GEODESY is the science that is concerned and measuring the shape and size of the Earth, together with the determination of the exact position of particular points on its surface by taking the Earth’s curvature into account. The earth can be modeled as Sphere, Ellipsoid and Geoid.
ELLIPSOID is a mathematical model which approximates the shape of the earth.
GEOID = Equipotential surface of the gravity field, also called “zero-level surface“. It corresponds, approximately to the Mean of Seas Levels, extrapolated below the continents
DATUM = geodesic referential. A datum is composed of an Ellipsoid + A fundamental point/datum point. A datum is specific to an area. It allows the best approximation of a projected area
Coordinate Reference Systems (CRS)
Coordinate systems generally have specific projection, projection settings, and datum. The exception is Geographic coordinates (latitude and longitude), which are not considered projected.
Three commonly used coordinate systems are: Latitude and Longitude, Universal Transverse Mercator (UTM), and State Plane Coordinate System (SPCS).
The projection defines the numeric coordinate system. The datum defines a reference ellipsoid used to compensate for the curvature of the Earth. Knowing which projection and datum apply to your source images or data files is very important. Changes in projection or datum can result in maps not overlaying properly.
-The Old men behind the Horizon-