What surveying does

Surveying measures and records the positions of points on, above or below the earth's surface, and sets out new points from a design. Every civil project starts and ends with it: a topographic survey gives the designer the existing ground, setting out puts the design on site, and as-built surveys record what was built. Measurements are distances, angles and heights, combined into coordinates.

Two basic principles run through all survey work. First, work from the whole to the part: establish a framework of accurately fixed control points, then fill in detail from them, so that errors stay local and do not accumulate. Second, fix each new point by at least two independent measurements from known points, so that one gross error is detected rather than hidden. Every survey also needs checks, such as closing a traverse back on itself or reading a level back to the starting benchmark.

Surveys are classed as plane surveys, where the earth's curvature can be ignored, typically for areas under a few hundred square kilometres, and geodetic surveys, which account for the earth's shape. By purpose there are topographic, cadastral (property boundary), engineering, route, hydrographic, mine and construction surveys. Instruments have moved from chains, compasses and dumpy levels to total stations, digital levels, GNSS receivers, drones and laser scanners, but the principles, sources of error and need for checks remain the same.