Automatic generation of elevation data over Danish landscape

7 The investigation
Images scale 1:25,000
Resolution (μm) Mesh size (m) Accuracy (m) ‰ of flight altitude
5 x 5 m 1.27 m 0.34‰
15μm
12.5 x 12.5 m 1.86 m 0.50‰
25 x 25 m 2.00 m 0.53‰
5 x 5 m Cannot be calculated!
30μm
12.5 x 12.5 m 1.46 m 0.39‰
25 x 25 m 2.37 m 0.63‰
5 x 5 m Cannot be calculated!
60μm
12.5 x 12.5 m 1.56 m 0.42‰
25 x 25 m 1.98 m 0.53‰
Table 7.3: The results, included gross errors for images in scale 1:25,000 at the three resolutions and mesh sizes.
As seen from tables 7.1 – 7.3, the results are not very good, when a ”raw” generation of the elevation
data from digital images is done. Regardless of scale, pixel size or mesh size, the accuracy lies between
approx. 1.3m to approx. 3.5m.
7.1.2 Accuracy after elimination of gross errors
For each calculation in the iterative process, more and more points are eliminated. It is therefore, interesting
to see how many points it has been necessary to eliminate, before the iteration has stabilised itself.
The gross errors are eliminated according to the principles described in Chapter 6, section 3. In the tables
7.4 – 7.6, the accuracy and the percentage of the flight altitude after elimination is shown, and the numbers
of eliminated points are given as percentages.
Images scale 1 : 5,000
Resolution (μm) Mesh size (m) Accuracy (m) ‰ of flight altitude Eliminated points (%)
5 x 5 m 0.11 m 0.14‰ 18.7 %
15μm 12.5 x 12.5 m 0.12 m 0.17‰ 21.0 %
25 x 25 m 0.18 m 0.25‰ 24.3 %
5 x 5 m 0.13 m 0.18‰ 18.7 %
30μm 12.5 x 12.5m 0.17 m 0.22‰ 21.8 %
25 x 25 m 0.22 m 0.30‰ 22.9 %
5 x 5 m 0.24 m 0.32‰ 14.3 %
60μm 12.5 x 12.5 m 0.24 m 0.33‰ 16.3 %
25 x 25m 0.31 m 0.41‰ 19.8 %
Table 7.4: The results without gross errors for scale 1:5,000 at the three resolutions and mesh sizes. The number of
eliminated points is given as a percentage.
Images scale 1 : 15,000
Resolution (μm) Mesh size (m) Accuracy ‰ of flight altitude Eliminated points (%)
5 x 5 m 0.32 m 0.14‰ 17.6 %
15μm 12.5 x 12.5 m 0.33 m 0.15‰ 18.7 %
25 x 25 m 0.36 m 0.16‰ 19.7 %
5 x 5 m 0.53 m 0.24‰ 14.0 %
30μm 12.5 x 12.5 m 0.51 m 0.23‰ 12.3 %
25 x 25 m 0.57 m 0.25‰ 25.3 %
60μm
5 x 5 m Cannot be calculated!
12.5 x 12.5 m 0.71 m 0.31‰ 10.8 %
25 x 25 m 0.80 m 0.35‰ 10.7 %
Table 7.5: The results without gross errors for scale 1:15,000 at the three resolutions and mesh sizes. The number
of eliminated points is given as a percentage.
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