Fieldwork with GPS in South America: PSAD 1956 (La Canoa) or WGS84 datum?

Provisional South American Datum of 1956

 

Maps published in a number of Andean countries in South America (i.e., Ecuador, Peru, Bolivia) are still often based on the Provisional South American Datum of 1956, La Canoa (Venezuela), abbreviated to “PSAD56”.

GPS fieldwork

If you would like to set your GPS to the PSAD1956 datum then look for settings in the GPS that allow you to change the Coordinate System Datum to “Prov South American” or “La Canoa”. Note that South American Datum of 1969 (SAD69) is not the same datum and it is seldom used.

Putting your GPS in PSAD1956 makes sense if you are doing fieldwork based on paper maps referencing the 1956 datum. However, if you plan to export your data to GIS, and you anticipate working with satellite imagery, regional topographic data, and other newly available datasets, you should instead use the modern, worldwide WGS1984 datum.

In either case you should be able to convert your data after-the-fact using GPS download software or a full-blown GIS. The important thing is: you should know what system you GPS is set to during fieldwork. Often if you export your data as XYZ or Shapefile from GPS download software the data appears without referencing in GIS software. If the data is NOT in WGS84 because you used PSAD56 in your GPS unit during fieldwork then make sure you indicate it as such.

 

Conversion in Metric coordinates

You can convert back and forth between PSAD56 and WGS84 in most Andean countries in the UTM coordinate system using these values (mean values for Andean countries).

From: Provisional South American 1956, Ellipsoid: International 1924
To: WGS84
Change in X: -288±17 meters
Change in Y: 175±27 meters
Change in Z: -376±27 meters
To go from WGS84 to PSAD56 reverse the sign on the above transformation.

This is the transformation known as " 1201: PSAD_1956_To_WGS_1984_1" and it applies to: Bolivia; Chile; Colombia; Ecuador; Guyana; Peru; Venezuela

For more information see this page:

Cartographic transformations in ArcGIS for South America

 

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