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Digital Image Conversion Utilities

By Brian May, St. Johns River Water Management District (bam@sjrwmd.state.fl.us); Stephen W. Hodge, Florida Resources and Environmental Analysis Center, Florida State University (shodge@freac.fsu.edu), Georgianna Strode, Florida Resources and Environmental Analysis Center, Florida State University (strode@freac.fsu.edu). This is a revission of an article originally posted at www.spatialnews.com/education/tutorials/drgprocessing1/

Digital Ortho Quarter Quads (DOQQs) and Digital Raster Graphics (DRGs) offer the greatest benefits when used in combination with other existing GIS data. In order for this to occur, the DOQQ, DRG, and GIS data formats must be compatible. In many cases, the DOQQ’s may need to be georeferenced to NAD83 or NAD27, resampled, reprojected, merged into a seamless image, or converted to another graphic format. The DRG’s may need to be reprojected, have the "collars" (the white area around the map) removed, and be merged into a seamless image. Florida State University has assembled a collection of DOQQ and DRG utility programs to easily perform these tasks using ARC/INFO. This paper offers a brief introduction to these concepts.
Introduction

Geographic Information Systems (GISs) have most often employed vector data because its flexibility allows users to easily attach attributes and perform queries and calculations. However, with the introduction of high-quality georeferenced raster images, GIS designers are increasingly incorporating raster layers to visually enhance their systems and provide a Geographic Information Systems with the benefits of vector data combined with the visual impact of raster images.

Digital Orthographic Quarter Quads (DOQQs) are scanned aerial photographs that are rectified to correct for camera lens distortion, tilt and pitch of the airplane and the terrain of the ground. The USGS–supplied images have a ground resolution of one meter. This resolution can be lowered to reflect the needs of the specific system. Digital Raster Graphics (DRGs) are scanned USGS topological sheets. Both DOQQs and DRGs can be used as visual backdrops in GIS systems. DOQQ’s and DRG's can be easily combined with other digital cartographic products such as digital elevation models (DEM) and Digital Line Graphs (DLG). The DOQQ and DRG show great potential as tools for validating and assessing other digital data.

In order to be used with existing GIS data, raster images must be compatible with other GIS data. In many cases, the DOQQ’s may need to be georeferenced to NAD83 or NAD27, resampled, reprojected, merged into a seamless image, or converted to another graphic format. The DRG’s may need to be reprojected, have the "collars" (the white area around the map) removed, and be merged into a seamless image.

Florida State University has assembled a collection of DOQQ and DRG utility programs to easily perform these tasks using ARC/INFO. This paper offers a brief introduction to these concepts. Copies of the DOQQ programs and instructions are available at http://www.freac.fsu.edu/tap/doqutil. DRG aml scripts can be obtained at http://sjr.state.fl.us/technical/gis/data.

Digital Orthographic Quarter Quads

Background

A digital orthophotograph is a digital image with the geometric qualities of a map. It is derived from a digitized aerial photograph corrected for image displacements caused by camera tilt and relief of terrain. The U.S. Geological Survey (USGS) is the lead Federal agency responsible for the collection and distribution of the DOQQs on a national basis.

The images are georeferenced with a UTM projection and both NAD27 and NAD83 for the datum. Each quadrangle covers one 7.5" area, broken into four quarters, hence the name "quarter quad". Florida has color infrared (three bands); other states may have black-and-white (one band) images. Each color quarter-quad is approximately 150 megabytes in size, totaling 600 MB for the quad, while a black-and-white quarter-quad is only 50 megabytes. The files are in Band Interleave by Pixel (BIP) format with a resolution of one meter.
v The BIP file contains ASCII text with information such as quad name, quarter piece, fly date, coordinates, etc. BIP files are in two different formats. Part of Florida is in the "old" format, which has a long record format. Most files are in the 80-character, or "new," record format. For more information, see the USGS "Standard for Digital Orthophotos" at http://mapping.usgs.gov/www/ti/DOQ/standards_doq.html.

Software Tasks Performed
Input: Three-band Digital Ortho Quarter-Quads on CD-ROM distributed by USGS
Output: Georegerenced images in the following formats: TIFF, ERDAS (LAN and IMG), BIL, BIP, JFIF(JPG), BMP, BSQ, or RLC.
Optional Functions

Projection from UTM to state plane (other projections available with Version 2.0)
Resampling at user-specified rate
Merging of four quarter-quads into one image (available with Version 2.0)
One-band image processing (available with Version 2.0)

Additional Features

Datum can be either NAD83 or NAD27
Three user-specified file paths (input, output, and working area) helps processing organization
No advanced knowledge of ARC/INFO is necessary

System Requirements

Designed for DEC Alpha (unix) and RS6000 workstations. (Not tested on SUN platforms.)
Designed for ARC/INFO 7.0 (Will run at the 6.x level with the exception of conversion to JPG format)
ARC/INFO GRID license is necessary for reprojection
At least 2 G of disk space for color images (600MB for black-and-white) is necessary for temporary workspace and final images
FORTRAN or C compiler (if using a system other than RS6000 or DEC Alpha (unix))

Creation of Header Files

The DOQQs files must have an accompanying header file in order to be georeferenced. Header files (.hdr) contains coordinate and pixel information necessary for a GIS program. Header files may be created for NAD27 or NAD83 datums. The sample header file below contains information on the image size, coordinates, and number of bands present. This information is used by the GIS software programs to both read and georeference the image.
Sample Header File

nrows 7613

ncols 6728

skipbytes 80736

ulymap 3298867.000000

ulxmap 390920.000000

totalrowbytes 20184

layout bip

xdim 1

ydim 1

nbands 3

Header files can be created by any program that is able to read the ASCII information from the raw DOQQ file and write the information to a header file. The DOQQ utilities include executable code for RS6000 and DEC Alpha (unix) machines. The FORTRAN and C source code is also included and can be compiled for other machines.

