Downloading and Formatting USGS GTOPO30 Elevation Data for GIS Use

Author:

Joseph J. Kerski, Ph.D.
Geographer:  Education/GIS
U.S. Geological Survey
Building 810 - Denver Federal Center - Entrance W-5 - Room 3000
Box 25046 - MS 507
Denver CO  80225-0046  USA
Tel: 303-202-4315
Fax:  303-202-4137
jjkerski@usgs.gov  

Below, sample image after data from GTopo30 was processed:

Description:

This is a tutorial that will aid you in downloading and formatting GTOPO30 for GIS-based analysis. 

For more information, see:

http://edcdaac.usgs.gov/gtopo30/README.asp

GTOPO30 is a global digital elevation model (DEM) resulting from a collaborative effort led by the staff at the U.S. Geological Survey's EROS Data Center in Sioux Falls, South Dakota.  The name GTOPO30 is derived from the fact that elevations in GTOPO30 are regularly spaced at 30-arc seconds (approximately 1 kilometer).  GTOPO30 was developed to meet the needs of the geospatial data user community for regional and continental scale topographic data. This release represents the completion of global coverage of 30-arc second elevation data that have been available from the EROS Data Center beginning in 1993. Several areas have been updated and the entire global data set has been repackaged, so these data supersede the previously released continental data sets. Comments from users of GTOPO30 are welcomed and encouraged. Please send your comments to Dean Gesch at gesch@edcmail.cr.usgs.gov or to Sue Greenlee at sgreenlee@edcmail.cr.usgs.gov.

GTOPO30 is a global data set covering the full extent of latitude from 90 degrees south to 90 degrees north, and the full extent of longitude from 180 degrees west to 180 degrees east. The horizontal grid spacing is 30-arc seconds (0.008333333333333 degrees), resulting in a DEM having dimensions of 21,600 rows and 43,200 columns. The horizontal coordinate system is decimal degrees of latitude and longitude referenced to WGS84. The vertical units represent elevation in meters above mean sea level. The elevation values range from -407 to 8,752 meters. In the DEM, ocean areas have been masked as "no data" and have been assigned a value of -9999. Lowland coastal areas have an elevation of at least 1 meter, so in the event that a user reassigns the ocean value from -9999 to 0 the land boundary portrayal will be maintained. Due to the nature of the raster structure of the DEM, small islands in the ocean less than approximately 1 square kilometer will not be represented.

To facilitate electronic distribution, GTOPO30 has been divided into 33 smaller pieces, or tiles. The area from 60 degrees south latitude to 90 degrees north latitude and from 180 degrees west longitude to 180 degrees east longitude is covered by 27 tiles, with each tile covering 50 degrees of latitude and 40 degrees of longitude.  Antarctica (90 degrees south latitude to 60 degrees south latitude and 180 degrees west longitude to 180 degrees east longitude) is covered by 6 tiles, with each tile covering 30 degrees of latitude and 60 degrees of longitude. The tiles names refer to the longitude and latitude of the upper-left (northwest) corner of the tile. For example, the coordinates of the upper-left corner of tile E020N40 are 20 degrees east longitude and 40 degrees north latitude. There is one additional tile that covers all of Antarctica with data in a polar stereographic projection. The are in the image above is from file W100N40, covering from 10 South to 40 North Latitude, and from 60 West to 100 West Longitude. 

1)  To access the data, go to:

http://edcdaac.usgs.gov/gtopo30/gtopo30.asp

2)  Click on a map tile that you want to download.

3)  Click on the download link.  Save the file to your computer.

4)  Use WinZIP or other extracting software.  If you are using WinZip to uncompress this file, please turn off (uncheck) the TAR File Smart conversion found under Options / Configuration (then under Misc. if using Winzip 8.0).   See:

http://LPDAAC.usgs.gov/gtopo30/faq_gtopo30.asp for more information.  

5)  Rename your resulting .dem to .bil according to the documentation.  

6)  Using ESRI's ArcToolbox: Access Image to grid and convert your BIL to a ESRI Grid.

7)  Access ESRI's ArcMap.  Add the Spatial Analyst extension and toolbar.  Click on Add data, and add your grid.

8)  The grid needs further processing, though, because some of the values are incorrect.  According to the documentation, IMAGEGRID does not support conversion of signed image data, therefore the negative 16-bit DEM values will not be interpreted correctly. After running IMAGEGRID, an easy fix can be accomplished using the following formula in Grid:

out_grid = con(in_grid >= 32768, in_grid - 65536, in_grid)

You can calculate the above formula using the Raster Calculator in the Spatial Analyst extension in ArcMap.  The ocean on the screen shot is elevation value -9999 and the land elevations are in meters.

The converted grid will then have the negative values properly represented, and the statistics of the grid should match those listed in the .STX file. If desired, the -9999 ocean mask values in the grid could then be set to NODATA with the SETNULL function.