UPPER COLCA ARCHAEOLOGICAL
RESEARCH PROJECT
Arequipa, Peru
Coordinate System and Projection
Updated: 08 March 2003
We are using World Geodetic System 1984 (WGS84) for all of our digital data on this project. This is the internal system of the GPS system worldwide, and it works best with Arcpad. A good explanation of Coordinate Systems and Projections can be found at Peter Dana's The Geographer's Craft website.
We are using UTM 19S projection for local maps so that metric measurements can be taken from the maps. When given the choice, however, store all digital data in WGS84 decimal Lat/Long (not UTM 19S) because this is the most versatile system for reprojecting into a variety of systems.
In order to make our data more compatible with data from elsewhere, say the Alca obsidian source in the Cotahuasi canyon in UTM 18S, we should store our data in unprojected decimal Lat/Long, WGS84.
Note that most Peruvian maps are in PSAD 1956 / La Canoa datum. In order to incorporate this data into our project you must first define the projection/coordinate system to what it currently is (PSAD56 La Canoa) and then transform it into the WGS84 system.
A general 3 parameter transforamtion useable in the central Andes for converting metric coordinates from PSAD56 to WGS84 in UTM system is as follows, together with an accuracy statement (from NIMA/NGA TR 8350.2):
ΔX (eastings) = –279 m ± 6 m
ΔY (northings) = +175 m ± 8 m
ΔZ (masl) = –379 m ± 12 m
Map Datums issue in South America
The American Society for Photogrammetry and Remote Sensing has published several short editorial articles on the coordinate systems in use in several Andean countries called "Grids and Datums". These articles contain a short history of the mapping programs in each country and an explanation of the transformations between PSAD 1956 and WGS 1984.
Peru
http://www.asprs.org/resources/grids/05-2006-peru.pdf
Bolivia
http://www.asprs.org/resources/grids/07-2001-bolivia.pdf
Ecuador
http://www.asprs.org/resources/grids/05-99-ecuador.pdf
Argentina
http://www.asprs.org/resources/grids/12-99-argentina.pdf