The DEM derived from laser altimetry was checked with a 
DEM generated photogrammetrically. Four sets of aerial photo- 
graphs (M=1:120,000) that cover a large part of the Jakob- 
shavns drainage basin were acquired in 1985 and 1986 
(Fastook et al., 1995). In our study photographs from 10 July 
1985 were used. As reported in (Csatho et al., 1996) the two 
DEMs agree well. Figure 4 illustrates the comparison graphi- 
cally. It confirms that the topography depicted by the two ap- 
proaches is very similar. 
For further analysis a more detailed DEM was measured. Con- 
tour lines from this DEM are superimposed on the orthophoto 
in Figure 4.a. No elevations could be measured in featureless 
areas (for example west of lake A). On the area of the ATM 
swath the surface elevation was accurately mapped by the laser 
altimetry. These elevations were used to estimate the accuracy 
of the photogrammetrically derived DEM. Between the parallel 
ATM swaths the photogrammetrically derived DEM was used 
to estimate the error introduced by the interpolation of the laser 
altimetry data. An RMS accuracy of 2.5 meter was obtained for 
the photogrammetry derived DEM. The laser altimetry derived 
DEM is very accurate over the swaths. The interpolated eleva- 
tions between adjacent swaths are less accurate (maximum error 
is 5 m). 
The temporal behavior of the lakes was also studied. Because 
the ice on the lakes melted during the photogrammetric flight 
mission it was possible to delineate the lakes on the orthophotos 
(for example Lake A in Figure 4.a). In addition, the smooth and 
horizontal lake surfaces can readily be identified from the laser 
altimetry data (for example Lake A and D in Figure 4.b and c). 
The comparison of the lake boundaries confirms that the larger 
surface depressions are tied to bedrock irregularities which 
means that is they do not advect with the moving ice. Analysis 
of the DEMs also shows that despite their proximity the lakes 
do not drain at the same time. : 
Ripples were mapped along the shorelines (for example features 
located west of Lake B in Figure 4.b and 4.c). These features 
are advecting with the moving ice downstream. Detailed maps 
were derived from laser altimetry data to study their shape and 
distribution (Csatho et al., 1996). 
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fi 
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Figure 4 
Lakes ai 
Ae