GNSS and control networks

Global navigation satellite systems, GPS, GLONASS, Galileo, BeiDou and India's NavIC, let a receiver find its position by measuring ranges to several satellites. A single handheld receiver is good to a few metres; surveying needs centimetres or better, which comes from carrier-phase measurements with corrections from a nearby base station, either in real time (RTK) or by post-processing, or from a network of permanent reference stations.

GNSS gives positions on an ellipsoid, a mathematical model of the earth such as WGS 84. Heights from GNSS are ellipsoidal heights h. Engineers need orthometric heights H, measured above the geoid, the surface that mean sea level follows, because water flows according to orthometric height. The two are linked by the geoid undulation N: H = h − N. Around India, N is negative over large areas, so the geoid lies below the ellipsoid there.

A control network is the skeleton of every large project: a set of permanent, well-marked points with accurately known coordinates and levels, tied to the national datum, from which all setting out is done. It is designed with redundancy, more observations than strictly needed, and adjusted by least squares so that errors are detected and spread properly. Survey of India maintains national control and reference stations.

GNSS needs open sky and suffers from multipath near buildings and under trees, so total stations and levels remain essential. Long linear projects such as railways and highways also need a projection with low scale distortion so that grid distances match ground distances.