Filling SRTM Voids: The Delta Surface Fill Method

In February 2000, the Space Shuttle Endeavour fl ew a single payload, 11-day mission (STS-99) in support of a joint project between the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA). The agencies designated this space fl ight as the Shuttle Radar Topography Mission, or SRTM. Prior to this mission, the only complete global digital topographic elevation data set was the United States Geological Survey (USGS) GTOPO 30 data, with one kilometer post spacing. The goal of this new joint project was to produce digital topographic data for 80% of the Earth’s surface at a post spacing of one arc second (approximately 30 meters) (JPL Fact Sheet 400713, 1998). This SRTM data is quickly becoming a useful source of elevation data so critical to modern imagery analysis and geospatial intelligence (GEOINT) requirements. However, SRTM data has various sized holes, or voids, resulting in incomplete datasets. This causes many analysis processes (e.g. orthorectifi cation, viewshed generation) to fail. Some of these voids can be attributed to the complex nature of IFSAR technology (Dowding et al., 2004), while topographic shadowing can cause others. A new technique to fi lling voids in SRTM digital elevation data is introduced here that shows improvement over traditional approaches, such as the Fill and Feather (F&F) method. In the F&F approach, a void is replaced with the most accurate digital elevation source (hereafter, “fi ll”) available with the void-specifi c perimeter bias removed. Then the interface is feathered into the SRTM, smoothing the transition to mitigate any abrupt change. It works optimally when the two surfaces are very close together and separated by only a bias with minimal topographic variance. The Delta Surface Fill (DSF) process replaces the void with fi ll source posts that are adjusted to the SRTM values found at the void interface. This process causes the fi ll to more closely emulate the original SRTM surface while still retaining the useful data the fi ll contains. There is no need for feathering with the DSF approach.