94
February 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
PSAD 56 and SAD 69 seem to be largely ignored by Colombia
although both datums are extended into other countries
through Colombia and are extensively used elsewhere. The
majority network of Colombia, which had consisted of 21
fixed stations and 679 adjustable stations, was adjusted to
the SAD 69 in 1972. However, a free adjustment was also
done in 1972 to the total network, which consisted of 1 fixed
station and 950 adjustable stations. This free adjustment
appears to be the system currently in use under the original
name of the Bogotá Datum of 1941. The World Geodetic
System of 1984 datum is used extensively in Colombia,
albeit just a tool to extend the classical datum.
In 1980, IGAC established a special “city grid” projection
for the City of Santa Marta, the oldest city in Colombia
(1525). This special system was similar in philosophy
to the systems developed for counties in Wisconsin and
Minnesota for the convenience of the local surveyors
and GIS applications. Specifically, this “city grid” was
established at a particular elevation that corresponded to
the average elevation of the city. Local surveyors could
then ignore the reduction to sea level correction when
transferring measured field distances to the city maps. The
system was still based on the Gauss Conform Transverse
Mercator, but the False Eastings and False Northings
were modified by truncating to only five digits from the
observatory at Bogotá.
The reference ellipsoid (International) was modified
to have its semi-major axis increased by 29 meters,
corresponding to the average elevation of the city above
mean sea level determined at Buenaventura. Curiously,
the system was reversed back to the standard national
coordinate system in 1994, while other cities were published
with their own special plane coordinate systems such as
Villavicencio 1994 (450 meters), Leticia 1994 (100 meters),
and Armenia 1995 (1,470 meters).
Although the Colombian definition uses the Gauss
Conform Transverse Mercator for these systems, the
Local Space Rectangular sometimes used in analytical
photogrammetric calculations could just as easily be used
with identical results. We commonly use such systems for
analytical rectification projects if the project area is free of
relief such as a plateau or a flood plain.
Note that these newcity systemswere orthophoto projects
and not just rectified photos. Grids used in Colombia
without special modifications to the ellipsoid include the
Ciudad de Bogotá (City) Grid and the Ciudad de Medellín
(City) Grid. The Grid System used for the Ciudad de
Cartagena is the standard “B” belt of the National System.
The contents of this column reflect the views of the author, who is
responsible for the facts and accuracy of the data presented herein.
The contents do not necessarily reflect the official views or policies of
the American Society for Photogrammetry and Remote Sensing and/
or the Louisiana State University Center for GeoInformatics (C
4
G).
This column was previously published in
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CERTIFIED MAPPING SCIENTIST, LIDAR
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Effective December 6, 2015, expires December 6, 2020
RECERTIFIED PHOTOGRAMMETRISTS
James G. Hill, Certification #R766
Effective August 22, 2015 August 22, 2020
Branko Hricko, Certification #R1468
Effective December 6, 2015, expires December 6, 2020
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Effective November 2, 2015 November 2, 2020
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RECERTIFIED MAPPING SCIENTIST, GIS/LIS
Paul K. Ogino, Certification #R179GS
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Lloyd Lawrence Coulter, Certification #R187RS
Effective December 18, 2015, expires December 18, 2020
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