Coordinate transformation between
Everest and WGS – 84 datums – A parametric approach
S.K.Singh
Geodetic & Research Branch, Survey of India, Dehradun-248001
Abstract
The local and global geodetic reference frames can be related through a seven parameter similarity transformation model but the accuracy of transformation depends upon the consistency and homogeneity of local terrestrial data. Indian geodetic network being more irregular and inhomogeneous, ten sub-regions transformation parameters were computed to convert the Everest coordinates to WGS-84 coordinates. The transformation could achieve the meter level accuracy, which is sufficient enough to deal with the matter of producing WGS-84 series maps from existing topo sheets on 1:50,000 or smaller scale. More sophisticated techniques and more amount of GPS data are required if one wishes to get sub-meter transformation accuracy.
Introduction
Survey of India has been recognized as one of the most important national agency for research and development activity in many diverse field such as space geodesy, astronomy, gravity and magnetism, seismo–tectonics etc besides its paramount importance in the field of Surveying and Mapping. Though department has been supplying topographic maps to the users, organizations and individuals since long time, there have been some problems on distribution of the maps due to security reasons.
There has been consistent demand from several quarters that the topographical data should be treated as national assets and to be made available general public without much of restriction. The GIS industry has also been raising on the issue on various platforms to speedily evolve a long-term policy on data sharing between government and non-government organizations.
The government realized the genuineness of the problem and set-up a committee of intellectuals and experts to review the existing map restriction policies. The committee have gone through the various aspects of data sharing and its implications to national security and suggested dual maps series in India.
As per the recommendations of the committee a new series of maps based upon WGS-84 datum will be introduced in SOI apart from existing Everest datum maps. The WGS-84 series maps will be openly made available to civil users whereas existing maps will continue to meet defence requirements. The main task before the surveying community was how to produce the new maps. The easiest approach is to convert the existing maps through a set of datum transformation parameters. These transformations parameters were required to be determined immediately which involved tremendous amount of field work. The job was taken up by Geodetic &Research Branch of Survey of India under the financial support from Deptt. of Science and Technology ( DST ), Govt. of India.
Everest & WGS-84 datums
The Everest Ellipsoid has been used for India and several adjacent countries for mapping activities. Named after Sir George Everest the ellipsoid was derived in 1830 and since then it has been used as basis for all types of control surveys. Sir George Everest paid careful attention to the measurement of bases and astronomical latitudes and longitudes were measured throughout the arc of the meridian especially at Kalianpur ( 240 7˘ ) where many observation were made ( Cook, A, 1990 ). Dimensions of Everest ellipsoid and its orientation at origin were carried out one by one at a number of times. Semi major axis (a), flattening (f) and North – South component of Deflection of Vertical ( Meridonal ) was defined by Everest in 1840 whereas East – West ( Prime – Vertical ) component was defined by Walker in 1878. Though Everest ellipsoid has been the best fitting mathematical surface for India and adjacent countries but it cannot be extended too far from the origin and hence its applications are rather limited. For this reason the Indian Datum is probably the least satisfactory of the major datums ( DMA, 1983 ).
The terrestrial reference system used by the U.S. Department of Defence ( DoD ) for GPS positioning is the World Geodetic System 1984 ( WGS-84 ). The navigation message broadcast by GPS satellites are expressed in this system. WGS-84 is a global geocentric coordinate system defined originally by DoD based on Doppler observations of the TRANSIT satellite system ( the predecessor of GPS ). WGS-84 was first determined by aligning as closely as possible, using a similarity transformation, the DoD reference frame NSWC–92Z and Bureau International de l’Heure ( BIH ) Conventional Terrestrial System ( BTS ) at the epoch 1984.0 ( BIH is the predecessor of the IERS and BTS is the predecessor of ITRF ). It was realized by the adopted coordinates of a globally distributed set of tracking stations with an estimated accuracy of 1-2 meters ( compare to 1-2 cm accuracy of ITRF ) ( Teunissen P.J.G., Kleusberg, A., 1998).
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