Geodesy

Coordinate Reference System (CRS)

A coordinate reference system  is a coor-dinate system which is referenced to the earth. CRS is one part of SRS. 

Spatial Reference System (SRS)

A reference system that defines coordinate reference system, datum using ellipsoid and either a geocentric, geographic or projection coordinate system.

Geographic Coordinate System (GCS)

A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. A GCS is often incorrectly called a datum, but a datum is only one part of a GCS. A GCS includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid).

Geoid

Geoid is gravitational surface of the earth which approximetes mean sea level.

Ellipsoid

Ellipsoid is locally best fitting elliptic 3D approximation of the earth's surface.

In geodesy only 2 axis (in equatorial radii?) are considered:

• semi-major axis (a)
• semi-minor axis (b)

In geodesy though ellipsoid is quite often described with these two paramters:

• semi-major axis (a)
• inverse flattening 1/f where f is a ratio of:
  f = (semi-major - semi-minor)/semi-major axis )

the third axis (c or Z) is ignored.

Spheroid

Spheroid  is a type of ellipsoid where semi-minor axis = sami-major axis (a =b).
Oblate spheroid is a spheorid where c < a, the Earth is an oblate spheroid.

Geodetic Datums

A geodetic datum defines the position and orientation of the reference ellipsoid relative to the centre of the earth, and the meridian used as zero longitude – the prime meridian. 

In short, datum is the shift of a spheroid, it is a point where spheroid touches the surface of the earth. Each spheroid has several datums.


Geodetic datum transformations include:

• shifts: dX, dY, dZ
• scaling
• rotations: X, Y, Z

Spatial Reference System usually has:

• SRS name
• unit type
• mapping projection parameters:
• projection name
• latitude of origin
• central meridian
• scale factor
• false easting
• false northing
• geographic CRS parameters:
• CRS name
• datum name
• Pprime meridian related info:
• name
• longitude
• spheroid related info:
• name
• semi-major axis
• semi-minor axis
• inverse flattening