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Software development & validation for Geodetic Datum Transformations
Venkat Ramanan,Vamsi Krishna,M. N. Kulkarni
Indian Institute of Technology, Bombay, Mumbai, India
Tibor Borsa
FOMI Satellite Geodetic Observatory, Penc, HUNGARY
Introduction
A common problem faced by most countries is the estimation of precise transformation parameters between their national geodetic datum, and the World Geodetic System 1984 (WGS84) global datum, used by the Global Positioning System (GPS). In this study, a coordinate transformation package has been developed for this purpose, under UNIX environment, using the C programming language.
Transformation parameters
There can be considerable difference in positions of the local ellipsoidal datum and the global datum, sometimes up to several hundred meters. So a country like India may require a lot of datums for accurate transformation of coordinates from the local datum to the global one and the vice-versa. A country like Hungary, on the other hand uses just one datum, HD-72. With the increasing exchange of geographic information local and globally, positions need to be available in terns of both a local and global datums. The process of mathematically converting positions from one datum to the other is known as datum transformation. Datum transformation parameters define functional relationship between two reference frames. The GPS derived coordinates (which are with respect to the global datum), and the local terrestrial coordinates (which are relative to the local datum of the region) of the lumped points may be processed together using an appropriate transformation model. This forms the required set of database for calculating the result, which would be a set of quantities termed as transformations parameters which could be used for converting coordinates from one datum to the other and vice-versa. The combination of data sets from two or more different sources is purely a matter of adjustment and the technique used in this case is "least square adjustment". Since cartographers work with projections systems, in general, the transformation from one projection system to any other projection system can be performed as it follows:
- From projections system1 to its corresponding datum, say datum1
- From datum1 to one of Global reference frames (Say to WGS-84]
- From global reference frame to the Datum of interest. Say datum2,
- From Datum2 to the projection system in goal.
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