8. October 1998Notes
- Geographic co-ordinates are not planar coordinates!
- 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!
- 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).
Changing geographic co- ordinates means:
- Map
Projection
Conversion from the curved surface to a plane.
- 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
- geodetic
coordinates
mathematically: perpendicular, based on the reference surface, specifically ellipsoid,
used for
large and medium scale mapping, and in geodesy.
- geographic
coordinates
mathematically: perpendicular, based on the reference surface, typically spheres,
used for small
scale mapping
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