DEM Generation

An introduction to automatic extraction of DEMs from digital stereomodels was given in Chapter 3.4.3. Generally, creating DEMs from SFAP is associated with the same procedures and difficulties as creating them from traditional photography, and various geoscientists, including the authors, have extracted detailed elevation models from SFAP images (e.g., Hapke and Richmond, 2000; Henry et al., 2002; Marzolff et al., 2003; Scheritz et al., 2008; Marzolff and Poesen, 2009; Smith et al., 2009). However, there are some specific characteristics of SFAP as compared to metric aerial photographs with practical consequences for photogrammetric processing that are briefly summarized in the following.

• Small-format digital photographs are usually taken in color with a Bayer pattern sensor (see Fig. 6-4). When using image matching algorithms for DEM extraction that work on monospectral data (single image bands), the green band will be likely to give better results as only 50% of the pixel values are interpolated by demo-saicking algorithms (unlike 75% in the red and blue bands).

• The DEM extraction is controlled by a set of strategy parameters defining search and correlation window sizes, correlation coefficient limits, etc. Because the default values for these parameters are designed for the standard airphoto case, they might not be ideal for the SFAP case. Slight adaptions of the values might improve point density and quality (Marzolff and Poesen, 2009; Smith et al., 2009). For example, the comparatively higher remaining y parallax resulting from lower triangulation accuracy may be taken into account by increasing the search size in y direction that limits the distance of the corresponding point search to the epipolar line. Higher image noise and resolution may result in low point density, which can be counteracted carefully by increasing the correlation window size and decreasing the correlation coefficient limit. An increased search size in x direction may also be necessary if high terrain differences, causing larger x parallaxes, are present in the scene (see also below).

• Differences in image scale, which are common for SFAP series, may hamper not only visual stereoscopic analysis ni.««; îï

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