A CRITICAL REVIEW OF GEODETIC AND GRAVIMETRIC METHODS OF GEODESY

Abstract

The main problem of geodesy consists of locating the position of a .station on the surface of the earth by comnuting its geodetic coordinates, namely latitude (S) longitude (L), and height (h) and finding the size and shape of the earth ellipsoid as accurately as possible. The geodetic coordinates can be obtained in different ways depending upon the accuracy requir@d for the result-ing map. The various methods available for this purpose can be named as geodetic, astronomic, astrogeodetic, astrofix, gravimetric, astro-gravitational and satellite methods. It is com on.knowledge that our plannot earth has a spheroidal shape with equitorial radius of about 6378 Km and a compression of about 1/300. The present work deals with the critical study of these various existing methods for providing control and computing the parameters i.e. semi-major axis and semi-minor axis and flattening of the earth ellipsoid, and finding out the most suitable method keeping in view the accuracy requirements, simplicity of the method and cost of equipment used in each method. In Chapter I, an introductory approach to the various geod-etic and gravimetric methods have been made. Developments in geodesy from the very beginning to the present stage has been given. Chapter II deals with the study of various geodetic methods of providing control and computing earth's parameters. Field work of triangulation, preparation of triangulation data and solution of spherical triangle to compute geodetic coordinates are discussed in detail. Various methods of providing control and their accuracies and scales upto which each method can be used up are mentioned. Various distance measuring instruments by electronic methods such as Hiran, Shoran, geodimeter, tellurometer and electro-tape, their ranoes and accuracies -stained by each are given. Various methods of computing the size and shape of the earth ellipsoid, viz, two -arc method, figure by several arcs and the area method of figure determina-tion are discussed in detail . The results obtained by different authors for the parameters of the reference ellipsoid by geodetic methods are given at the end of this Chapter. In Chanter III an attempt has been made to describe the methods of gravity measurement in detail. A brief description of types of gravity m-asurements, international gravity formula, variation of gravity with lititude and gravity reference system to be used is given. Pendulum ap7aratus, different gravimeters and the method of gravity measurement has been discussed at length. A brief description is also made of the gravity observations at sea and of the tidal effect. The rrinciple and use of Eotves torsion Balance in (3,V) the gravity measurements has been described. Chapter IV, deals with the study of different isostatic and non-isostatic methods used for the reduc-tion of gravity measurements as described in Chapter III Recent methods of isostatic reduction viz., the carto-graphic method and mass-line method have been described in detail and comparison of different reduction methods used is also given. A brief description is made of the gravimetric deflection of the vertical and a detailed procedure of computing gravity anomalies in unsurveyed areas has been explained. The gravity material necessary for the computations of mean free-air anomalies and that currently available have been mentioned. Dimensions of the earth ellipsoid obtained by different authors by gravimetric methods and a practical procedure to find the shape of the geoid are described. In Chapter V a detailed description of the satellite methods used for the computation of geodetic coordinates and the earth's shape and size has been made. A detailed description of the most important .gtaatt.c systbms by obsoisatcrns fic41_ problem of connection of two4satellites have been made. The accuracy aspects of the satellite methods and the results of size and shape of the earth's ellipsoid and flattening the earth are given. The accuracies and the detailed results of different methods have been compared and are given in the concluding Chapter