The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
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value 0 in both Refemce3D and GSI DEM were masked out
during the statistic calculation. The two islands in tile N43E145
were masked out because there was no reference DEM
available from GSI DEM.
N43El44
N43E145
Whole
area 1
area 2
Mean (m)
0.8
0.3
-0.2
0.3
STDEV (m)
7.1
10.5
2.3
4.9
Min (m)
-142
-142
-23
-71
Max (m)
183
183
21
54
LE90(m)
11
16
4
7
Table 3a. Reference3D compared against DEM of GSI
Two typical areas with different slopes in tile N43El44 were
selected for detail assessment (Figure 6). Area 1 is a mountain
area with a complex terrain including mountains, lakes and
ravines and some locations have steep slopes (more than 50
degrees) that associate with worse elevation accuracy. Area 2 is
agricultural land of low slopes (less than 20 degrees) that
associate with better elevation accuracy (Table 3a).
The difference intervals (Table 3b) show that the differences for
over 93% of the cell in the mountain area are globally within
20m range, to be compared against the 10m / 18m / 30m
specification of Reference3D along slopes.
Figure 6. area 1 (mountain) and area 2 (flat)
Difference
intervals
N43E144
N43E145
Whole
Area 1
Area 2
-5 / +5m
73.6%
53.9%
97.6%
83.6%
-10 / +10m
89.7%
76.2%
99.9%
95.6%
-15 / +15m
95.2%
87.7%
100.0%
98.3%
-20 / +20m
97.7%
93.7%
100.0%
99.3%
Note that Area 1 is very steep (max slope >50 degrees)
4.4 Qualitative analysis
The maximum difference occurred in a steep relief. A detail
check was done for the places with maximum and minimum
difference (183m and -142 m) between the two DEM by
comparing their horizontal profiles (white lines in Figure 7).
Figure 7. Profile positions (white lines) of maximum and
minimum difference
Figure 7 shows that the largest errors arise over steep volcano
slopes. The elevations of Reference3D and GSI DEM cross the
profiles (white lines in Figure 7) are depicted in Figure 8 (Max
error profile) and Figure 9 (Min error profile).
Figure 8. Profile of the place with maximum positive
difference: 183m over the shore of a volcanic lake
gray zone: the MQu mask
It is interesting to note that Figure 8 shows that the maximum
difference occurs over a steep slope that is depicted as regular
by GSI DEM, whereas Reference3D shows more details. The
affected area shows a 250m elevation difference within a very
short distance, about 200m long, ie 4 GSI posting values only.
Table 3b. Reference3D compared against DEM of GSI.