GTOP030 Tutorial

These instructions indicate how to construct a custom digital elevation map (DEM) using the GTOPO30 data. In this tutorial, we will create a map of the North Atlantic Seaboard which runs from 100W50N to 50W5N (see image below).

• Download and install [[1]]. This will place a FWTools Shell icon on your desktop. From within this command line environment, a number of useful GDAL functions can be accessed.
• Goto GTOPO30 site and download the four DEM maps which span that area we are interested in: w100n90.tar.gz, w100n40.tar.gz, w060n40.tar.gz, w060n90.tar.gz
• Start the FWTools Shell by double clicking the FWTools Shell icon on your desktop.
• To extract the area spanning 100W50N to 50W5N from these four DEMs use gdal_merge:

gdal_merge -o north_atlantic.tif -of GTiff -ul_lr -100 50 -50 5 W060N40.DEM W060N90.DEM W100N40.DEM W100N90.DEM

• This will result in a large (~62MB) GeoTiff file. Generally, it is best to work with smaller files within GenGIS. We can subsample this GeoTiff file using gdal_translate:

gdal_translate -of AAIGrid north_atlantic.tif -outsize 25% 25% north_atlantic.ascii

• This will create an ~10MB ASCII file suitable for use within GenGIS. We have switched to the ASCII file form because GDAL seems to have trouble producing subsampled GeoTiff files.

Note: GDAL does not overwrite files! If you wish to run either of these commands again with different parameters you either need to change the output file name or delete the previously made file.

Map of North Atlantic Seaboard from 100W50N to 50W5N.