Many GIS users are familiar with the NASA WorldWind virtual globe utility as the less well-known cousin of the extremely popular Google Earth application. WorldWind appeared around 2004 (about the same time as Google Earth) but was overshadowed into near obscurity by the massive marketing power of Google. Unlike Google Earth, WorldWind is an open source application, and has been quietly adding capabilities, thanks in large part to its connection with NASA and its access to their data. NASA also released the WorldWind SDK, an API designed to let developers access and extend the core capabilities. NASA posts demonstration Java applets at its Java SDK site. Some of these are very interesting and useful. This article examines one of them, the 'Exporting Surface Imagery and Elevations' utility. Using this program, it is possible to download co-registered satellite imagery along with SRTM DEM data. This greatly facilitates draping the image over the DEM to create terrain models. There is a slight glitch to doing this as the elevation files are downloaded mirror flipped over the central East-West latitude line, but PANCROMA has a utility to correct this.
After navigating to the Demo Applications page , scroll down to the 'Exporting Surface Imagery and Elevations' utility. You will need to check some of the relevant data layers in order to best navigate the virtual globe. I found that checking the following layers was helpful:
NASA Blue Marble Image
Blue Marble (WMS) 2004
MSVirtual Earth Aerial
USDA NAIP USGS
Note that only the first four data layers are relevant for areas outside of the United States. The USDA NAIP and USGS NAIP USGS data layers seem to have the highest resolution, apparently around 2m. As far as I can tell, the Urban Ortho Image layer and the USGS topographic map data layers have not been implemented in the demonstration applet. An example USGS NAIP image dowloaded from WorldWind is shown below.
After selecting your area of interest, you can save both the image file and the elevation files separately. These can be used by other applications like the excellent 3DEM for preparing terrain models by draping the image over the elevation model.
As mentioned, an issue with this data is that the elevation files are saved mirror flipped around the center latitude line. PANCROMA has a utility for correcting this problem. The elevation data is saved by NASA in GeoTiff format using IEEE floating point 32 bit representation for the elevation postings. It is necessary to flip the elevation postings without changing the georeferencing in order to use the data. To produce a corrected file, select 'File' | 'Open' and open the elevation file. Then select 'DisplayOne File' | 'Display GeoTiff IEEE FP 32 Elevation Image' | 'Flipped'. A rendering of the processed file will appear. You can save the file by selecting 'File' | 'Save Grayscale Image' | 'GeoTiff'. The file will be saved as a GeoTiff DEM (not a raster image).
As an example of how to build a terrain model, I downloaded a mountainous scene in central Tibet. The raw and flipped DEM renderings are shown below.
The satellite image is also shown (this does not have to be flipped):
I draped the satellite image over the flipped DEM using the free 3DEM utility. The results are shown below.
Although not designed as a data source, the NASA WorldWind SDK demonstration applet can provide useful data for many applications.