) connect the 
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Section 3 - Relative orientation. In this step, the user 
will digitise at least six well-distributed, corresponding 
cardinal points in both images. With these and the 
digitised object points as indata, PC-TAS will calculate 
the relative orientation using the coplanarity equation. 
This is an iterative process which is supervised by the 
user. 
The new model coordinates and the parallaxes will be 
displayed (upon command) and the user will be able to 
compare the values with those generated before the 
relative orientation. 
Section 4 - Absolute orientation. The user will now 
enter the ground control point coordinates and finally 
identify and digitise the points in the two images and 
perfom the absolute orientation. In its current form, PC- 
TAS allows for a maximum of 10 control points to be 
digitised. After some possible re-digitising and/or 
exclusion of points, the absolute orientation parameters 
will be calculated. The absolute orientation is performed 
in two steps: 
- Helmert transformation in planimetry. 
- Levelling in elevation. 
Section 5 - Object coordinates. Simply by pressing a 
button, the model coordinates (x y z) of the digitised 
object points will be transformed to the ground 
coordinate system (XY 2). 
Results sheet. To view all significant results from the 
analytical measurements, the user may switch to the 
Results sheet. This sheet is also a good help for gross 
error detection, or for the assisting teacher when 
evaluating the measurements. 
113 
4. TEACHING EXPERIENCE 
The Department of Geodesy and Photogrammetry has 
been using PC-TAS as a part of the exercises since 
December 1994. At least 200 students have already tried 
this system, and their opinion is that PC-TAS is easy to 
use, much because of the differently coloured Excel 
cells, telling the user where to digitise and where to look 
for specific results. The most common reaction is, 
however, a fascination over how much you can do with a 
simple Excel sheet. 
Our experience with PC-TAS shows that it performs well 
and that it is well suited for our needs. The only 
drawback is that, due to the nature of the Excel sheet, it 
is impossible to write-protect the cells where the 
digitisation is done. An enterprising student can easily 
destroy the links if he/she starts to copy and paste cells 
without knowledge of the correct procedures. 
It should be noted that PC-TAS is not a complete 
substitute for the stereoplotter exercises, it is merely a 
way for the students to get some basic practice and 
knowledge before trying the analytical stereoplotters, 
thus reducing the time spent at those exercises. It should 
also be mentioned that the quality of the measurements 
with PC-TAS is not as good as with analytical 
stereoplotters. This is not surprising because of the 
digitiser resolution, paper print stability and viewing 
magnification. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996