Triangulation is a technique for measuring distance or height based on the use of trignometry. If the length of one side of a triangle is known or measured, and two of the angles of the same triangle are measured, then the lengths of the other two sides can be computed.
Prior to the advent of the Global Positioning System, triangulation was the only practical way of surveying over long distances. A base line would be measured as accurately as possible, and then a triangulation network would be extended from the ends of the base line. At each point of the network, a theodolite was used to make accurate measurements of the angles between sight lines to other stations. After the observations were obtained, a computation determined the locations of each of the stations.
See the Transcontinental Triangulation for a historical example.