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Error Estimates for WGS-84 and Everest (India-1956) Transformation
 Mr. B.K. Srivastava General Manager (Cartography) Mr. K. Ramalingam Member (Planning) Airports Authority of India, New Delhi, India Abstract WGS-84 is an earth-fixed, earth-centred global reference frame including an earth model as defined by a set of primary and secondary parameters. WGS-84 system has come into existence only towards the end of 20th Century, and prior to that the local coordinate system (Everest or Indian System) had been in use in India for more than 150 years. Thus, most of the maps, records and data are available in the form of Everest system only. Till such time all the records are transformed to WGS-84 and new maps are printed, the local system is also likely to continue along with WGS-84. Ready-made softwares like MADTRAN or DATUM, use the standard and average values of transformation parameters for conversion of coordinates of WGS-84 and Everest system. Therefore, some extent of errors gets introduced during such mathematical transformations, which is required to assessed. Similarly, it is also desirable to determine whether coordinates of one system could be used instead of other system without any transformation for non-precision activities in the field of aviation. Accordingly a study was conducted to examine the above aspects of WGS–84 and Everest Coordinate Systems. During observations, it was found that there was a maximum error of about 1 arc-second between precise observations and mathematical transformation. Thus, it was concluded that it could be possible to use the software for non-precision activities in aviation field where maximum positioning error ranging upto 50 meters is acceptable. It was also observed that if the local coordinates are used in place of WGS-84 coordinates or vice versa, there could be a maximum positioning error ranging upto 500 metres. Thus, for activities where an error tolerance of ½ km to 1 km is acceptable, there may not be any requirement of carrying out any transformation from one system to other and the coordinates of one system could be conveniently used in place of other. In the field of aviation, the methodology could be safely applied for representing coordinates of En-route Navigational Aids, En-route Reporting Points, Control Zones, Control Areas, Terminal Control Areas (TMAs) and Flight Information Region (FIR) boundaries and to some extent even for Restricted Areas in WGS-84 System. |