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PLEASE NOTE: This is a beta release of the Antarctica DEM. If any data anomalies are noticed, please send an E-mail to either Mike Oimoen at: email@example.com, or Sue Jenson at: firstname.lastname@example.org. We will look into them, and they may be addressed in the next release.
The Antarctica data is provided in two projections.
Antartic DEM in geographic (lat/lon) coordinates: 30 arc-second spacing
ant_dem_lkms1 UL = 180 W, 60S. LR = 90W, 90S (3600 rows x 10800 columns) ant_dem_lkms2 UL = 90 W, 60S. LR = 0W, 90S (3600 rows x 10800 columns) ant_dem_1kms3 UL = 0 W, 60S. LR = 90E, 90S (3600 rows x 10800 columns) ant_dem_1kms4 UL = 90 E, 60S. LR = 180E, 90S (3600 rows x 10800 columns)
Antarctic DEM in polar stereographic coordinates (meters)
ant_dem_1kmps UL = -2700000 x 2699000. LR = 2699000 x -2700000 (5400 rows x 5400 columns)
Note: Both DEMs are referenced to the WGS84 ellipsoid. The standard latitude of the polar stereographic DEM is 71S, and it central meridian is 0.
Data are distributed as 16-bit straight raster image files in a latitude/ longitude coordinate system, and also in a polar stereographic coordinate system.
Image files are identified by the .bil.gz extension. Each image file of the Antarctica data set is compressed using the GNU "gzip" utility. If you do not have access to gzip, the FTP server will uncompress the file as you retrieve it. To do this, simply leave off the ".gz" extension when retrieving the file (NOTE: This option is not available through MOSAIC). For example, to retrieve the file "af_1k_dem1.bil.gz" without compression just use "get af_dem_lks1.bil". Note that the uncompressed files are typically five times larger than the compressed versions and so will take five times longer to transmit. The gzip program is available via anonymous FTP at the following sites: prep.ai.mit.edu:/pub/gnu wuarchive.wustl.edu:/systems/gnu
Each image file is accompanied by five ancillary files (header file, world file, statistics file, coordinate file, and data descriptor record ). The format of each ancillary file is described below:
Header File (.hdr)
The header file can be used as input to the ARC/INFO Image Integration application. Information contained within this file can also be used with other image display packages such as IMDISP. The header file is named after the image with an .hdr extension. The following list identifies the keywords used in the header file (ARC/INFO User's Guide, Image Integration, 5-4 to 5-10):
byteorder: byte order in which image pixel values are stored M = Motorola byte order (high order byte first--Sun, HP, etc.) layout: organization of the bands in the file bil = band interleaved by line (note: single band image. This field is required for image integration in ARC/INFO. nrows: number of rows in the image
ncols: number of columns in the image nbands: number of spectral bands in the image (will always be 1) nbits: number of bits per pixel (will always be 16) bandrowbytes: number of bytes per band per row (will always be double the number of columns) totalrowbytes: total number of bytes of data per row (will always be double the number of columns) bandgapbytes the number of bytes between bands in a BSQ format image, the default is 0. nodata value used for masking purposes (will be ignored by the Image Integration application; it is not a standard ARC/INFO keyword) ulxmap: longitude of the center of the upper-left pixel ulymap latitude of the center of the upper-left pixel xdim x dimension of a pixel in geographic units (decimal degrees) ydim y dimension of a pixel in geographic units (decimal degrees)
Example Header file:
byteorder M layout BIL nrows 2410 ncols 2401 nbands 1 nbits 16 bandrowbytes 4802 totalrowbytes 4802 nodata -999 bandgapbytes 0 nodata -9999 ulxmap 30.0000000000000000000000000 ulymap 20.0000000000000000000000000 xdim 0.0083333333333333285900000 ydim 0.0083333333333333285900000
World File (.bilw)
The world file can be used for image-to-world transformation when displaying an image using the ARC/INFO Geographic Information System (GIS) Image Integration routine. The world file is named after the image with a .blw extension. The following is an example of a world file with a description of each record (ARC/INFO User's Guide, Image Integration, 2-3 to 2-5):
0.0083333333333333285900000 xdim x dimension of a pixel in geographic units 0.0000000000000000000000000 rotation term (will always be zero) 0.0000000000000000000000000 rotation term (will always be zero) -0.0083333333333333285900000 negative y-scale; dimension of a pixel in geographic units 30.0000000000000000000000000 longitude of the center of the upper-left pixel
20.0000000000000000000000000 (latitude of the center of the upper-left pixel)
Note: ARC/INFO will ignore ulxmap, ulymap, xdim, and ydim in header file if a world file is present during ingest.
