The geoid-ellipsoid-map transfer process is illustrated by Figure 1
(for details, see Iliffe, 2000).
In spite of the existence of a standard international ellipsoid, many mapping agencies
prefer to use a national or continental ellipsoid that better fits the surface of their
country. They also use different map projections that minimize the distortions for the
geometry of their country. Furthermore, different projects or organizations within
a same country often use different map projections over a same zone depending of
the total area to be covered by their maps. Selecting the most appropriate ellipsoid
and map projection allows them to minimize (for the entire zone covered by a map
series) the distortion between map measurements and the measurements made on
the Earth (i.e., with regard to the geoid).
Furthermore, there are different spatial referencing systems to determine the position
of objects on maps. One may use a latitude-longitude-height international ellipsoidal
coordinate system, an x-y coordinate system based on a map projection,
an x-y-z coordinate system from a 3D digital terrain model, a street address or
street-intersection, a place name, a distance-direction to a landmark, a route-direction-
distance-offset linear referencing systems, and so on.
Figure 1. From one object measured on the Earth to different map representations
Spatial Online Analytical Processing (SOLAP) 0
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