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Many areas in Colombia are characterized by rugged terrain conditions and 
steep slopes often offer a challenge for the use of orthophotos. The 
practical limits for the use of the stereo-orthophoto technique, with respect 
to accuracy and image quality, are assessed as part of the pilot project. 
The accuracy of coordinates and elevations, provided by the stereo-orthophoto 
technique, will have to be considered in order to decide to what extent the 
stereo-orthophoto technique can be used to provide cadastral information for 
built-up areas such as villages and smaller towns. It is expected that for 
the larger towns and cities a higher accuracy, provided by a more 
sophisticated photogrammetric system, will be needed. 
Equipment for data acquisition, data processing and display of information, 
which was installed at IGAC as part of the pilot project, and which will be 
able to handle the amount of cadastral information, typical for one cadastral 
region in Colombia, comprises the following: 
Analytical Plotter 
The analytical plotter (US-2) is intended primarily to provide by aerial 
triangulation the control coordinates for the production of stereo-orthophotos 
and digital elevation models. The dedicated computer (PDP 11/34) of the 
instrument will, during the second phase of the pilot project, be connected to 
the data bank computer to allow on-line acquisition of digital data for areas 
that require higher accuracy boundary information. The data acquisition will 
be carried out under control of the data collection software, developed as 
part of the pilot project. 
Orthophoto Production Equipment 
À Gestalt Photo Mapper II system is used for the production of orthophotos, 
stereomates and digital elevation models. The system has been equipped with 
two printing units for simultaneous recording of the orthophoto and stereomate 
images. The height data needed for the generation of the stereo-orthophotos 
is recorded as a regular grid of elevation data which is used for the plotting 
of contourlines on a computer-controlled plotting table. 
Stereocompilers 
Four Stereocompilers are available to collect the digital information from 
stereo-orthophotos (fig. 1).. 
The Stereocompiler offers the possibility of plotting a manuscript on a 
transparent overlay on the orthophoto, during the digitizing process (van 
Wijk, 1982). The operator observes the plotted information superimposed over 
the stereoscopic model, which provides him with an up-to-date record of the 
digitized data and reduces the chance of errors due to double digitizing and 
omissions. 
The Stereocompilers are on-line with the data bank computer and each one is 
equipped with an alphanumeric terminal which provides a communication link 
between the operator and the computer and allows the data acquisition to be 
performed under computer control. 
Digitizing Table 
A Gentian Hi-state digitizing table with a 90 cm x 120 cm digitizing area is 
used for digitizing soil type boundaries from field interpretated orthophotos