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Soil types are identified by specialized soil survey teams, involving field 
inspection as well as laboratory analysis. The soil boundaries are marked on 
the orthophotos for subsequent digitizing. The stereo interpretation of the 
terrain surface, offered by the stereo-orthophotos, has been recognized to 
contribute to the accuracy of the interpreted soil type boundaries. 
The interpreted information is transferred to orthophoto mosaics, copies of 
which are kept in the cadastral regional office as an up-to-date cadastral 
record. Other copies serve as a guide for the digitizing operation. 
Digitizing 
The data acquisition is based on individual parcels and is carried out 
according to the information on the field interpretated orthophotos. All 
data, pertaining to a certain parcel, such as its boundaries, cultivation 
outlines and buildings, are digitized before proceeding with the following 
parcel. Strings of coordinates form segments which are parts of boundary 
lines located between points common with adjacent parcels or land use units. 
Segments, in turn, make up closed polygons. For parcels extending beyond the 
window which delineates the area to be digitized in a stereo model, polygons 
are closed with a "virtual" segment along the window. 
The digitizing is carried out under computer control, so that the measured 
model coordinates can be transformed into the ground control coordinate system 
in real-time and are used as such during the entire data acquisition process. 
This on-line procedure offers the possibility of using the same strings of 
coordinate values to define segments shared by adjacent parcels. The operator 
defines a segment as "old" when it was digitized as part of a previous polygon 
and identifies it by pointing at its end points and one intermediate one. The 
original segment then becomes part of the new polygon. The segment fíle in 
the computer memory consists consequently of a unique set of segments which 
can be used repeatedly to form different polygons. A special code is used 
before the digitizing of a segment is started to indicate whether the segment 
is new, old, virtual or internal, the last expression being used for segments 
common to two different cultivation types within a parcel. At the end of each 
segment a code is used to indicate whether the last segment point is a new one 
or whether ít has already been digitized as part of a different parcel or 
field. These features, combined with the condition that all data make up 
closed polygons, ensure that each boundary poínt is defined in the data bank 
by a single set of coordinates, even when such a point belongs to different 
parcels or cultivation fields. 
As mentioned before, the Stereocompiler offers the possibility of producing a 
graphical overlay of the digitized information on the orthophoto 
simultaneously with the digitizing process. The operator observes this 
graphical record superimposed over the stereo model and it provides him with 
an up-to-date record of the digitized information. A previously digitized 
segment will, for example, be recognized and can consequently be identified as 
"Old". Windows are marked as a preparatory step on the overlays to assure 
that the required overlap is obtained for the data digitized in adjacent 
models.  Cadastral information, such as parcel ídentification numbers and 
identified boundaries, can also be indicated on the overlays and can thus be 
observed superimposed over the stereoscope model. The operator consequently 
has all information, needed for the digitizing operation, displayed in the 
instrument's field of view. 
The graphical record observed by the operator during the digitizing process 
also reduces the chances of errors, omissions and double digitizing. Since