Control net

A geodetic control network (also geodetic network, reference network, control point network, or control network) is a network, often of triangles, which are measured exactly by techniques of terrestrial surveying or by satellite geodesy.

A geodetic control network consists of stable, identifiable points with published datum values derived from observations that tie the points together.

Classically, a control is divided into horizontal (X-Y) and vertical (Z) controls (components of the control), however with the advent of satellite navigation systems, GPS in particular, this division is becoming obsolete.

Many organizations contribute information to the geodetic control network.

The higher-order (high precision, usually millimeter-to-decimeter on a scale of continents) control points are normally defined in both space and time using global or space techniques, and are used for "lower-order" points to be tied into. The lower-order control points are normally used for engineering, construction and navigation. The scientific discipline that deals with the establishing of coordinates of points in a high-order control network is called geodesy, and the technical discipline that does the same for points in a low-order control network is called surveying.

After a cartographer registers key points in a digital map to the real world coordinates of those points on the ground, the map is then said to be "in control". Having a base map and other data in geodetic control means that they will overlay correctly.

When map layers are not in control, it requires extra work to adjust them to line up, which introduces additional error. Those real world coordinates are generally in some particular map projection, unit, and geodetic datum.

In "classical geodesy" (up to the sixties) control networks was established by triangulation using measurements of angles and of some spare distances. The precise orientation to the geographic north is achieved through methods of geodetic astronomy. The principal instruments used are theodolites and tacheometers, which nowadays are equipped with infrared distance measuring, data bases, communication systems and partly by satellite links.

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