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.