Coordinate string records o Attribute code records o Text records (not currently used) The actual sequence of records in an optional distribution format DLG file is as follows: l. Header records Ten file identification and description records Accuracy records (not currently used) Control point identification records (one per control-point) Data category identification records (one per data category in the file) 2. Data records Node identification record Repeated Node-to-line linkage record(s) for each Attribute code record(s) node within a Text record(s) data category Area identification record Repeated Repeated Area-to-line linkage record(s) for each for each Attribute code record(s) area within a data Text record(s) data category catagory Line identification records Repeated Coordinate string record(s) for each Attribute code record(s) line within a Text record(s) data category Descriptions of the contents of the various types of records in an optional distribution format DLG are contained in the following tables. APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued FILE IDENTIFICATION AND DESCRIPTION RECORDS Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte* Comment 1 1 Banner ALPHA A72 1 72 Descriptive text. 2 1 Name of digital ALPHA A40 1 40 When practical, the name of cartographic unit the source map followed by the State two-character designators. --- --- Filler --- --- 41 41 l space 2 2 Date of original ALPHA Al0 42 51 Year of original source source material material followed by latest revision date if applicable; for example, l956, l965. 2 3 Date qualifier ALPHA A1 52 52 Qualifier to discriminate revision date if present. (P=photorevision, I=photo- inspection, L=limited revision). 2 4 Scale of original INTEGER*4 I8 53 60 Scale denominator of source source material material, for example, 24000. --- Filler --- 61 63 3 spaces *The logical record length for the optional distribution format is 80 bytes, with 8 spaces of blank fill in bytes 73-80 of each record which may be used for a record sequence number. APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued FILE IDENTIFICATION AND DESCRIPTION RECORDS--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 2 5 Arbitrary quad ALPHA --- 64 66 Quad number assigned for USGS number (24K files) internal use. 2 Filler --- --- 67 72 6 spaces 3 Filler --- --- l 41 41 spaces 3 1 Largest primary ALPHA A4 42 45 Largest primary contour contour interval interval, followed by the interval unit (1=feet, 2=meters). Present only if two or more primary contour intervals exist. (Selected overlays.) 3 2 Comma ALPHA A1 46 46 Comma separator 3 3 Largest primary ALPHA A4 47 50 Largest primary bathy- bathymetric contour metric interval, followed interval by the interval unit (1=feet, 2=meters, 3=fathoms). Present only if two or more primary intervals exist. --- --- Filler 1 space 3 4 Smallest primary ALPHA A4 52 55 Smallest or only primary contour interval contour interval, followed by the interval unit (1= feet, 2=meters). (Selected overlays.) 3 5 Comma ALPHA A1 56 56 Comma separator APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued FILE IDENTIFICATION AND DESCRIPTION RECORDS--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 3 6 Smallest primary ALPHA A4 57 60 Smallest or only primary bathymetric contour bathymetric interval, interval followed by the interval unit (1=feet, 2=meters, 3=fathoms). 3 7-9 Coded Flags ALPHA A1 61 63 3 flags for future use 3 10 Coded Flag ALPHA A1 64 64 Database coded edge flag 3 11 EDGEWS ALPHA A1 65 65 Status flag for west edge, values are: b = unchecked, 0 = passed, 1 = alignment discontinuity, 2 = attribute discontinuity, 3 = attribute and alignment discontinuity. 3 12 EDGEWR ALPHA A1 66 66 Reason for EDGEWS, values are: b = unchecked, 4 = adjacent data do not exist, 5 = adjacent data unavail- able, 6 = temporal discon- tinuity, 7 = attribute mis- match valid, 8 = paneling unauthorized. 