| |
A large- or intermediate-scale map (see map image or map) describing the salient physical and cultural features of a place or region. Derived from the Greek words topos (place) and graphein (to write), the word topographic originally referred to the work of landscape illustrators, whose pictorial treatment of terrain evolved from more primitive symbolic representations (Harvey, 1980). Contemporary topographic maps — the products of land survey (the third phase of topographic mapping) — typically describe the shape of the land with contour lines (see isoline) and use varied abstract symbols (see symbolization) to represent roads, railways, political boundaries and hydrographic features such as rivers, streams and lakes (Keates, 1972). These maps also contain toponyms (place names), which usually reflect traditional local usage. If scale permits, topographic maps might show individual buildings and other large structures as well as woodland, marshland and other types of land cover that might hide troops or impede their movement. Because national defence is a primary use, totalitarian regimes often severely restrict the distribution of detailed maps that might prove useful to insurgents or invaders.
Modern topographic maps are the outgrowth of systematic national surveys initiated in the eighteenth and nineteenth centuries, when advances in measurement technology coincided with recognition by central governments that a detailed, geometrically precise cartographic inventory was essential for efficient civil administration (Konvitz, 1987). Trigonometric surveying (see surveying) enabled topographers to construct planimetric maps, which show reliable distances between locations projected perpendicularly onto a horizontal reference plane, or datum. After measuring or calculating differences in elevation, topographers described the three-dimensional land surface with either hachures (short strokes running uphill and representing relative slope by variations in width or spacing) or contours (lines of constant elevation). For those who understand how to read them, contours afford a concise description of the terrain\'s shape and elevation. For less \'savvy\' users, cartographers occasionally incorporate hill shading or other dramatic pictorial enhancements (Imhof, 1982).
Because of the need for myriad details, topographic maps are rarely published at scales smaller than 1:250,000. In the developed world, where scales of 1:50,000 or larger are common, they provide a cartographic base for geological, soils and planning maps, and occasionally for cadastral maps, which describe property boundaries. More generally, topographic maps serve as source materials for compiling physical-political reference maps at smaller scales.
Aerial photography proved an enormous boon to topographic mapping. Although air photos are perspective views with appreciable distortion of horizontal distances in areas of moderate or high relief, a pair of overlapping photos provides a stereoscopic image of terrain useful for identifying features on the ground and measuring differences in elevation. Photo-grammetric plotting substantially reduced the need for direct measurement and field observation, and aerial photographic interpretation afforded an efficient method of compiling and updating topographic maps. In the 1970s, efficient automated removal of horizontal distortion encouraged production of orthophotomaps — geometrically accurate images of terrain that complement the traditional but comparatively abstract topographic \'line map\' (Thrower and Jensen, 1976). (MM)
References Harvey, P.D.A. 1980: The history of topographical maps: symbols, pictures and surveys. London: Thames and Hudson. Imhof, E. 1982: Cartographic relief representation. Berlin: Walter de Gruyter. Keates, J.S. 1972: Symbols and meaning in topographic maps. International Yearbook of Cartography 12: 168-8 1. Konvitz, Josef W. 1987: Cartography in France, 1660-1848: science, engineering, and statecraft. Chicago: University of Chicago Press. Thrower, N.J.W. and Jensen, J.R. 1976: The orthophoto and orthophotomap: characteristics, development and application. The American Cartographer 3: 39-56. |
|
