GTOPO30 Frequently Asked Questions

• Correct Corruption
• How to read with ArcInfo 7.1.1
• How are the GTOPO30 data formatted
• What is the byte order for GTOPO30 data
• What is the difference between GTOPO30 DEM and USGS DEM
• How are elevation values converted from binary to ASCII in ArcInfo or ArcView
• What vertical exaggeration should be used when creating a shaded releif image in ArcInfo

What corrects corruption to GTOPO30 data after it is unzipped and untarred using the PC software Winzip?

If using Winzip on a PC, the "Tar File Smart CR/LF Conversion" default needs to be overwritten (or turned off). This is under the configuration menu on options. One of the possible problems, if this setting is active, is carriage return characters are converted to CF/LF pairs resulting in a larger DEM file which creates the corrupted files.

As a reference, all unzipped DEM files from 60 degrees south to 90 degrees north latitude and 180 degrees west to 180 degrees east longitude will have exactly 57,600,000 bytes. The Antartica DEM files will have 51,840,000 bytes and the ANTARCPS file will have 58,320,000 bytes when unzippped.

How are downloaded GTOPO30 data read with ArcInfo 7.1.1?

Once the file is unzipped and untarred, rename the files to all lower case. ArcInfo 7.1.1 will only recognize files in lowercase. Also change the DEM extension to a bil extension, e.g. filename .bil (This is necessary for all versions of ArcInfo.)

How are the GTOPO30 data formatted?

The GTOPO30 DEM file is provided as 16-bit signed integer (Motorola) data in a simple binary raster format. There is no header or trailer bytes imbedded in the image data. The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.).

What is the byte order for GTOPO30 data?

The DEMs are provided in Motorola byte order. The Intel byte order, which stores the least significant byte first ("little endian"), is used on DEC Alpha systems. Motorola byte order stores the most significant byte first ("big endian"). Systems such as Sun and Silicon Graphics workstations use the Motorola byte order. PCs and Macs are Intel byte order. Computers, which address bytes in the Intel byte order, will need to "swap bytes" of the DEM data unless the application software being used performs the conversion during ingest. Users can check the system range of DEM values stored on their system by looking at the statistics file (.STX) provided for each tile.

What is the difference between the GTOPO30 Arc Second Digital Elevation Model (DEM) data set and the 1-degree United States Geological Survey (USGS) DEM?

The GTOPO30 DEM is 16-bit signed integer data in simple binary raster format. The 1-degree USGS DEM, which is 3-arc second data, is provided in ASCII format. Both have geographic coordinate projection (latitude and longitude.) The GTOPO30 data set was generated by generalizing the USGS 3-arc second data for North America and other regions.. This was required by the Defense Mapping Agency (DMA, now National Imagery and Mapping Agency (NIMA)) in order to use the higher resolution source data.

How are the GTOPO30 elevation values in the DEM files converted from binary to ASCII format in ArcInfo or ArcView?

The GTOPO30 binary files can be converted to an ASCII format by creating a Grid in ArcInfo or ArcView.

1) Ensure the DEM files have been renamed with bil extension (see related FAQ.)

2) Run Imagegrid. Imagegrid does not support the conversion of signed image data, therefore the negative 16-bit signed values will not be interpreted correctly without the formula in step #3.

3) Run the following formula in Grid:

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

The statistics of the Grid should now match those listed in the .STX file.

NOTE:

The GTOPO30 data masks the oceans with -9999 values. If desired, use the Setnull function and set these masked values to: nodata.

4) Type the command: gridascii. The GTOPO30 elevation values will now be in ASCII format. NOTE: The command: asciigrid will convert the ASCII values to grid.

What vertical exaggeration should be used when creating a GTOPO30 shaded relief image in ArcInfo?

When creating a GTOPO30 shaded relief image in ArcInfo, it is necessary to enter a value for the vertical exaggeration. Start by entering a small number (eg .005) and adjust as needed. The exaggeration factor needs to be small because the horizontal grid is in decimal degrees and the z factor is in meters.