GTOPO30 is a global digital elevation data set with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). GTOPO30 was derived from several raster and vector sources of topographic information. GTOPO30 was completed in late 1996. Most users soon after switched to using NOAA's GLOBE data set, which is similar to GTOPO30. In modern times, SRTM3 terrain elevation data is usually preferred, although SRTM3 data cuts off at latitudes beyond 60 North or South. Note that although GTOPO30 files use the .dem extension (as well as other file extensions in the GTOPO30 file set) they must be imported using the GTOPO30 DEM Manifold dataport, a different dataport than used for USGS DEM format. Manifold can normally autodetect which type of DEM is being imported. a different importer than the USGS 1:250K-scale DEMs.
GTOPO30 files import into Manifold with projection information preserved and with invisible pixels at sea level. This makes them handy for use as components in maps showing large areas, since seas may be displayed in layers below with cartography that is not hidden by the GTOPO30 terrain.
To import from GTOPO30 DEM format:
Choose File-Import from the main menu.
In the Import dialog browse to the folder containing data of interest.
Double-click the file ending in .dem for the data of interest.
A table and an image will be created.
The image shows the terrain elevation data. We can double-click on the image to view it.
For a more interesting display, we first create a new data source using a Bing street maps imageserver as shown in the Example: An Imageserver Tutorial topic. We then create a map and drag and drop the Bing layer into the map, and then we drag and drop the e060n40 image into the map.
We use the Style pane to color the image and to apply hill shading, as seen above.
The result is a much more interesting and understandable display, seen somewhat zoomed in.
Zooming in even further to Mount Everest, the red peak in the center of the window, we see one of the locations where GTOPO30 data has lower resolution.
Legacy - GTOPO30 DEM format is fairly rare, with few sites providing downloads: even the the "long term" USGS archive web pages such as https://lta.cr.usgs.gov/GTOPO30 no longer provide GTOPO30 data in the original format. Using USGS's "Earth Explorer" web site GTOPO30 terrain elevation data can be downloaded in GeoTIFF form. Manifold can easily handle GTOPO30 data for the entire Earth if merged into a single image using the Edit - Merge - Merge Images command.
Downloads for NOAA GLOBE are currently at http://www.ngdc.noaa.gov/mgg/topo/gltiles.html
Example: Spectacular Images and Data from Web Servers