The establishment of a new 3-D control framework for GB based on modern space techniques
Christie, Ross R.
Surveying World. - Vol. 1, nr. 1 (November 1992) ; p. 41-43 : fig.
The horizontal and vertical control frameworks have been treated as quite separate entities until fairly recently. The reason is that horizontal frameworks are purely geometrical and relate to a well-defined datum point. Vertical frameworks, on the other hand, generally relate to mean sea level which is not so easy to define, and comprise of direct measurements across geopotential surfaces. As a result, they are affected by the direction of gravity at each instrumental set-up. The advent of the GPS and a greater use of Satellite Laser Ranging (LSR) and Very Long Baseline Interferometry (VLBI) have led to precise geometrical measurements being made in all three dimensions. Computations of centimetric geoid models have enabled the ellipsoid height differences determined through GPS measurements to be converted to orthometric height differences, as observed using conventional spirit levelling techniques. The time has now come to re-examine the traditional separation of horizontal and vertical control and look at ways in which three dimensional frameworks will be used in future.
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