Refinement of Dense Digital Elevation Models

The Topographical Survey Division of the Surveys and Mapping Branch, Energy, Mines and Resources, Canada, employs dense digital elevation model (DEM) data for the production of hypsographic overlays for conventional and orthophoto maps. The Gestalt Photomapper (GPM II/3), installed as a production system since summer of 1977, uses computer controlled electronic scanning and image correlation techniques to produce both orthophotos and dense digital elevation models at high speeds. Discontinuities caused by built-up areas and water bodies, poor image quality, shadows, and steep terrain slopes create anomalies in the DEM which require special data filtering and refinement techniques. A complex DEM post data-processing system has now been developed to remove such anomalies and inlcudes various sub-programs which operate in Interactive and/or Batch processing modes. The key components of the system are: * interactive filtering of erroneous observations from the raw OEM's produced by GPM II/3, and editing noise from scanning over water surfaces, * generating of elevations from a global grid oriented to the UTM coordinate system, * edit of DEM's for elevation biases, and the enhancement of the hypsographic data to achieve cartographically acceptable contour/ stream re-entrants, * generating of a structured global DEM data base with the capability of integrating height information from other sources, * production of hypsographic overlays for conventional and orthophoto maps. In order to illustrate the advantages of using the refinement process, several samples are included. INTRODUCTION The Gestalt Photomapper (GPM-II), installed at the Topographical Survey Division in 1977, use electronic image correlation to measure parallaxes and produces automatically and simultaneously three products: an orthophoto negative, a contour overlay and a digital elevation model (DEM). The GPM divides the stereomodel into about one thousand 9 by 8 mm scanning area. Depending on the mask size used to print the orthophoto and contour overlay, a matrix (40 by 40, 32 by 32, or 24 by 24) of elevations (patch) in each scanned area is generated. The grid spacing of the matrix is fixed at 182 Mm at the photo scale.