Geodesy


Geodetic infrastructure in India

Dec 2005 | Comments Off on Geodetic infrastructure in India
 
Geodetic Infrastructure in India is inadequate. It needs a fresh look and complete revision
   

The Great Trigonometrical Survey of India was completed in 19th century under leadership of the great surveyors- Lambton and Sir George Everest. It is inconsistent and inadequate. Accuracy of the network is only of the 1st order or less. First order was defined as better than only 1 in 50,000 only. Reference surface and Datum- The Everest Spheroid was given by Sir George Everest in 1830. Center of Everest Spheroid is about a km away from the center of gravity of the Earth; hence it is non-geocentric. Thus it is inaccurate and unsuitable under present circumstances. Leveling network of India has inconsistencies. Gravity observations were not carried out and not taken in to consideration. It was not appropriately adjusted. Indian Absolute Gravity Datum does not exist. Absolute gravimeters have not yet been used to define Gravity Datum in India. Topographical maps are on Polyconic projection. Assumptions and approximations accepted make it a non-projection. The earth is assumed to be fiat and there are no distortions of any kind. The projection has created problems in digitization, compilation and integration of maps. Design of the Grid adopted in India is not satisfactory. Distortion at central parallel is 1 in 824, which is quite high. There is archaic Restriction policy, which is not transparent and hinders research and development India has to make a choice between chaos and development. These problems have been discussed in detail in this paper. How India should go about to establish new geodetic infrastructure for systematic development and research, has been described in this paper.

Geodetic Infrastructure in India is inadequate. It needs a fresh look It needs a fresh look and complete revision. Indian Geodetic Datum, The Reference Surface, Horizontal Control, The Vertical Datum, Height Control, Indian Geoid and Mean sea level, The Gravity datum, Projection System for Maps, and Indian Grid, are all inadequate or inappropriate.

Indian Geodetic Datum is based on Everest Spheroid as Reference Surface and Kalyanpur in Central India as initial point. Center of this reference surface is estimated to be about 1 km away from the center of gravity of the Earth. The datum is thus a local datum and in error. Scientific and Defense studies of vital National importance cannot be based on such a system. It is therefore extremely necessary that the Indian Geodetic Datum should be redefined at the earliest. The project on Redefinition of Indian Geodetic Datum should be taken up in right earnest and completed within one year. Horizontal Control in India resulted from The Great Trigonometrical Survey. The different triangulation series are inconsistent between each other. It has not been properly adjusted. Stations are burdened with varying degrees of error and many points are either destroyed or in need of repair. It is therefore urgently necessary that fresh observations be carried out to get a set of vectors by GPS and other means and least square adjustment be carried for the whole country at one go to get control points of zero, 1st, and 2nd order in 2 to 3 years. Vertical Datum for Heights in India was chosen as the Mean Sea Level at a group of nine tidal observatories situated at Indian ports. Level network in India is of moderate to high precision at different places. No gravity observations were carried out at that time. Network has not been properly adjusted. It is required that a fresh datum for required that a fresh datum for heights based on Mean Sea level at one tidal observatory say Mumbai. High Precision Leveling should be carried out afresh throughout India to get a network along with gravity observations and adjusted so as to have 1st order vertical control. It is necessary to have a Gravity Datum in India. Presently there is no absolute gravity station in India. Gravity values in India are based on relative gravity observations based on gravity datum/ s in other countries. It is therefore necessary to establish some absolute gravity stations, adopt a gravity datum and carry out relative gravity observations in order to get a 1st order gravity network. No satisfactory Indian Geoid is available. To get heights above Mean sea level by GPS observations we need a geoid that can give geoidal undulation of accuracy of 25 cm or better. A project should be taken up immediately to obtain geoid by gravimetric as well as astro-geodetic methods. Projection for topographical maps in India is Polyconic. Each individual sheet is projected individually assuming no distortion along parallels and neglecting distortion along meridians. Meridians and parallels are assumed as straight lines. These assumptions make it a perfect projection, which is not possible. We can say that The Earth/Ellipsoid has been assumed to be ?at for individual sheets. This creates problems in digitization, integration and compilation of maps. It is therefore necessary to abandon so called polyconic projection and adopt either Lambert Conformal Conic or Transverse Mercator Projection designing suitable zones. Indian Grid on Lambert Conformal Conic Projection for superimposition on topographical maps was designed during British days. It has 9 zones with scale factors at central parallel as 649/650 and 823/824. The designis unsatisfactory as scale error at central parallel should be 1 in 2000 or better. The grid has been restricted which is irrational as the grid parameters are available to everyone outside India including Pakistan. It is therefore necessary to design grids on Transverse Mercator or Lambert Conformal conic Projection with suitable zones similar to State Plane Coordinate systems in USA. It is suggested that each Indian state should have a grid for that state and all mapping is carried out on such grid for civil use. Structure of Monuments (geodetic stations, bench marks etc.) is not of permanent nature in many cases and many have been destroyed. Monuments that are not fixed to bedrock are not suitable for geodetic monitoring of crustal movements. New and suitable permanent monuments need to be constructed before any new project regarding geodetic infrastructure is taken up.

Indian Geodetic Datum

Indian Geodetic Datum is based on Everest Spheroid as reference surface given by Sir George Everest, then Surveyor General of India in 1830. Kalyanpur in central India was chosen as initial point or origin. Coordinates of initial point and azimuth of a

table1

line were obtained by astronomical observations and leveling. The reference surface was however defined peace-meal at various times. Astronomical observations were carried out at least twice. More precise observations carried out later were accepted. Hence meridional and prime vertical de?ection of vertical, were de?ned at Kalyanpur. Parameters of the datum are given in table1.

Value of Semi major axis and semi minor axis were given in feet as 20,922,931.80 feet and 20,853,374.58 feet respectively. These values were converted into metres using different conversion factors resulting in many values of a and b of Everest spheroid. The of?cial conversion factor for India is 0.3047996 and should be accepted.

It is estimated that the center of Everest spheroid is about 1 km away from the center of gravity of the earth; hence it is not a geocentric datum. We therefore conclude that it is inaccurate system and needs rede?nition. The present datum is especially not suited for many geodetic, geodynamic, geophysical and defence applications. One has either to use WGS84 for Geodetic monitoring of crustal movements, plate tectonic movements, development and deployment of missiles and many other scientific applications or redefine Indian Geodetic System/Datum. A project to redefine Indian Geodetic Datum was taken up in 1989 but nothing much appears to have been done so far. Redefinition project should be taken up and given highest priority. It should be time bound and completed in 2 to 3 years. In USA a similar project was taken up in 1974 by National Geodetic Survey and completed in 1983. It is known as NAD 83 and is used for all applications including Geodynamic and defense.

 

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