Table 2. Summary of the Differences 
Between 12 Control Point Elevations 
and Elevations Determined by SoftTest 
  
  
  
  
Using Single Point Matching 
Maximum +3.1m 
Minimum Difference 0.0 m 
Average Difference +0.74 m 
RMS Difference 1.79 m 
  
  
  
  
Table 3. Summary of the Discrepancies in 
Meters Between the DEMs Produced by 
  
  
  
  
WISDOT and by SoftTest 
Average Difference -1.7m 
RMS Difference 3.8m 
Maximum Difference | 20.8 m 
Minimum Difference | 0.0 m 
  
  
  
  
screen. The measured elevations should be close 
to the elevations used to orient the photographs. 
Table 2 summarizes the differences found 
between these values. 
A DEM was produced with a grid 
spacing of 15m that cover the whole model. To 
check the accuracy of the DEM, a source of 
higher accuracy must serve as a reference. We 
selected approximately 50  well-distributed 
points within the model. Using a semi- 
analytical — stereoplotter, the Wisconsin 
Department of Transportation (WISDOT) 
generated elevations for the 50 points. The 
results of the test can be found in Table 3. 
Since this is the first real test to the system, 
some errors in the model are more than 
expected. More investigation will take place in 
the near future to find the source of the errors. 
Digital orthophotographs are used by 
many professionals in the mapping field such as 
surveyors, engineers, planners, and foresters. 
The next test examined the accuracy of digital 
orthophoto produced by the SoftTest. 
164 
  
Table 4. Discrepancies in Control Points in 
The Easting and Northing Directions of 
Model 194-193 on SoftTest and 
Orthomax Workstation. 
OrthoMAX SoftTest 
E |Ngps-N E |Ngps-N 
-2.1 0.2 -2.8 -3.0 
1.8 11 -2.0 -2.6 
-2.1 0 -2.8 "2. 7 
0.3 -0.8 -1.9 -3.5 
-0.9 0.4 -2.1 -2.3 
-1.6 2.1 -2.8 -2.1 
0.4 1.7 -2.8 -3.0 
-2 0.6 -2.7 -3.1 
0.3 -0.7 -0.4 -1.4 
-1 -1.5 -0.7 -3.2 
-1.8 -2.4 -2.0 -2.1 
E133 1.36 2:2 2.7 
The accuracy assessment was carried 
out using some of the control points as check 
points. This was accomplished by reading the 
coordinates of the control points on the digital 
orthophotos and comparing them with the 
reference coordinates determined by GPS. The 
results are shown in table 4 and figure 3 where 
each one displays the discrepancies between 
the measured and reference coordinates of the 
control points. 
It is clear from the figure that there is an 
error tendency toward south-west. At this stage, 
we do not know what the cause of the error 
tendency. However, the magnitude of the error is 
not that big. SoftTest still is not as good as 
OrthoMax. Again, more tests will be done to 
know the sources of the errors and we will try to 
avoid them. 
4. CONCLUSIONS 
The results of the tests with this new 
software were mixed. Generally we found that 
the software works as expected, but the errors 
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