# Package org.apache.sis.referencing.datum

Relationship of a `CoordinateSystem` (an abstract mathematical entity) to the earth or other system. An explanation for this package is provided in the OpenGIS® javadoc. The remaining discussion on this page is specific to the SIS implementation.

The root class in this package is `AbstractDatum`. Various subclasses are defined for various kinds of relationship to the Earth or time (geodetic, vertical, temporal), or to platforms (mobile or not) (engineering, image). Some of those SIS subclasses provide additional methods that are not part of OGC/ISO specifications:

- `DefaultEllipsoid​.getAuthalicRadius()`
- `DefaultEllipsoid​.getEccentricity()`
- `DefaultPrimeMeridian​.getGreenwichLongitude(Unit)`
- `DefaultGeodeticDatum​.getBursaWolfParameters()`
- `DefaultGeodeticDatum​.getPositionVectorTransformation(GeodeticDatum, Extent)`

Datum shifts
Three classes are provided in support of coordinate transformations between different datums:

- `BursaWolfParameters` performs an approximation based on a translation, rotation and scale of geocentric coordinates.
- `TimeDependentBWP` is like `BursaWolfParameters`, but varies with time for taking in account the motion of plate tectonic.
- `DatumShiftGrid` is used for more accurate transformations than what `BursaWolfParameters` allows, by interpolating the geographic or geocentric translations in a grid (e.g. NADCON or NTv2) instead than apply the same transformation for every points.

Since:
  0.4

Defined in the `sis-referencing` module

- Class Summary
  Class | Description
  --- | ---
  AbstractDatum | Specifies the relationship of a Coordinate System to the earth.
  BursaWolfParameters | Parameters for a geographic transformation between two datum having the same prime meridian.
  DatumShiftGrid<C extends Quantity<C>,​T extends Quantity<T>> | Small but non-constant translations to apply on coordinates for datum shifts or other transformation process.
  DefaultEllipsoid | Geometric figure that can be used to describe the approximate shape of the earth.
  DefaultEngineeringDatum | Defines the origin of an engineering coordinate reference system.
  DefaultGeodeticDatum | Defines the location and orientation of an ellipsoid that approximates the shape of the earth.
  DefaultImageDatum | Defines the origin of an image coordinate reference system.
  DefaultParametricDatum | Defines the origin of a parametric coordinate reference system.
  DefaultPrimeMeridian | Defines the origin from which longitude values are determined.
  DefaultTemporalDatum | Defines the origin of a temporal coordinate reference system.
  DefaultVerticalDatum | Identifies a particular reference level surface used as a zero-height surface.
  TimeDependentBWP | Parameters for a time-dependent geographic transformation between two datum.