Because the Earth is a sphere, any representation of its surface on a flat sheet of paper involves distortion. This distortion is relatively insignificant for maps showing small parts of the Earth, such as city maps, but quite considerable for maps of whole countries or continents.
The decision facing a map-maker, therefore, is not whether to have a distortion on a map, but what type of distortion. Over the centuries, various geometrical schemes have been worked out for representing the curved surface of the Earth on map sheets; these schemes are known as map projections. All projections have certain advantages and disadvantages, and the selection of one or the other depends chiefly on the needs of the user.
The size and shape of the country being mapped determines the most suitable projection for its system of topographic maps. Very large countries such as Canada must be divided into strips, usually called zones, which are projected onto a plane in orderly fashion. One such system of strip projection is the Transverse Mercator. It is called transverse because the strips run north-south rather than east-west along the equator, as in the standard Mercator projection. A special type of Transverse Mercator is the Universal Transverse Mercator (UTM) Projection. The Canada Centre for Mapping and Earth Observation (CCMEO) uses the Universal Transverse Mercator Projection for its popular National Topographic System (NTS) series at 1:50 000 and 1:250 000 scales. Tied in with this projection is the rectangular grid, a special system for finding and identifying points on maps. All topographic maps also carry the familiar lines of longitude and latitude, but these are not well suited for quick and simple point location and identification.
This material updated from The Universal Transverse Mercator Grid, Department of Energy, Mines and Resources Canada, Surveys and Mapping Branch, Ottawa, © 1969, The Queen's Printer.
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