Digital photogrammetric determination of alpine snowpack distribution for hydrologic modeling

This paper discusses progress in the use of digital photogrammetry for producing two products: 1) orthophotos, from which maps of snow-covered area (SCA) can be produced, and 2) maps of the spatial distribution of snow depth, determined by subtracting a snow-free DEM from a snow-covered DEM. SCA is commonly used by snow hydrologists for modeling and other purposes. When aerial photography is used as a source for SCA, it is important that the photography is first ortho-rectified. In this paper, it is shown how digital photogrammetry can be used to produce orthophotos on a desktop computer, using ground control obtained from maps and a digital elevation model (DEM). The digital photogrammetric determination of snow depth has proved to be a much more difficult task, and results so far have been unsuccessful. Strict requirements for ground control accuracy appear to be the major constraint for this method. A brief overview of digital photogrammetry is provided, followed by a discussion of testing results from Sky Pond, an alpine basin in Rocky Mountain National Park, Colorado.