8. October 1998Notes
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Geographic
co-ordinates are not planar
coordinates!
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Geographic co-ordinates are based on a specific reference
system (e.g. sphere or ellipsoid).
-
Geographic co-ordinates define the perpendicular direction on the
surface of the reference system.
-
Geographic co-ordinates are ambiguous!
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Geographic co-ordinates may define different points on the earth's
surface. This depends on their reference system (the geodetic datum
problem).
- One
point on the earth's surface may have many geographic co-
ordinates, depending on their reference system (geodetic
datum).
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Changing geographic co- ordinates means:
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Map Projection
Conversion from the curved surface to a
plane.
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Geodetic Datum Transformation
Changing
geographic co- ordinates between different reference systems or
reference surfaces.
Terminology confusion with geographic
coordinates
3 types of co-ordinates define different
perpendiculars:
-
astronomical coordinates
physically:
perpendicular, based on the gravity
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geodetic coordinates
mathematically:
perpendicular, based on the reference surface, specifically
ellipsoid, used for large and medium scale mapping, and in
geodesy.
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geographic coordinates
mathematically:
perpendicular, based on the reference surface, typically
spheres, used for small scale mapping