Reprojection to State Plane

The reprojection process uses the GRID add-on package to ARC/INFO.

A disadvantage to the projection to state plane is a thin black border around the image due to the difference in shape between UTM and State Plane projections. This does not affect image quality and it can be corrected with a clipping process or by merging the images to a single image.

Resampling

Resampling reduces the image resolution and file size. This is useful when not as much detail is needed or file size is important. A typical use of a resampled DOQ might be for backdrops or general location maps. Figures 1, 2, and 3 show samples of 1-, 3-, and 6-meter resolutions.

Other Issues

There can be color differences between quarter-quads due to film differences, atmospheric conditions, and other reasons. This does not pose a problem for the software, but can present visual and analytical problems.

Reprojection is a time-consuming process. Conversion of one quad on a 500 MZ computer is about 6 hours depending on other processes. The same task on a slower computer can take weeks.

Digital Raster Graphics

Background

The USGS DRGs are digital representations of the 1:24,000, 1:100,000, and 1:250,000 map series. The USGS has scanned these maps and is now providing them on CD-ROM and over the Internet. They are nice to look at by themselves, but most people want to include them into their GIS. There are two problems that must be overcome. The first is that the quads are in the UTM NAD27 projection and coordinate system. Most organizations will need to project the quads. The other problem is that the maps were originally designed to be stand-alone and not part of a layer in a GIS. To get the most use out of the quads, the map collar must be clipped off and the quads must appear as a seamless map layer within the GIS. Figures 4 and 5 show a portion of a clipped and unclipped DRG. The edges of the clipped DRGs have a 100-meter overlap from adjacent quads. This overlap is visible on both the south and east borders in Figure 5. For a more complete description of DRGs, see http://www.labins.org/drg.htm#descr.

System Requirements for Utilities

The DRG utilities run on either UNIX or NT machines using ARC/INFO with the GRID add-on package to ARC/INFO. They are not tested on versions of ARC/INFO prior to 7.0.



Removing the Map Collar

An AML named scan24_proc1.aml was created for removing the map collar. The AML uses a 1:24,000 quad index coverage to reselect polygons to clip the GRID format DRG.

The basic methodology is as follows:

1.Using cursors, get the 4-digit unique quad number from the index24 coverage. (the USGS quads use a LAT/LONG and alpha-numeric numbering scheme).
2.Reselect the quad polygon from the index24 coverage (based on NAD27 tiles!)
3.Clip the DRG with the quad
4.Rename the quad to the your standard

Creating a Seamless DRG Layer

A separate AML named scan24_proc2.aml was developed for creating the images that became part of the seamless DRG layer.

The basic methodology is as follows:

1.With a simple looping AML, put all of the clipped DRGs that will be needed into a single directory.

NOTE: There have been problems putting more than 150 GRID format DRGs in one directory. GRIDs were randomly becoming corrupted when more were copied into the directory. Therefore, it is recommended to do this process in batches.

2.Get the coordinates for the quad box and add a 100-meter buffer
3.Use the coordinates with the generate command to generate a polygon coverage
4.Calculate which quads will be needed
5.Merge all available surrounding quads (up to 8 plus the quad itself) and re-clip the grid using the generated polygon coverage
6.Convert the DRG from GRID to TIFF image
7.Clean up

Creating the Image Catalog

The image catalog allows you to seamlessly browse the 1:24,000 DRGs without having to know which quad to load into ArcView or Arcplot.

Following are the two ARC/INFO commands necessary to implement the catalog.

1.CREATECATALOG drg24
2.ADDIMAGE drg_list drg24

In ArcView, image catalogs are selected by adding an image theme type and you must go into the INFO directory where the catalog is stored in order to see the catalog.

In ArcPlot, image catalogs are treated like a single image on the command line, e.g. IMAGE drg24

DRG Conclusion

The result is a set of DRG images that are squared off using the surrounding DRGs. The net effect, when used in conjunction with an image catalog, is the appearance of a seamless DRG layer in the GIS. On-screen, the 1:24,000 DRGs look best at a scale less than 1:24,000. This is because the screen is not as high resolution as the printed map is. The features on the map therefore do not have enough pixels on the screen to properly represent themselves. The 1:100,000 and 1:250,000 maps are similar in this respect. The TIFF image format using packbits compression is the same format that the USGS uses for the DRGs. The amount of compression varies by quad, depending on the variation in color on the quad. To give you an idea of disk space requirements, 275 quads in compressed TIFF format is taking up 2200 MB of space. File sizes range from around 3 MB to 23 MB. The average appears to be around 8 MB. The process outlined above is time-consuming, but the computer does most of the work and the results are well worth it.

Acknowledgements

Digital Orthographic Quarter Quads

The original concept and programs were developed by Dr. Steve Dicks of the Southwest Florida Water Management District. The United States Forest Service (USFS) has contracted with by the Florida Resources and Environmental Analysis Center (FREAC) of the Florida State University (FSU) for enhancements to the original programs.

Digital Raster Graphics

The DRG scripts were written by Brian May bam@sjrwmd.state.fl.us of the St. Johns River Water Management District.
Thanks to Brian May of the SJRWD for this contribution. All rights reserved - This article is the property of the SJRWMD and has been provided by them to The GeoCommunity. Any copying or reproduction of the article in whole or in part is strictly prohibited.


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