3 13 EDGENS ALPHA A1 67 67 Status flag for north edge, values = b,0,1,2, or 3 as above. 3 14 EDGENR ALPHA A1 68 68 Reason for EDGENS, values are b,4,5,6,7, or 8 as above. 3 15 EDGEES ALPHA A1 69 69 Status flag for east edge Values are b,0,1,2, or 3 as above. APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued FILE IDENTIFICATION AND DESCRIPTION RECORDS--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 3 16 EDGEER ALPHA A1 70 70 Reason for EDGEES. Values are b,4,5,6,7, or 8 as above. 3 17 EDGESS ALPHA A1 71 71 Status flag for south edge. Values are b,0,1,2, or 3 as above. 3 18 EDGESR ALPHA A1 72 72 Reason for EDGESS. Values are b,4,5,6,7, or 8 as above. 4 1 DLG level code INTEGER*2 I6 1 6 Code=3, DLG-3 4 2 Code defining ground INTEGER*2 I6 7 12 Code=l, UTM planimetric reference system 4 3 Code defining zone in INTEGER*2 I6 13 18 Codes for UTM coordinate ground planimetric zones are given in appendix reference system C. 4 4 Code defining units INTEGER*2 I6 19 24 Code=2, meters of measure for ground planimetric coordinates throughout the file 4 5 Resolution REAL*4 Dl8.ll 25 42 The true ground distance corresponding to one unit (0.00l inch at map scale) in the file internal coor- dinate system used in data collection. Scale Resolutions 1:24,000 0.61 M 1.25,000 0.635 M 1:48,000 1.22 M 1:62,500 1.587 M APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued FILE IDENTIFICATION AND DESCRIPTION RECORDS--continued Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 4 6 Number of file-to- INTEGER*2 I6 43 48 Usually 4. map transformation parameters 4 7 Number of accuracy/ INTEGER*2 I6 49 54 Currently=0, none included miscellaneous records 4 8 Number (n) of sides INTEGER*2 I6 55 60 n = 4. in the polygon which define the coverage of the cell. Number (n) also defines the number of control-points 4 9 Number (q) of categories INTEGER*2 I6 61 66 Value of q may be from 1 to in the DLG file 32. The value will be 1. --- --- Filler --- --- 67 72 6 spaces 5-9 1 Projection parameters REAL*8 3D24.l5 l 72 Three parameters on each of for map transformation 5 records. Parameters for the UTM projection are given in Appendix C. 10 1 Internal file-to- REAL*4 4Dl8.ll 1 72 A transformation of this map projection type is not required, since transformation coordinates are expressed parameters in a ground planimetric coordinate system (usually UTM). These parameters are however, valid for trans- formation as described in record B.1, data element 1, of the standard format. APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued CONTROL-POINT IDENTIFICATION RECORDS Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 1-n 1 Control-point label ALPHA A2 1 2 "SW," "NW," "NE," or "SE" for four quadrangle corners. Filler 3 6 4 spaces 2 Latitude REAL*4 Fl2.6 7 18 In degrees and decimal degrees. 3 Longitude REAL*4 Fl2.6 19 30 In degrees and decimal degrees. Filler 31 36 6 spaces 4 X coordinate REAL*4 Fl2.2 37 48 In units in the appropriate zone of the ground plani- metric coordinate system. 5 Y coordinate REAL*4 Fl2.2 49 60 In units in the appropriate zone of the ground plani- metric coordinate system. --- --- Filler --- --- 61 72 l2 spaces APPENDIX B.--Optional DLG Distribution Format (Record Contents)--continued DATA CATEGORY IDENTIFICATION RECORDS Record Data Type Starting Ending Number Element Contents (Fortran Notation) Format Byte Byte Comment 1-q 1 Category name ALPHA A20 1 20 The first 4 characters are unique. 2 Attribute format codes INTEGER*2 I4 21 24 Blank or zero (0) indicates default (2I6) attribute formatting in major-minor pairs. 3 Number of nodes refer- INTEGER*2 I6 25 30 Number of nodes referenced enced in file in file as start and end nodes of lines. 4 Actual number of INTEGER*2 I6 31 36 Only if some or all node nodes in file records were excluded from the file, would this number be different from data element 3. Filler