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DATA ACQUISITION 
Photographs taken at various Scales, selected according to the density of 
information to be collected and the required accuracy are used for the data 
acquisition. Photo scales vary between 1:50 000 and 1:6 000. The smaller 
scales (up to 1:10 000) are used for the rural areas. For these areas the 
scales of line and orthophoto maps, produced as part of the pilot project, 
range from 1:25 000 to 1:2 000. Urban areas are photographed at scales 
1:20 000 to 1:6 000, depending on the size of the town. Map products for 
these areas may be at scales of 1:5 000 to 1:1 000 (IGAC, 1980). 
The photogrammetric data acquisition process consists of different stages, 
which are briefly described as follows: 
Aerial Triangulation 
The control points needed for producing the stereo-orthophotos and the digital 
elevation models, and for digitizing and plotting operations are determined on 
the analytical plotter using on-line aerial triangulation software developed 
at NRCC. The subsequent block adjustment is also based on NRCC software. It 
is foreseen that, in the near future, the orientation parameters, derived from 
analytical aerial triangulation will be used directly for the relative and 
absolute orientation process on the Gestalt Photo Mapper. 
Production of Stereo-Orthophotos and Digital Elevation Models 
The stereo-orthophotos and digital elevation models are produced by automatic 
image correlation on the Gestalt Photo Mapper. The orthophoto and the 
stereomate are recorded simultaneously on the two printing stages. X- 
parallaxes, introduced in the stereomate, permit three-dimensional 
interpretation and the derivation of terrain heights from the stereo- 
orthophotos. Providing that the orthophoto and the stereomate are produced 
from the two different photographs of the stereo overlap, also small details 
such as buildings and trees can be interpreted and measured three- 
dimensionally. 
The X-parallaxes in the stereomate are usually generated according to the 
base-to-height ratio of the stereo-overlap. For areas of high relief, which 
are common in Colombia, a smaller X-parallax to height ratio could be used to 
reduce image quality problems occasionally encountered in the stereomate for 
areas with large elevation differences. 
The orthophotos are enlarged to the required map scale (2 to 5 times 
magnification) and combined into orthophoto mosaics according to the cadastral 
map sheet system. This allows for a convenient use of the orthophotos in the 
cadastral regional offices and offers the possibility of a systematic sheet by 
sheet revision process. 
Field Identification 
Enlarged orthophotos and stereomates are used for identification of the 
cadastral information in the field.  Cadastral survey teams mark the parcel 
and field boundaries on the orthophotos based on information observed in the 
field, obtained from the cadastral records, and provided by the property 
owners. .In addition, information on cultivation types, buildings, road clas- 
sifications, drainage patterns, etc. is indicated. Attribute information on 
parcel name or address, ownership, owner identification, cadastral value, date 
of change in ownership, etc. is collected for each parcel on special forms.