12.10 Height Measurement (DEM Generation)

12.10 Height Measurement (DEM Generation)

Topographic mapping or DEM (Digital Elevation Model) generation is possible with a pair of stereo images.

The height accuracy h depends on the parameters of base-height ratio (B/H) and the accuracy of parallax which may be approximated by ground resolution G, as indicated as follows.

h = H/B G

Table 12.10.1 shows the theoretical accuracy of height determination for Landsat MSS, Landsat TM and SPOT HRV (panchromatic). In the case of SPOT HRV, with maximum base length and a B/H ratio of about 1, the height accuracy will be about 10 m the same as the ground resolution, which will be suffcient to produce topographic maps with contour lines of 40 meters interval.

There are two methods of topographic mapping or DEM generation; using operator based analytical plotters with special software, and automated DEM generation by stereo matching.

Usually rectified images for a pair of stereo images are initially pre-processed using ground control points.

Then stereo matching is applied to determine the conjugate points, which give x-parallax or difference of height to be converted to the height or elevation.

Figure 12.10.1 shows a pair of stereo images of SPOT HRV panchromatic data. Figure 12.10.2 shows a three dimensional view with the use of a DEM generated by stereo matching (see Figure 12.10.3 as an example of a bird's eye view image).