# Package org.apache.sis.referencing.crs

Coordinate Reference System (CRS) definitions as coordinate systems related to the earth through datum. 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 for this package is `AbstractCRS`. Coordinate Reference System (CRS) can have various number of dimensions, but some restrictions apply depending on the CRS type:

Common Coordinate Reference System types
Dimension | CRS type examples | Remarks
--- | --- | ---
3 | Geographic, Geocentric | ISO 19111 uses the same class, `GeodeticCRS`, for those two cases.
2 | Geographic, Projected | `GeographicCRS` can also be 3D.
1 | Vertical, Temporal. |
Any | Compound | Often used for adding a time axis to the above CRS.

Apache SIS extensions
Some SIS implementations provide additional methods that are not part of OGC/ISO specifications:

- `AbstractCRS​.forConvention(AxesConvention)`
- `DefaultTemporalCRS​.toDate(double)`
- `DefaultTemporalCRS​.toValue(Date)`

In addition Apache SIS provides two distinct classes for geographic and geocentric CRS where OGC/ISO defines a single `GeodeticCRS` type. OGC/ISO distinguishes the geographic/geocentric cases according the type of the coordinate system associated to that CRS:

- A geodetic CRS associated to an ellipsoidal CS is geographic.
- A geodetic CRS associated to a spherical or Cartesian CS is geocentric.

SIS keeps the geographic and geocentric CRS as distinct types since such distinction is in wide use.

Since:
  0.4

Defined in the `sis-referencing` module

- Class Summary
  Class | Description
  --- | ---
  AbstractCRS | Coordinate reference system, defined by a coordinate system and (usually) a datum.
  DefaultCompoundCRS | A CRS describing the position of points through two or more independent coordinate reference systems.
  DefaultDerivedCRS | A coordinate reference system that is defined by its coordinate conversion from another CRS (not by a datum).
  DefaultEngineeringCRS | A 1-, 2- or 3-dimensional contextually local coordinate reference system.
  DefaultGeocentricCRS | A 3-dimensional coordinate reference system with the origin at the approximate centre of mass of the earth.
  DefaultGeographicCRS | A 2- or 3-dimensional coordinate reference system based on an ellipsoidal approximation of the geoid.
  DefaultImageCRS | A 2-dimensional engineering coordinate reference system applied to locations in images.
  DefaultParametricCRS | A 1-dimensional coordinate reference system which uses parameter values or functions.
  DefaultProjectedCRS | A 2-dimensional coordinate reference system used to approximate the shape of the earth on a planar surface.
  DefaultTemporalCRS | A 1-dimensional coordinate reference system used for the recording of time.
  DefaultVerticalCRS | A 1-dimensional coordinate reference system used for recording heights or depths.