Contouring

A contour is a line joining points of equal elevation on the ground. A contour map shows the shape of the land on flat paper: closely spaced contours mean steep ground, widely spaced ones mean gentle ground, V-shaped contours pointing uphill mark valleys and those pointing downhill mark ridges. The vertical interval between contours is chosen from the purpose, scale and terrain, for example 0.5 m for site planning and 5 or 10 m for regional maps.

Contours can be surveyed directly by tracing points at a chosen level on the ground, which is accurate but slow, or indirectly by measuring spot levels on a grid or on sections and interpolating between them. Indirect methods dominate modern practice. Total station, GNSS, drone photogrammetry and LiDAR all produce dense point clouds from which software builds a digital terrain model, a triangulated surface, and draws contours automatically.

Contours are used to choose road and canal alignments at a ruling gradient, to find catchment areas and reservoir capacities, to estimate earthwork from cross-sections or surface-to-surface comparison, to plan drainage and to check intervisibility. Some characteristics help to read and check maps: contours never cross except at overhangs, never end inside the map, are perpendicular to the direction of steepest slope, and close on themselves somewhere, possibly beyond the sheet.

Good practice combines methods: a GNSS or total station control framework, a drone or LiDAR survey for the bulk of the surface, and ground checks in vegetation, water and along breaklines such as kerbs, drain edges and embankment toes, where automatic surfaces are least reliable.