The Statistics File (.stx)
The statistics file contains the band number, the minimum pixel value (elevation expressed in feet above mean sea level), the maximum pixel value, the mean, and the standard deviation. The statistics file is named after the image with an .stx extension.
Example Statistics File:
1 489.0 8427.0 2875.5212789045699500000000000 1179.4763719888228400000000000
The Coordinate File (.tik)
The coordinate file is named after the image with a .tik extension. The following list identifies the keywords used in the coordinate file.
ulxmap: longitude of the center of the upper-left pixel ulymap: latitude of the center of the upper-left pixel urxmap: longitude of the center of the upper-right-pixel urymap: latitude of the center of the upper-right-pixel llxmap: longitude of the center of the lower-left-pixel llymap: latitude of the center of the lower-left-pixel lrxmap: longitude of the center of the lower-right-pixel lrymap: latitude of the center of the lower-right-pixel xdim: x dimension of a pixel in geographic units (decimal degrees) ydim: y dimension of a pixel in geographic units (decimal degrees)
Example Coordinate File:
ulxmap 30.0000000000000000000000000 ulymap 20.0000000000000000000000000 urxmap 50.0000000000000000000000000 urymap 20.0000000000000000000000000 llxmap 30.0000000000000000000000000 llymap 0.0000000000000000000000000 lrxmap 50.0000000000000000000000000 lrymap 0.0000000000000000000000000 xdim 0.0083333333333333285900000 ydim 0.0083333333333333285900000
The Data Descriptor Record (.ddr)
This file has been included to allow the image to be read directory by EDC's LAS software. To take advantage of this, the image file extension will have to be renamed from .bil to .img. This file contains all of the information contained in the other data files.
Example Data Descriptor Record:
IMAGE NAME:af_dem_lks1.bil NL:4201 NS:1201 NB:1 DTYPE:INTEGER*2 LAST MODIFIED: DATE: TIME: SYSTEM:ieee-std PROJ. CODE:(0)GEOGRAPHIC Valid:VALID ZONE CODE:0 Valid:INVALID DATUM CODE:0 Valid:INVALID PROJ. PARM: Valid:VALID A: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00 B: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00 C: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00 D: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00 E: 0.00000000000000E+00 0.00000000000000E+00 0.00000000000000E+00 CORNER COOR: Valid:VALID ULcorner:-5.00000000000000E+00 -1.50000000000000E+01 URcorner:-5.00000000000000E+00 -5.00000000000000E+01 LLcorner:-4.00000000000000E+01 -1.50000000000000E+01 LRcorner:-4.00000000000000E+01 -5.00000000000000E+01 PROJ. DIST:8.33333333333333E-03 8.33333333333333E-03 Valid:VALID PROJ. UNITS: degrees Valid:VALID INCREMENT:0.00000000000000E+00 0.00000000000000E+00 Valid:INVALID MASTER COOR:0 0
MINIMUM:1.00000000000000E+00 Valid:VALID MAXIMUM:6.54800000000000E+03 Valid:VALID
DATA SOURCE: SENSOR TYPE: CAPT. DIRECTION: DATE: TIME:
Antarctic Digital Elevation Model (DEM) data are produced from contour and coastline data supplied with the Antarctic Digital Database (ADD), produced under the auspices of the Scientific Committee on Antarctic Research (SCAR). The ADD contours were compiled from source maps at scales ranging from 1:200,000 to 1:5,000,000.
Once the data are pre-processed, they are used as input into the Australian National University Digital Elevation Model (ANUDEM) generation program developed by Michael Hutchinson, Centre for Resources and Environmental Studies, Australian National University. This program first reads input elevations, windows the data to the specified map limits, then generates a grid at a 1 km point spacing in a polar stereographic projection.
