Error and Uncertainty Degrade Topographic Corrections of Remotely Sensed Data

Chemical and biological composition of surface materials and physical structure and arrangementof those materials determine the intrinsic reflectance of Earth's land surface. The apparent reflectance—asmeasured by a spaceborne or airborne sensor that has been corrected for atmospheric attenuation—depends alsoon topography, surface roughness, and the atmosphere. Especially in Earth's mountains, estimating propertiesof scientific interest from remotely sensed data requires compensation for topography. Doing so requiresinformation from digital elevation models (DEMs). Available DEMs with global coverage are derived fromspaceborne interferometric radar and stereo-photogrammetry at ∼30 m spatial resolution. Locally or regionally,lidar altimetry, interferometric radar, or stereo-photogrammetry produces DEMs with finer resolutions.Characterization of their quality typically expresses the root-mean-square (RMS) error of the elevation, butthe accuracy of remotely sensed retrievals is sensitive to uncertainties in topographic properties that affectincoming and reflected radiation and that are inadequately represented by the RMS error of the elevation. Themost essential variables are the cosine of the local solar illumination angle on a slope, the shadows cast byneighboring terrain, and the view factor, the fraction of the overlying hemisphere open to the sky. Comparisonof global DEMs with locally available fine-scale DEMs shows that calculations with the global productsconsistently underestimate the cosine of the solar angle and underrepresent shadows. Analyzing imagery ofEarth's mountains from current and future spaceborne missions requires addressing the uncertainty introducedby errors in DEMs on algorithms that analyze remotely sensed data to produce information about Earth's surface.