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Digital Maps Reach New Heights

July 2005
By Henry S. Kenyon

 
The topographic data collected by the Shuttle Radar Topographic Mission (SRTM) allows the U.S. government to generate altitude maps for 80 percent of the Earth’s surface.
Space-based radar topography allows development of advanced guidance, tracking software.

A high-resolution global elevation map soon will allow warfighters to develop and use a variety of navigation, communications and engineering applications. Twice as accurate as previous geographic data systems, it can generate detailed topographies of 80 percent of the planet’s surface, government scientists say.

Collected in 2000 as part of an ambitious effort by NASA and the National Geospatial-Intelligence Agency (NGA) to create the most extensive and accurate global elevation map ever made, the data from the Shuttle Radar Topography Mission (SRTM) was gathered using a large radar aerial deployed from the space shuttle Endeavour. After four years of processing, the data is now available for use in mapping and navigation applications, explains Barry Heady, NGA support lead, commercial partnerships division, SRTM, St. Louis.

The map data has a resolution of one arc second, or one elevation reading every 30 meters (98 feet). This elevation data represents what Heady calls foundation data, or the underlying information upon which other mapping and navigation products are built. He notes that foundation data is used extensively in imagery products. For example, imagery systems use a process called orthorectification to compensate for the natural distortion caused by angle offsets when images are not captured from directly above. “You can use the elevation data to take out the displacement in the image that’s caused by elevation,” he explains.

The mapping data was used to develop an NGA mapping product called Controlled Image Base Standards Information Base. In addition to using SRTM material, the agency combined imagery and mapping data from commercial firms such as DigitalGlobe Incorporated and Space Imaging Incorporated. All of this data is combined to varying degrees and is used in the agency’s software products, he says.

Elevation data is used extensively to create contour information for charts. Heady shares that it is a key part of all U.S. military aviation mission planning systems. “The Air Force, Navy, Marines and Army—anybody that’s going to fly—don’t fly these days unless they have an elevation layer to run their mission planning systems. Everything is planned prior to the mission, so the elevation data layer is essential,” he maintains.

But current products using SRTM data have some flaws, warns Louis Fatale, a civilian analyst with the U.S. Army Corps of Engineers’ Engineering Research and DevelopmentCenter’s TopographicEngineeringCenter, Alexandria, Virginia.

Fatale notes that when the NGA initially released the data, some of it was in an interim format that was not free of errors. “But people were clamoring for it, so they put it out, and now it’s become mixed in,” he says. First impressions with new technologies are important, and Fatale notes that if users get a map cell and do not have a good experience with it, they become disillusioned. Other issues include a lag in notifying users about how to access the data and a lack of awareness about the SRTM data.

The Army recently released a report highlighting the best ways to use the NGA’s mapping data in different applications. Because of the recency of this information, Fatale believes it is too soon to discuss the use of the mapping data in existing systems accurately. However, he is sanguine about its potential for use in line-of-sight systems, contour mapping, terrain profile and slope mapping software.

Written from a warfighter perspective, Fatale claims that the report explains and outlines how to use the new data in the field. He notes that it is critical to avert data voids because users will need to know the location of these blank spots. “It’s important to identify where the noise is. There’s noise throughout the data,” he says.

Fatale explains that the SRTM data has many applications, but that users must understand its limitations. “It’s a great data set. It’s a huge improvement over current elevation data in coverage, homogeneity and accuracy. But understand its limitations before you use it,” he warns.

Prior to the SRTM mission, the most accurate global elevation maps had only about one-ninth the resolution. The new data is considered to be of medium resolution, or level 2. Higher resolution maps are available, but they cover only very small geographic areas and are usually produced for specific missions, Heady explains. He adds that the updated information supports mission planning activities more efficiently than does the older material. “Prior to SRTM, we had approximately 1,000 one-degree cells of  level 2 data. SRTM produced about 14,200 cells of one-degree data, a multifold increase of the data holdings at that resolution,” he says.

