D.H. Maling (Auth.)-Coordinate Systems and Map Projections-Pergamon (1992) Partie6

Preface

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have now evolved into Geographical or Land Information Systems. Much of the work may be done using graphics packages to produce a suitable monitor display rather than a paper map. The subsequent stages of map use by comparison and evaluation of different mapped images for a particular purpose are to be found in handling GIS or LIS layers. It means that the conventional paper map is going to be replaced more and more, so that it seems possible that in another 20 years most maps as we know them will have become rarities to be consulted in libraries. However, it is unrealistic to imagine that all GIS work can be handled by microcomputer. Since the files comprising individual layers in such systems may each comprise millions of pixels, there is need to process such data economically and in terms of transforming them geometrically, so that one layer is properly registered to another. Because of the demands upon space and storage, different and more economical numerical methods are needed to handle very large files than was traditionally used in mathematical cartography. The so-called rubber-sheeting methods, based upon numerical interpolation between control points, has divorced much of the work from the classic methods of computing map projections. Although some of the methods are considered towards the end of the book, the treatment is by no means exhaustive. Moreover, before plung­ ing headlong into these methods, it is wise to heed Paul Curran's warning (Curran, 1987) that although the current geographical information sys­ tems bandwagon has much to offer by way of models and analysis: It has generated a plethora of empirical studies in which vast amounts of data have been sandwiched together, just because it was computationally possible to do so.

The principal growth area for new surveying practices has been at sea, where the absence of visible marks at the surface, and the need to operate out of sight of land, has led to the development of a new branch of the s\xh]tci-marine geodesy. The impetus for this development has of course been economic; the need is for extremely accurate surveys to locate trial borings, well-heads, pipelines and drilling rigs required for the com­ mercial exploitation of the offshore oilfields. Because some of the most valuable sites are to be found in places far beyond the conventional and practical limits of national control surveys, the need to relate such surveys to properly defined projection systems has become an important aspect of locating points or boundaries on the sea bed. Like the first edition, the present book is concerned with principles and practical methods rather than with the formal description of the 50 or so individual map projections which have been commonly used. Thus it is not until Chapter 10 that the derivation of any specific map projection is described in any detail. Here only three are described, and primarily to demonstrate the methods of analysis which may be employed to define a map projection to meet a specific requirement. Far more important than