{@linkplain org.opengis.referencing.datum.Datum Geodetic datum} (the relationship of a {@linkplain org.opengis.referencing.cs.CoordinateSystem coordinate system} to the earth). The following is adapted from OpenGIS® Spatial Referencing by Coordinates (Topic 2) specification.

A datum specifies the relationship of a coordinate system to the earth or, in some applications to an Engineering CRS, to a moving platform, thus creating a coordinate reference system. A datum can be used as the basis for one-, two- or three-dimensional systems.

Five subtypes of datum are specified: geodetic, vertical, engineering, image and temporal. Each datum subtype can be associated only with specific types of coordinate reference systems. A geodetic datum is used with three-dimensional or horizontal (two-dimensional) coordinate reference systems, and requires an ellipsoid definition and a prime meridian definition. It is used to describe large portions of the earth's surface up to the entire earth's surface. A vertical datum can only be associated with a vertical coordinate reference system. Image datum and engineering datum are both used in a local context only: to describe the origin of an image and the origin of an engineering (or local) coordinate reference system.

### Vertical datum

Further sub-typing is required to describe vertical datums adequately. The following types of vertical datum are distinguished:

- Geoidal
  The zero value of the associated (vertical) coordinate system axis is defined to approximate a constant potential surface, usually the geoid. Such a reference surface is usually determined by a national or scientific authority and is then a wellknown, named datum. This is the default vertical datum type, because it is the most common one encountered.

- Depth
  The zero point of the vertical axis is defined by a surface that has meaning for the purpose the associated vertical measurements are used for. For hydrographic charts, this is often a predicted nominal sea surface (i.e., without waves or other wind and current effects) that occurs at low tide. Examples are Lowest Astronomical Tide and Lowest Low Water Spring. A different example is a sloping and undulating River Datum defined as the nominal river water surface occurring at a quantified river discharge.

- Barometric
  A vertical datum is of type "barometric" if atmospheric pressure is the basis for the definition of the origin. Atmospheric pressure may be used as the intermediary to determine height (barometric height determination) or it may be used directly as the vertical ordinate, against which other parameters are measured. The latter case is applied routinely in meteorology.

  Barometric height determination is routinely used in aircraft. The altimeter (barometer) on board is set to the altitude of the airfield at the time of take-off, which corrects simultaneously for instantaneous air pressure and altitude of the airfield. The measured height value is commonly named "altitude".

  In some land surveying applications height differences between points are measured with barometers. To obtain absolute heights the measured height differences are added to the known heights of control points. In that case the vertical datum type is not barometric, but is the same as that of the vertical control network used to obtain the heights of the new points and its vertical datum type.

  The accuracy of this technique is limited, as it is affected strongly by the spatial and temporal variability of atmospheric pressure. This accuracy limitation impacts the precision of the associated vertical datum definition. The datum is usually the surface of constant atmospheric pressure approximately equating to mean sea level (MSL). The origin or anchor point is usually a point of known MSL height. The instruments are calibrated at this point by correcting for the instantaneous atmospheric pressure at sea level and the height of the point above MSL.

  In meteorology, atmospheric pressure routinely takes the role as vertical ordinate in a CRS that is used as a spatial reference frame for meteorological parameters in the upper atmosphere. The origin of the datum is in that case the (hypothetical) zero atmospheric pressure and the positive vertical axis points down (to increasing pressure).

- Other surface
  In some cases, e.g. oil exploration and production, geological features, i.e., the top or bottom of a geologically identifiable and meaningful subsurface layer, are sometimes used as a vertical datum. Other variations to the above three vertical datum types may exist and are all bracketed in this category.

### Image datum

The image pixel grid is defined as the set of lines of constant integer ordinate values. The term "image grid" is often used in other standards to describe the concept of Image CRS. However, care must be taken to correctly interpret this term in the context in which it is used. The term "grid cell" is often used as a substitute for the term "pixel".

The grid lines of the image may be associated in two ways with the data attributes of the pixel or grid cell (ISO CD 19123). The data attributes of the image usually represent an average or integrated value that is associated with the entire pixel.

An image grid can be associated with this data in such a way that the grid lines run through the centres of the pixels. The cell centres will thus have integer coordinate values. In that case the attribute "pixel in cell" will have the value "cell centre".

Alternatively the image grid may be defined such that the grid lines associate with the cell or pixel corners rather than the cell centres. The cell centres will thus have noninteger coordinate values, the fractional parts always being 0.5. ISO CD 19123 calls the grid points in this latter case "posts" and associated image data: "matrix data". The attribute "pixel in cell" will now have the value "cell corner".

This difference in perspective has no effect on the image interpretation, but is important for coordinate transformations involving this defined image.

### Prime meridian

A prime meridian defines the origin from which longitude values are specified. Most geodetic datums use Greenwich as their prime meridian. A prime meridian description is not needed for any datum type other than geodetic, or if the datum type is geodetic and the prime meridian is Greenwich. The prime meridian description is mandatory if the datum type is geodetic and its prime meridian is not Greenwich.

### Ellipsoid

An ellipsoid is defined that approximates the surface of the geoid. Because of the area for which the approximation is valid - traditionally regionally, but with the advent of satellite positioning often globally - the ellipsoid is typically associated with Geographic and Projected CRSs. An ellipsoid specification shall not be provided if the datum type not geodetic.

One ellipsoid must be specified with every geodetic datum, even if the ellipsoid is not used computationally. The latter may be the case when a Geocentric CRS is used, e.g., in the calculation of satellite orbit and ground positions from satellite observations. Although use of a Geocentric CRS apparently obviates the need of an ellipsoid, the ellipsoid usually played a role in the determination of the associated geodetic datum. Furthermore one or more Geographic CRSs may be based on the same geodetic datum, which requires the correct ellipsoid the associated with any given geodetic datum.

An ellipsoid is defined either by its semi-major axis and inverse flattening, or by its semimajor axis and semi-minor axis. For some applications, for example small-scale mapping in atlases, a spherical approximation of the geoid's surface is used, requiring only the radius of the sphere to be specified.