International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
http://www.ipf.tuwien.ac.at/products/produktinfo/scop/scop_dt
m. sheet.htm).
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Figure 1: Density N of the original set of terrain points serving
to derive the ALS-DTM (width of the analyzing grid: 5m)
2.2 Root Mean Square Error (RMS)
At the position XY of individual points of the original data set
("terrain points"), the elevation as measured is compared to the
corresponding elevation. of the DTM surface. Of these
discrepancies, per cell of the analyzing grid, RMS (root mean
square) values will be computed. It represents the accuracy of
the original elevation values as measured by data acquisition —
provided that a suitable analyzing grid size is chosen with
regard to terrain surface characteristics and point density. This
accuracy 1s varying depending on the quality of data
acquisition. In case the RMS thus computed in some cell of the
analyzing grid is smaller than the estimate G4-prioi AS Mentioned,
it will be replaced by the estimate. Cases of such replacements
occur in cells with few terrain points in them and/or 1f the
algorithm applied in interpolating the DTM 1s without filtering
of random errors (e.g. a triangulation of the given point set).
Figure 2 visualizes the RMS of the terrain points applied to
compute a laser DTM with the program package SCOP. The
maximum RMS value is 52cm; the quadratic mean of the RMS
values equals 5.1cm. In 64% of the cells the RMS has been
replaced bÿ O4-prion Estimated as 5cm. The area shown in Figure
2 and in the figures to follow is identical with the area
represented in Figure 1. In cells of the analyzing grid without
original terrain points no RMS is indicated; these cells are
marked black.
114
0 005 01 02 03 >05 Im]
Figure 2: RMS of terrain points in relation to an ALS-DTM,
black: no data available (width of the analyzing grid: Sm)
2.3 Curvature 1/r
DTM quality is highly influenced by local terrain curvature.
Therefore, it is computed for all points of the DTM grid. The
algorithm applied is based on differential geometry (Appendix
A at the end of this paper), and it yields the maximum value of
curvature; figure 3 visualizes these values. They vary between
-0.61m' and 0.78m ^. The curvature overlay thus computed
yields a detailed visualization of the geomorphology of the
terrain (Figure 9 shows the corresponding orthophoto). In
agriculturally fields, curvature is very low; it is considerably
larger 1n residential areas, and also in woods. Extreme curvature
values are typical along break lines.
EEE s
0.8 0 08 Im
Figure 3: Curvature values 1/r in an ALS-DTM
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