Coordinate Transformation between Everest and Wgs – 84 Datums
Analysis of results
From the very beginning it was suspected that a single set of transformation parameters for the entire country may not give the level of accuracy required for topographical applications due to inconsistency and irregularity in Indian triangulation network. However to form a definite opinion about it, initially the model was applied to all the 300 points but 20% of the points which were kept reserved for checking the quality of transformation process. The strategy did not work and as expected the transformation parameters derived from the model when applied on check points resulted in large positional errors.
In order to minimize errors in transformed coordinates resulting due to non – homogeneity of Everest coordinates it was decided to produce separate 7-parameter sets for sub-regions. The entire region covered by GPS observations was divided into 10 Zones (Refer Fig. 2) to meet the requirement of consistency in the GT C oordinates. The transformation parameters, computed zone-wise, have brought out the significance of improvement in accuracy of transformed coordinates. For most of the stations the the transformation accuracy was improved to the level of +3m, which is well within permissible limit on 1:50,000 scale and would serve the purpose of converting existing topo maps to WGS-84 maps.
The various levels of positional errors in transformed coordinates has been shown zone wise in Figures 3-12. The effect of consistency in Everest Coordinates on accuracy of transformation is clearly visible in Figure-9, which represents zone-5. This is the only zone in which most of the stations belong to primary series eminating from Calcutta longitudinal series. G.T. Stations falling in all other zones are either of secondary order accuracy or from a cluster of independently adjusted series. However, the transformation derived from this exercise are accurate enough to accomplish the goal of producing WGS-84 series maps on 1:50,000 or smaller scales.
Fig.3 Positional Error of Transformed Coordinates in Zone - 1
Fig.4 Positional Error of Transformed Coordinates in Zone - 2
Fig.5 Positional Error of Transformed Coordinates in Zone – 3A
Fig.6 Positional Error of Transformed Coordinates in Zone – 3B
Fig.7 Positional Error of Transformed Coordinates in Zone – 4A
Fig.8 Positional Error of Transformed Coordinates in Zone – 4B
Fig.9 Positional Error of Transformed Coordinates in Zone - 5
Fig.10 Positional Error of Transformed Coordinates in Zone - 6
Fig.11 Positional Error of Transformed Coordinates in Zone - 7
Fig.12 Positional Error of Transformed Coordinates in Zone - 8
