Establishing geodetic control for Topographical survey
Control surveys establish a common, consistent network of physical points that are the basis for controlling the horizontal and vertical positions of transportation mprovement projects and facilities. Corridor control surveys ensure that adjacent projects have compatible control. Project control surveys provide consistent and accurate horizontal and vertical control for all subsequent project surveys — photogrammetric, mapping, planning, design, construction, and right of way.
Surveying is based on geodesy. With precise survey instrument, the Earth must be treated as an ellipsoid instead of a sphere. The ellipsoid that is chosen for geodetic survey is called reference ellipsoid. The shape of the Earth, so called geoid, is expressed as an equipotential surface of gravity. Usually, position on the ground is measured in longitude, latitude on the reference ellipsoid and height on the geoid. For convenience to measure the three dimensional values, a lot of monuments whos e longitude, latitude or height is known are settled as referen ce point all over the countries. Control point surveying is a surveying to establish the reference point network, determine the coordinates of a new point based on the reference point network. Some precise measuring instruments such as total station and level are used for the surveying. In addition, rapid development of the space geodesy enables us to establish an international reference frame covering the whole globe.