Published on *The Nature of Geographic Information* (https://www.e-education.psu.edu/natureofgeoinfo)

Surveyors have developed distinct methods, based on separate control networks, for measuring horizontal and vertical positions. In this context, a horizontal position is the location of a point relative to two axes: the equator and the prime meridian on the globe, or x and y axes in a plane coordinate system. Control points tie coordinate systems to actual locations on the ground; they are the physical manifestations of **horizontal datums**. In the following pages, we review two techniques that surveyors use to create and extend **control networks** (triangulation and trilateration) and two other techniques used to measure positions relative to control points (open and closed traverses).