CAD METHODS FOR RESAMPLING

Abstract

Advances in computer technology have made it possible to integrate the various data sets together, with a wide range of information. Geographic information systems (GIS), a new technology- is becoming essential tools for analyzing and graphically transferring knowledge about the world. Resampling is the means by which a geometric transformation is actually applied to the input data. Map sheets, satellite imageries and aerial photographs represent parts of a large continuum and are required to be resampled for compatibility, before integrat-ing them together. Resampling is often performed to convert the map grid to uniform latitude/ longitude grid and the viceversa. In GIS, resampling is applied to input raster data/vector data, for image registration, rectification and mosaicking purposes. The problem of rolling fit in map mosaicking is a serious problem in countries like India where the polyconic projection is used in the construction of the maps. To avoided this prob-lem, the map details needs to be transformed using a suitable projection transformation method. In analytical transformations, the coordinate positions of the source map are converted into their geographical coordinates and then the projection coordinates for the new map are com-puted from these geographical coordinates. But the object of most of digital mapping is not to compute the transformation of just one point, or even a graticule comprising several hun-dred points, but is applied to all map details, and this may well involve repetition of the entire procedure hundreds of thousands or even millions of times. Thus the analytical method of transformation is a cumbersome, tedious and time consuming one. The Finite Element Method, as described, and largely pioneered by Zienkiewicz, is used in design, structural analysis and many more applications in civil engineering. Though this method was originally developed for structural analysis, but the general nature of the theory on which it is based has made possible its successful application in other fields of engineering also.In finite element methods, the variation of displacements etc. in an element is expressed as a function of its nodal values. The representation of geometry in terms of (non linear) shape functions can be considered as a mapping procedure for transformations from local coordinate system into global Cartesian coordinate system. Thus the finite element method is found useful for making two dimensional transformations and would serve very well as a tool in remote sensing and GIS. In this thesis work, an attempt has been made to use finite element interpolating functions for resampling of raster as well as vector images.