The SRTM mapping data is stored in an NGA database and distributed to customers in several ways. U.S. Defense Department users can access the data online via the NGA gateway. Topographic information also is passed through the Defense Logistics Agency, which distributes the data to military commands on digital versatile discs. Heady observes that the SRTM maps are the NGA’s first publicly available data set. He adds that this information is accessible through the U.S. Geological Survey’s EROSDataCenter.

Although the mission mapped much of the planet, the map data covers a swath of terrain between 60 degrees north latitude and 56 degrees south latitude. The extreme northern and southern parts of the planet remain unmapped.

Heady notes that the government is especially interested in high-resolution maps of the terrain at latitudes above 60 degrees north. But issues of cost and scale exist. Methods are available to map higher and lower latitudes, but these systems cover only small areas of the surface with their sensors. Platforms capable of global-scale coverage are available but not at the required resolution and accuracy. A system must be efficient to operate on a large scale, and this efficiency does not exist today, he says.

Several radar satellite missions scheduled to deploy in the near future have attracted the NGA’s interest. One such mission features tandem radar spacecraft. The platforms may be able to provide global coverage to augment the SRTM data. This mission also could produce data at a 10-meter (33-foot) resolution. Heady adds that radar is the best type of sensor for global elevation and terrain mapping because it can operate day or night and is unaffected by clouds.

 
The accurate terrain data collected by the SRTM will allow the U.S. Defense Department to develop more sophisticated navigational software systems with 30-meter (98-foot) resolution.
However, no systems currently deployed have the resolution or coverage of the SRTM data. Heady predicts that this type of mapping mission will probably never fly on the space shuttle again primarily because it collected almost all the data it was designed to collect. The shuttle also is not designed to orbit above 60 degrees north latitude because of its Florida launch site. He believes that any future program with NGA participation will have to cover the higher reaches of the Northern Hemisphere. “Another shuttle mission is probably not an option,” he says.

During the February 2000 space shuttle Endeavour flight, the mapping was conducted with a modified C-band radar deployed at the end of a 60-meter (200-foot) mast. The Endeavour also deployed an X-band radar antenna in its cargo bay, which allowed the sensor to conduct interferometric mapping where the radar returns to both antennas at slightly different times. The difference in the signal timing is used to determine the elevation of the planet’s surface.

Heady explains that the mission was jointly conducted by NASA and the NGA. All the data was stored on tape aboard the Endeavour and taken to NASA’s Jet Propulsion Laboratory in Pasadena, California, for processing. The agency conducted the raw signal processing before handing the data over to BAE Systems and The Boeing Company to covert into the digital terrain elevation data (DTED) mapping product. He notes that the firms modified the data by flattening out bodies of water and making sea level a uniform elevation for the map. The Jet Propulsion Laboratory processed the data from 2000 to 2002 before passing it to the two contractors hired by NGA. The final mapping product was finished in mid-2004.

Deploying and stowing the antenna proved to be major challenges for the mission. The collapsed radar antenna was stowed in a canister in the Endeavour’s cargo bay. To operate, it had to extend to its full length and lock into place. “That was a big unknown right there. If the mast had failed, the mission was a failure,” Heady observes.

Another issue was stability. To allow data to be gathered properly, the antenna and the shuttle had to remain stable. The final concern was stowing the antenna at the end of the nine-day mission. It was fitted with explosive bolts, allowing the entire structure to be jettisoned if it could not be retracted, but Heady adds that the goal was to return with the equipment. “It actually exceeded expectations. While there was a lot of concern prior to the mission, it really performed flawlessly.”

 

Web Resources
National Geospatial-Intelligence Agency: www.nga.mil
Shuttle Radar Topography Mission: www2.jpl.nasa.gov/srtm
EROSDataCenter: http://edc.usgs.gov
U.S. Army Corps of Engineers’ Engineering Research and DevelopmentCenter, TopographicEngineeringCenter: www.tec.army.mil