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New NASA-METI Topographic Data Should Improve Coverage Study Accuracy

This week NASA and Japan's Ministry of Economy, Trade and Industry (METI) released a new digital topographic map of Earth based on data from NASA's Terra spacecraft. The new global digital elevation model was created from nearly 1.3 million individual stereo-pair images collected by the Japanese Advanced Spaceborne Thermal Emission and Reflection Radiometer, or Aster, instrument on Terra. NASA said the data is available online to users everywhere at no cost.

"This is the most complete, consistent global digital elevation data yet made available to the world," said Woody Turner, Aster program scientist at NASA Headquarters in Washington. "This unique global set of data will serve users and researchers from a wide array of disciplines that need elevation and terrain information."

Propagation studies are only as accurate as the data used to create them. USGS terrain data is used for most broadcast coverage studies. One of the problems with the USGS digital elevation databases is they do not include man-made structures or trees. As a result, coverage studies based on USGS terrain data show coverage in areas blocked by tall buildings.

The accuracy of coverage studies can be improved through the use of the Shuttle Radar Topography Mission (SRTM) data (see the Google Map I created using SRTM data showing the coverage of KNBC). You can clearly see the impact of buildings in Los Angeles and Century City. If the color overlay disappears, zoom out.

While the SRTM data allows more accurate studies (as least for the landscape and buildings that existed in February 2000), anyone who has used it knows there are voids in the data where radar reflections weren't available or were wrong. They can be filled in with the other databases, which may limit accuracy. SRTM data is available for 80 percent of the Earth's land mass, between 60 degrees north latitude and 57 degrees south latitude.

Michael Kubrick, SRTM project scientist at JPL, explains, "The Aster data fill in many of the voids in the shuttle mission's data, such as in very steep terrains and in some deserts. NASA is working to combine the Aster data with that of the Shuttle Radar Topography Mission and other sources to produce an even better global topographic map." The Aster data covers 99 percent of the Earth's land mass, from 83 degrees north latitude to 83 degrees south latitude with a resolution of 30 meters (98 feet).

NASA's news release had links to two sources for the data, but I wasn't successful in finding a way to download the data from either of them. I'll keep checking and also see if there is a way to get this new elevation data into a format that can be used by the open-source SPLAT! propagation software.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.