#### 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)

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

)

#### 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