GPS Locations and Costa Rican Topo Maps

Over the past several years I have been working with Sally Horn and our U.S. and Costa Rican collaborators on several projects in Costa Rica related to paleoenvironmental research. Such research requires accurately locating geomorphic and archaeological features in the field, using GPS and the 1:50,000 scale topographic maps produced by the Instituto Geográfico Nacional. Doing so is a challenge to the uninitiated, because GPS locations (including elevation) appear to be just plain wrong when you match them to the map. A position collected on the north side of a stream plots on the south side on the map, or a position well to the east of a settlement plots in its center! From such experiences, some of our collaborators have concluded that their GPS units must be of poor quality or broken. No doubt biologists and other field researchers have found themselves similarly befuddled. In this report I explain the nature of the problem in non-technical terms, and I provide both low-tech and high-tech solutions. I also review the steps to plot latitude and longitude on Costa Rican topographic maps, and how to use these maps to determine latitude and longitude in the coordinate system used for the maps