A triangulation station, or "tri-station", is a type of ["horizontal control"] ["station"]. Coordinates of triangulation stations were obtained by precisely measuring the angles between the lines of sight to other stations, and then computing their positions by using trigonometry (corrected for the nonspherical earth) and an accurately measured base line. [:triangulation:Triangulation] was used for long-distance surveys where it was impractical to directly measure distances between stations.