Elevation data are generalized by accepting a maximum of 4 data points per grid cell and discarding any remaining points. Contour data are generalized by accepting a maximum of one line per grid cell. The program then employs a multi-grid method that calculates grids at successively finer resolutions until the specified grid resolution is achieved. During this process, drainage conditions are imposed to remove sinks where possible. Values at grid points not occupied by data points are calculated by Gauss-Seidel iteration with over relaxation (SOR method) subject to an appropriate roughness penalty and ordered chain constraints. The ordered chain constraints are obtained from the stream line, sink point, and contour line data and through automatic drainage enforcement as calculated by the program. Starting values for the first coarse grid resolution are calculated from a least squares plane fit to the data points. Values for each succeeding grid are linearly interpolated from the preceding grid (M.F. Hutchinson, unpub. data, 1991).
After DEM generation, the resultant data was transformed into geographic coordinates using ARC/INFO.
The Antarctic DEM is distributed in two forms to address the complications inherent in equiangular point spacing near the poles.
ant_dem_1kmps.bil presents elevation information at a 1 km point spacing based on a polar stereographic projection.
ant_dem_1kms1.bil, ant_dem_1kms2.bil, ant_dem_1kms3.bil, and ant_dem_1kms4.bil present the DEM data in geographic (lat/lon) coordinates, with a point spacing of 30 arc seconds.
The horizontal datum is WGS84. Elevation values are expressed in meters above mean sea level.
The absolute accuracy of the ADD vector information is difficult to assess, as the ADD was produced from a broad variety of cartographic sources. However, the accuracy of the grid generated from these data will be no more accurate than this source. The accuracy for the grid has not been measured or calculated.
Data Availability Procedures for Obtaining Data
Haiti, Madagascar and Africa datasets are available through an Internet anonymous File Transfer Protocol (FTP) account at the EDC (at no cost).
Orders for tape copies can be placed through the GLIS Inventory SUMMARY RESULTS screen or by contacting the EROS Data Center, Customer Services section, at the following address:
Customer Services EROS Data Center Mundt Federal Building Sioux Falls, SD 57198 TEL: (605) 594-6151 FAX: (605) 594-6589
Customer Services can also be contacted for information about DCW DEM data that are in progress. As additional geographic areas become available they will be announced in the GLIS News.
Products and Services
DCW DEM data are available from the EROS Data Center on unlabeled magnetic tape. Africa, North America, Madagascar, and Haiti datasets can be obtained through anonymous FTP. Instructions for accessing the anonymous FTP account can be found under Procedures for Obtaining Data.
Applications and Related Data Sets
Moderate resolution (100 meter to 1 kilometer) topographic data have applications in many diverse land science fields such as geology and geophysics, ecology, soil science, botany, and glaciology. Topographic data are also critical to procedures used for correcting and/or presenting remotely sensed satellite and other global data.
The elevation data available here are a result of the global 30 arc-second elevation data project being carried out by staff of the USGS' EROS Data Center (EDC) in Sioux Falls, South Dakota and the co-located United Nations Environment Program / Global Resource Information Database office (UNEP/GRID). The broad goal of this project is the completion of global 1 km elevation data for the land surface and the systematic extraction of derivative information to assemble a global data base of topographic elevation, slope, aspect, hydrologic flow paths, and watersheds. The primary collaborators and data contributors to date are the USGS, NASA, UNEP/GRID, USAID, DMA, the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, and the Geographical Survey Institute of Japan.
Defense Mapping Agency, 1992, Development of the Digital Chart of the World: Washington, D.C., U.S. Government Printing Office.
Environmental Systems Research Institute, 1991, Image Integration ARC/INFO User's Guide: Redlands, California, Environmental Systems Research Institute, Inc.
Hutchinson, M.F., 1988, Calculation of hydrologically sound digital elevation models: Proc. Third Inter. Symp. Spatial Data Handling, Columbus, Ohio, August 17-19.
Hutchinson, M.F., 1989, A new method for gridding elevation and stream line data with automatic removal of pits: J. Hydrol, 106, 211-232 p.
Hutchinson, M.F., 1991, A continental hydrological assessment of a new grid- based digital elevation model of Australia: Hydrological Processes 5, 45-58 p.