248 
A close look at the Figure Of Merit (FOM) values, 
overlapped to the image, indicates that poor matchings 
are actually concentrated along breaklines showing that 
the self-diagnosis tool of the system is (on visual 
evidence) in good agreement with the error plot. 
The same area (but with a mesh size of 5 m) was 
surveyed with system B (which has as available tuning 
parameters only the correlation window size and 
correlation coefficient threshold). A low resolution grid 
(we used a 50 and a 20 m mesh size, which amounts to 
130 and 800 points respectivly) taken from the 
reference data was fed to the program to support it in 
the search for homologous points. The system output 
around 13000 points in about Ih 30’ on an equivalent 
machine. The statistics are less satisfactory than in the 
proceeding case: the RMS of the discrepancies is 7 and 
6 m for the 50 m and the 20 m grid respectively; 83% 
and 87% of the errors are within 1 RMS in both cases. 
The distribution of DEM errors is shown for the two 
cases in Fig. 4 and 5. As it is apparent, compare to 
System A not only errors are larger, but also they 
extend further from the breaklines. As far as the 
support given by the grid is concerned, the is a rather 
small improvement only. This suggest that the system 
may not be adequate to the task. 
The procedure described in Section 2.3 for an 
independent check based on two orthophotos has been 
implemented in a software program and applied to the 
stereo pair, in a smaller area. About 2000 points have 
been matched and the corresponding ground 
coordinates computed by intersection. The RMS of the 
differerences with System A are in the same range as 
those found from the reference values, but larger, a 
comparison with the DEM from manual plotting 
showed a small percentage of outliers, likely due to 
insufficient robustness of our matching algorithm: no 
further comparison were therefore performed with this 
data set. 
4. CONCLUSIONS AND PROSPECTS 
The work is really still in progress, since the second 
flight due next summer will provide new, more 
redundant data, where multi-image techniques will be 
applied. The preliminary results seems to point out that 
in such a demanding environment only sophisticated 
systems do provide acceptable results. It should be 
stressed, nevertheless, that contour lines do not provide 
an ideal reference set in breaklines areas, that is. where 
the largest errors are concentrated: their nominal 
accuracy along step edges cannot be maintained. Also 
wooden areas, where it is known that results are 
unreliable, should be perhaps taken out from the 
comparison. Besides, a few buildings in the southern 
area are not taken into account in the reference DEM, 
so positive discrepancies there are indeed not errors. If 
this is the case, these preliminary figures would 
improve and. as far as System A is concerned, it may 
be proved that it is suited to the taks. A definite answer 
will be only available after the completion of a 
topographic survey with a laser scanning theodolite. 
5045700.0&-c_, 
5045600.00- 
5045500.00- 
5O454OG.0G-K 
5045300J 
5045200.00- fecs 
p 
i'yj 
5045100.00- riv 
5045000. 
5044S00.0Q-> 
JMfO, 
5044800.00- p//// jI f{ \ 
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gnji f -Tf'.rs fw 
1602300.00 1602500.00 
1602700.00 
1603100.00 1603300.0 
-50.00 to -6.00 
-6.00 to 6.00 
6.00 to 30.00 
Fig. 4 - DEM error plot on system B (5 m grid. 20m support grid)