In: Wagner W„ Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
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the landscape shows a disconnected area. Figure 8 (a)~(d) show 
the reference DEM and the filtered DEM by three filters 
respectively. We can see that the erosions and smoothness were 
produced at the disconnected areas which generated by 
TerraScan and ASF. The disconnected area generated by 
DDASF is closer to reference DEM. 
Erosions due to over-filtering 
Figure 8. Filtering results of Sample 23 by three different 
filtering methods. 
3.2 Filtering results of Sample 53 
Sample 53 is located at suburb areas and contains many features 
of discontinuity preservation. There are cliffs with large 
elevation-difference over the lower-right landscape and terraced 
filed over the upper-left landscape. Figure 9 (a)~(d) show the 
reference DEM and the filtered DEM by three filters 
respectively. We can see that there is less difference between 
the DEMs produced by TerraScan and DDASF while DEM 
generated by ASF reveals a quite difference at the discontinued 
areas. This is due to the over-filtering situation. Figure 10 
represents the differences between reference DEM and DEMs 
which generated by the different filtering methods. From Figure 
10(b) and (c), DDASF shows an improved result when a 
directional window is implemented. Generally DDASF can 
generate the closest DEM over the discontinued areas in this 
case, since the black(represents larger difference) areas in 
Figure 10(c) are thinner than others. 
(a) Reference DEM (b) DEM by TerraScan 
(c) DEM by ASF (d) DEM by DDASF 
Figure 9. Filtering results of Sample 53 by three different 
filtering methods. 
(a) Difference between (b) Difference between ASF 
TerraScan DEM and DEM and Reference DEM 
Reference DEM 
Figure 10. Difference between reference DEM and generated 
DEM 
Besides, the classification accuracy and RMSE are also 
calculated to evaluate the three methods. It is noticed that a 
good classification accuracy on a sample does not stand for a 
well RESM on the corresponding sample, too. For example the 
total error of Sample 53 produced by TerraScan is minimum