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  • The SRTM global Mapping Mission... almost 2 years later!

    By GeoCommunity Staff - editor@geocomm.com - (Jan. 24, 2002)
    Printer Friendly Version

    Can you beleive that it's been almost 2 years since we provided detailed coverage of the SRTM mission? Our original coverage was composed of detailed mission goals, we featured exclusive commentaries from our Space correspondent (Philip Chien), and we tracked the mission from start to finish. It's been roughly 2 years and many of you are now wondering about the status and availability of the data. This SRTM feature serves to bring you up to date and will help you locate and download the derivative SRTM data products. If you are using the sRTM data products, please tell the editor about it. We'd love to hear how/what you're doing with the data as well as your comments & feedback.

    SRTM Recap

    The Shuttle Radar Topography Mission (SRTM) data products result from a collaborative mission by the National Aeronautics and Space Administration (NASA), the National Imagery and Mapping Agency (NIMA), the German space agency (DLR) and Italian space agency (ASI), to generate a near-global digital elevation model (DEM) of the Earth using radar interferometry.

    The SRTM data flight occurred Feb. 11-22, 2000 on STS-99 and successfully fulfilled all mission objectives. Following a lengthy calibration and validation phase, the 12 terabytes of raw data are currently being processed into digital elevation maps.

    Displaying spectacular new 3-D images and animations of California from space, scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., recently announced the release of high-resolution topographic data of the continental United States gathered during the February 2000 Shuttle Radar Topography Mission -- a mission that is creating the world's best topographic map.


    Figure 1: SRTM data compared to GTOPO30


    A look through some of the supplied documents reveal details of how the data processing is occurring on two parallel tracks:
    1. Systematic processing of the global data set on a continent-by-continent basis, with North America first. As each continent is completed, the data will be delivered to NIMA where it will be edited, verified, and brought into conformance with National Map Accuracy Standards. These 'finished' data will then be returned to NASA for distribution to the public through the United States Geological Survey. Current plans call for the first continental data to be available by Summer, 2002, with the whole job completed by the end of 2002.

    2. Processing of smaller data sets covering sites of scientific interest designated by SRTM Principal Investigators. Each site covers a number of 1 degree by 1 degree latitude and longitude 'cells', and each processed data set consists of unedited digital elevation maps, images, and ancilliary data. As these data sets are completed, they will be made available to the scientific community and the general public.

    How To Access Publicly Available SRTM Data

    SRTM data for a number of sites within the United States are now available for download via File Transfer Protocol (ftp). Users should be aware that these data are intended for use with Geographic Information System (GIS) or other special application software, and are not directly viewable in a browser. Various types of visualizations (perspectives, stereo pairs, anaglyphs, etc.) generated using these data are available from the main page.

    Also, users should be aware that the digital elevation maps and images are unedited and are intended for scientific use and evaluation. They are outputs directly from the SRTM interferometric radar processor, and for example may contain numerous voids (areas without data), water bodies may not appear flat, and coastlines may be ill-defined. As of January 24, 2002, the following data was available via the SRTM ftp site:
    • California (Wed Jan 23)
    • Kodiak_Island (Mon Jan 14)
    • Madison_County (Mon Jan 14)
    • Michigan_Forests (Mon Jan 14)
    • Oregon_Coast (Mon Jan 14)
    • Pinon_Canyon (Mon Jan 14)
    • Salt_Lake_City (Tue Jan 22)
    • Utah_Southeast (Mon Jan 14)
    • Washington_state (Mon Jan 14)
    Data are available either from the USGS server:
    ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/
    or from JPL at
    ftp://fringe.jpl.nasa.gov/sipub/SRTM_Data/

    Sample Image (Pasadena, CA)
    The image was created from three datasets: the Shuttle Radar Topography Mission (SRTM) supplied the elevation data; Landsat data from November 11, 1986 provided the land surface color

    Once a coverage area has been selected, users will be presented with a directory such as: N33W115. File & directory names refer to the latitude and longitude of the lower left corner of the tile - e.g. N33W115 has its lower left corner at 33 degrees north latitude and 115 degrees west longitude. Two sub-directories (eg. 1arcsec/ & 3arcsec/) will then provide users access to SRTM-1 or SRTM-3 data. SRTM data are distributed in two levels: SRTM-1 with data sampled at one arc-second intervals in latitude and longitude, and SRTM-3 sampled at three arc-seconds.

    Data are divided into one by one degree latitude and longitude tiles in "geographic" projection, which is to say a raster presentation in no projection at all but easy to manipulate and mosaic. xxx_yyy denotes the individual data take during which the data were acquired, where xxx = orbit number and yyy = serial number of acqisition on that orbit. Note: extensive meta-data is provided in the .ANN file

    For Users of ARC/INFO

    Users of ARC/INFO or ArcView can display the DEM data directly after renaming the file extension from .HGT to .BIL. However, if a user needs access to the actual elevation values for analysis in ARC/INFO the DEM must be converted to an ARC/INFO grid with the command IMAGEGRID. See the readme

    not to be confused with DTED data (see also http://edcftp.cr.usgs.gov/glis/hyper/guide/usgs_dem) , the SRTM-1 and SRTM-3 data differ from DTED in a number of ways:
    1. They are unedited, and may contain numerous voids (regions with no data) and other spurious point such as anomalously high ("spike") or low ("well") values.

    2. Coastlines of water bodies are typically not well delineated and may not appear "flat".

    3. They have not been evaluated for conformance with National Mapping Accuracy standards.

    4. SRTM-1 and SRTM-3 differ significantly in data format from the DTED standards.

    5. Elevations are given relative to the WGS84 ellipsoid or to the reference surface that was used to measure ground control points. Final DTED products will give elevations relative to the WGS-84 geoid.

    6. The SRTM-1 and SRTM-3 topography mosaics do not utilize the continental-scale block adjustments that will be used for the final global mosaics, and thus may contain residual offsets and tilts.

    Source: SRTM user documentation

    Detailed documentation with technical specification of the SRTM data can be found at
    ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/..SRTM_Topo.txt

    About the SRTM Mission

    The Shuttle Radar Topography Mission was flown aboard Space Shuttle Endeavour February 11-22, 2000. It used modified versions of the same instruments that comprised the Space Shuttle Imaging Radar-C/X-Band Synthetic Aperture Radar that flew twice on Endeavour in 1994.

    The mission collected 3-D measurements of Earth's land surface using radar interferometry, which compares two radar images taken at slightly different locations to obtain elevation or surface-change information. To collect the data, engineers added a 60-meter (approximately 200-foot) mast, installed additional C-band and X-band antennas, and improved tracking and navigation devices.

    The Shuttle Radar Topography Mission supports NASA's Earth Science Enterprise, Washington, D.C., a long-term research and technology program designed to examine Earth's land, oceans, atmosphere, ice and life as a total integrated system.

    Feedback from SRTM Data Users

    CubeWerx Inc. SRTM Demo - The folks at cubeWerx have loaded the DEM data for the published regions into their spatial warehouse. They are viewable in a online demo, with a shaded relief algorithm applied. The link below jumps to the demo at an appropriate zoom level etc, with some context data in the background. The data is displayed on top of the GTOPO30 data set. The SRTM data has been aggregated in the warehouse, so you users quickly view the entire data set, or zoom into any area. A screen shot is provided below.

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