Establishing Precise Ground Control Points Using DGPS Measurements and Different Processing Parameters

Abstract

The Most Traditional Source of Ground Control Point's (GCP)geodetic Coordinates has been the topographic maps. However, with the advent of the global Positioning System (GPS), it has become very easy to establish the GCP's Coordinate in the Field using a geodetic GPS Receiver. This Source of GCP's Give the Coordinates in a Fixed Datum (Say, WGS84) Whereas the Topographic maps in our Country are in different Datum i.e. Modified Everest, Once the Data is Collected Using GPS it has to be Processed with Few Reference Stations (IGS) so as to generate the Final Coordinates. Many Softwares, Both freeware as well as Commercial, are available for its processing, but the problem of phase ambiguity resolution over large baselines and the Various ionospheric and tropospheric Correction modules becomes deciding when one wants to choose among them. In this Study we have tried to compare the result of two Campaigns Processed Using Two Different GPS Post Processing Softwares, Carried Out in a Hiily and another over a plain terrain. It has been observed that in the positional coordinate the deviations are not much but in the height the difference found was as high as 4-5m. The difference is attributed to the detailed ionospheric and tropospheric corrections as well as the use of various earth rotation parameters taken in the standard software as compared to the freeware. Also the difference of datum and use of various tectonic plates for the computation of station velocities as Considered in the Standard packages has contributed to this difference.