Geodetic Control Framework

Why are They Needed?

All GIS are based on an assumption that the coordinates of the points being used are known, or can be calculated. This process would be comparatively easy on a flat earth, but because the earth is a solid body, of somewhat irregular shape, the process of creating coordinates for the GIS user is not a simple one. To assist us in performing these operations, surveyors over the years have created geodetic control frameworks (or networks), a series of rigid, formalized grids that allow the computation of position.

A control framework is a geometrical mesh that is created mathematically on the earth's surface to allow [Page 177]computations of position. Traditionally, navigation and surveying measurements provide the coordinates of a “control” point in latitude, longitude, and height, with the units of measurement being degrees, minutes, and seconds for latitude and longitude and either meters or feet for the height. A process known as map projection involves the conversion of geodetic coordinates (latitude, longitude, and height), which are in three dimensions, to x- and y-coordinates (or eastings and northings), which are in two dimensions, for use in mapping. An understanding of map projection is also required of the GIS user in the process of going from the three dimensions of the earth to the two dimensions of the paper map. A true representation of shape, area, and distance cannot always be maintained, and some characteristics will be lost. It is therefore necessary to recognize the characteristics of the map projection being used and be consistent throughout the project. For many GIS applications, some variant of the Universal Transverse Mercator (UTM) map projection is used.

Who Creates Them, and How?

Customarily, the creation of such frameworks was the responsibility of national mapping organizations, such as the U.S Coast and Geodetic Survey or the Ordnance Survey of Great Britain. Triangles are created across the country of interest, in a process known as triangulation or trilateration. Before the advent of satellite positioning methods, this was done by using theodolites to measure angles and tapes or electronic distance-measuring instruments to measure distances. Latitudes and longitudes were then calculated for selected points on the ground known as control points, and these points were often marked by a concrete monument with a bronze disc containing the number of the control point. The coordinates, obtained by computation, were then made available to the public upon request.

Such control points formed a mesh or a grid over the area of interest, and coordinates were calculated for various “accuracies” of surveys. The accuracy needed would depend upon the ultimate use of the survey control points. Traditionally, such accuracies were divided into four: first order, which consisted of the primary control network of the country or area of interest and was used for highly accurate surveys, such as the deformations of dams or the tracking of movement due to seismic activity, and subsequent second, third, and fourth order, which were used for most mapping purposes. Most accuracies are given in terms of parts per million, or the size of an error area around the point, and first-order surveys can have accuracies of better than a centimeter. The use of geometric computations to obtain latitudes and longitudes from measurements of angle and distance was, in fact, a result of the fact that accurate latitudes and longitudes could rarely be measured directly.

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