Digital Elevation Models (DEM) comprise valuable source of data required for many engineering applications. Contour lines, slope - aspect maps are part of their many uses. Moreover, a DEM theme is an essential layer to be included in most GIS analysis operations. Recent scientific achievements aim to automate acquiring DEMs with the most possible speed and accuracy. This paper studies two methods of DEM data extraction from sample aerial stereopairs, the analytical plotter method versus the digital photogrammetric method (DP). The DEM output of the analytical plotter is used as a reference. This is compared to the work performed using the DP method on the sample photos in digital format. Comparison covers various steps of image orientation followed by DEM collection. Numerical outputs of speed and accuracy are presented and discussed. The tests show that errors in automatically extracted DEMs may result from areas representing ground surface of poor texture or contrast conditions, or it may appear due to imperfect photographic processing. Editing time of the product is a major factor especially in urban/ forest areas. Depending on the specified needs of the user, an erroneous DEM output might be accepted as appropriate digital surface model DSM. Statistical tests detect marginal error types in the output. The paper gives conclusions about some problematic sources with recommendations to improve the product.
Keywords: DEM, stereopair, digital photogrammetry