Establishment of Local Control Points for Mapping using GPS
Dr. Jayanta Kumar Ghosh, Ojasa and Goyal
Civil Engineering Department, IIT Roorkee
Uttaranchal, 247 667 India
Email: gjkumfce@iitr.ernet.in
Abstract
Establishment of local control points is very important for carrying out surveying and mapping of the area for any engineering project. GPS is already been used as an important tool for establishment of control point accurately, quickly and economically in a versatile way. The coordinates of the terrestrial sites thus obtained are in the WGS 84 reference frame. But, surveyors are not, usually, interested in coordinates of terrestrial points referring to a global frame; rather the results are preferred to be in a local coordinate frame. Since the WGS 84 is a geocentric system and the local system usually is not, transformations from global coordinates to local level coordinates are required.
To prepare a map for an area, at Jamrani (Nainital, Uttaranchal) Dam construction site, GPS observation was taken to establish a network of control points. The coordinates of control points thus found were in WGS 84 system and were transformed to local coordinate system by multiplying a with a coordinate transformation matrix. The BENCH MARK available at the site is considered as origin of the local coordinates system, and geographic North, East and Vertically up direction are considered as the three reference axes. The coordinates thus found served excellently in preparation of map of the area (of about 800 metre X 300meter) using data from terrestrial field observation.
The vertical positions of control points thus calculated is compared with those of Orthometric height based on datum transformation. These are found to be excellently correlated. Thus, it can be concluded that for small scale surveying and mapping, control point can be established very accurately and precisely simply by transforming the GPS observed global coordinates to its equivalent local coordinates corresponding to the North, East and Vertically up direction.