LARGE SCALE GEODETIC LEAST SQUARES ADJUSTMENT BY DISSECTION AND ORTHOGONAL DECOMPOSITION Gene

Very large seal@ matrix problems currently arise in the context of accurately computing the coordinates of points on the surface of the earth. Here g.eodesists adjust the approximate values of these coordinates by computing least squares solutions to large sparse systems of equations which result from relating the coordinates to certain observations such as distances or angles between points. The purpose of this paper is to suggest an alternative to the formation and solution of the normal eqgations for these least squares adjustment problems. In particular, it is shown how a block-orthogonal decomposition method can be used in conjunction with a nested dissection scheme to produce an algorithm for solving such ,problems which combines efficient data management with numerical stability. As an indication of the magnitude that these least squares adjustment problems can sometimes attain, the forthcoming readjustment of the North American Datum in 1983 by the National Geodetic Survey is discussed. Here it becomes necessary to linearize and solve an overdetermined system of approximately ~,OOO,OOO equations in 400,000 unknowns a truly large-scale matrix problem.