Mapping and Surveying. Surveying and Reference Systems

Mapping may always have been a human instinct. Originally a mental process, it was associated with spatial awareness for survival—seeking food and shelter and avoiding danger. As these tasks became more complex and as people formed communities, cognitive knowledge may have—instinctively—been externalized as sketch maps in sand or in the dust on a cave floor. Such thinking aids offered a unique form of graphic storage and display and also allowed spatial ideas to be shared, compared, and analyzed.

Although this technique of sketching out elements of local environments may have been practiced for forty thousand years or more, the evidence for it is lost. Most early map artifacts (dating from around 2000 все) are often pictorial in character, expressing ideas and beliefs, and structured without precise measurement.

More specific survey methods were employed by the Egyptians in the Nile River valley around 1400 все for property mapping, and the science was advanced at various stages by the Romans, Greeks, and Arabs during the first millennium ce. Not until the eighteenth century did mapmakers begin to develop and apply rigorous scientific survey procedures for military, hydrographic, and national mapping.

Human ancestors must have carefully surveyed their environments visually to expand their spatial knowledge, but surveying has since matured into an established discipline for determining the positions of points (and, by implication, lines and aerial features) at or near the earth's surface, primarily for the production of maps.

Maps are now commonplace, but without accurate surveying this situation would not exist. Surveying is employed both on land (topographic) and sea (hydrographic). The principles are the same, but the techniques differ in detail. Although sea navigation charts were among the earliest maps to be used by professionals, this article will concentrate on maps of the land.

Surveying and Reference Systems. Maps are plotted from information surveyed for both control frameworks (an array of points whose positions and elevations have been measured to a high degree of accuracy) at regional and global scales and for the recording of landscape detail. One rule for preserving survey accuracy is to work from the whole (the control) to the part (the detailed mapping).

This also reflects the main types of surveying: geodetic, where the earth's curvature is taken into account, and plane, where the reference surface is assumed to be flat and horizontal. To determine location on a plane surface requires only a system of rectangular (x, y) coordinates. The equivalent for the globe is a graticule (grid) of lines, on the earth's reference sphere, of latitude, parallel to the equator, and longitude, at right angles to the equator and passing through the poles.

For the production of small-scale maps the earth can be assumed to be a sphere, although in reality it is flattened at the poles. Acknowledging this, surveyors use a reference ellipsoid (a mathematical shape approximating the true shape of the earth), which approximates reality. However, they also require a separate surface to which ground-surveyed positions in the vertical and horizontal can be referenced. If the earth were of uniform composition and covered with ocean its outer surface would approximately match the ellipsoid.

However, the uneven distribution of rock and sea disturbs this simple model and gives, instead, the irregular gravity based surface known as the geoid. When map survey measurements are plotted on a flat surface the earth graticule must be transformed to a flat plane. This is done using various forms of map projection. The graticule coordinates of the specific map projection adopted for the country being surveyed can then be converted into the plane coordinate system of that country.

Control Surveys. The holistic approach is provided in geodesy (the study and measurement of the precise shape and size of the earth), and geodetic surveying accounts for Earth curvature in the measurement and computation of the precise locations of widely spaced reference points and the distances between them. Smaller control surveys are also carried out, notably at the national level for mapping programs.

Control is required for both horizontal and vertical measurements. The classic procedure for establishing horizontal control for a large area begins with the determination, using astronomical methods, of the precise latitude and longitude coordinates of two stations defining a baseline that is also measured precisely. A third point is then selected to form a large triangle, and this triangulation is extended to cover the area to be mapped. This primary control can still be identified in some landscapes from the distribution of fixed triangulation pillars (or survey "monuments") that mark out the network.

Although with lower precision, control can also be extended across smaller areas (especially where intervisibility, clear line-of-sight between two points, is poor, such as in forests or urban areas) by creating a traverse of a series of lines through the landscape determined by the direct measurement of distances and angles. The method of trilateration, where triangle side lengths alone are measured, is also possible, but until the introduction of electronic distance measurement (EDM) techniques it was not in extensive use.

Traditionally, vertical control was related to what is called the "mean sea-level datum" for a particular region (the local position of the geoid surface), commonly the average of tidal ranges measured over a nineteen-year period. This is connected using precise leveling techniques to a primary network of local reference points (benchmarks) across the region. In Great Britain, mean sea level was calculated at Newlyn, Cornwall, between 1915 and 1921 as the ordnance datum, with an accessible reference point, the observatory benchmark, established 4.751 meters higher.