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1.
oodn ys ms ndG od sy
1.1 The Origin – Reference Surfaces and Elevations in Mapping
T here are three basic surfaces that are pertinent to good mapping, shown here, these are:
Terrain : The topographic surface of the ground or seabed
Geoid : An equipotential surface that is irregular and approximates to Mean Sea Level (MSL)
Ellipsoid : A regular model surface that approximates the geoid, created by rotating an ellipse about the polar axis. Used to simplify the computational complexity of the Geoid
Figure 1: The three reference surfaces in geodesy.
Terrain: The terrain is the surface we walk on or the seabed. This surface is irregular. It is the surface we have to set upou su ym su ngd c s,such s ‘o ls on’o G c . h n u of h su f c wlldc h d c onofg y po n . n moun nous n, h c lwlld fl c n ow ds h m n‘centreofm ss’of the mountains. The Geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level (MSL). An equipotential surface is a one where gravity is an equal force everywhere, acting normal to the surface.The geoid is an irregular surface that is too complex for calculation of coordinates. The Ellipsoid: The ll pso d s ‘mod l’ of h Earth that permits relatively simple calculations of survey observations into coordinates. The ellipsoid provides h m h m c l b s s of g od sy. o h h G o d s n ‘ c u l phys c l surfaceliketheterrain,but h llpsod s ‘ h o c l’su f c h sd sgn d o‘m ch’ h g od sclos ly s possbl n h ofop ons. o lso h h nom l o h g od(whch s‘ c l’) sno h s m s h normal to the ellipsoid.
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