The measurement of surface features and configuration of an area or a region, and the graphic expression of those features. Surveying is the art and science of measurement of points on, above, or under the surface of the Earth. Topographic maps show the natural and cultural features of a piece of land. The natural features include configuration (relief), hydrography, and vegetation. The cultural features include roads, buildings, bridges, political boundaries, and the sectional breakdown of the land. Topographic maps are used by a wide variety of people, such as engineers designing a new road; backpackers finding their way into remote areas; scientists describing soil or vegetation types, wildlife habitat, or hydrology; and military personnel planning field operations. See also Cartography; Map projections.
Topographic maps that show natural and cultural features only in plan view are called planimetric maps, while maps that show relief are called hypsometric maps. Contour lines join points along a line of the same elevation across the ground. Contours show not only the elevation of the ground but also the geomorphic shape of features. See also Contour.
A digital terrain model (DTM) is a computer-generated grid laid over the topographic information, which can then be rotated, tilted, and vertically exaggerated to give a three-dimensional view of the ground from different perspectives, including oblique representations. This technology is an excellent presentation tool: it utilizes the advances that have been made in computer mapping and drafting software.
Prior to starting collection of data, a network of known horizontal and vertical control points must be established. The network also allows measurements made from several different locations in the same coordinate system to fit together into the same reference datum (the basis for the coordinate system). Field methods that are used to make measurements include ground surveys, geographic positioning systems, and hydrographic surveys. Photogrammetry and remote sensing techniques involve the use of photography to obtain reliable measurements. Photographs can be taken from airplanes, helicopters, and even satellites; thus the term remote sensing is applied to this technology. See also Aerial photograph; Hydrography; Photogrammetry; Remote sensing; Surveying instruments.
