terminology

# Terminology

This terminology is generally sourced from Appendix A of 'Datums and Map Projections' (Iliffe & Lott 2008) based off the ISO 19111 terms.

Term Comment
Coordinate A sequence of n numbers designating the position of a point in n-dimensional space
Coordinate conversion Change of coordinates from one CRS to another in which the two CRSs are based on the same datum
Coordinate operation Change of coordinates with a 1-to-1 relationship, from one CRS to another. Comprised of coordinate conversion and coordinate transformation processes
Coordinate reference system (CRS) A coordinate system that through a datum, has defined position, scale and orientation with respect to an object (usually the Earth)
Coordinate system Set of rules defining how coordinates are assigned to points
Coordinate transformation Change of coordinates from one CRS to another, where the CRSs datums differ
Datum Defines the position of the origin, the scale and the orientation of the axis of a coordinate system with respect to an object
Easting Distance in a CRS eastwards from a north-south reference line, positive eastwards
Ellipsoid A closed surface formed by the rotation of an ellipse about a main axis. The figure of the Earth is usually modelled as an ellipsoid which is slightly squished from the shape of a sphere at the poles. Shape usually described by terms a and b (semi-major and semi-minor axis lengths) or a and f-inverse (inverse flattening)
Ellipsoidal height (h)Distance of a point from the ellipsoid, measured along the perpendicular from the ellipsoid to the point
Geodetic CRSA CRS where position is specified by geodetic latitude, geodetic longitude and (for 3d case) ellipsoidal height
Geodetic latitude Angle from the equatorial plane to the perpendicular to the ellipsoid through a given point, positive northwards
Geodetic longitude Angle from the prime meridian plane to the meridian plane of a given point, positive eastwards
Geoid Equipotential surface of the Earth's gravity field, which is everywhere perpendicular to the direction of gravity and which best fits mean sea level
Geoid-ellipsoid separation (N)Height of the geoid above the ellipsoid
Gravity-related height (H)Height dependent on the Earth's gravity field, such as orthometric or normal height, which both are approximations of the distance of a point above mean sea level. H = h + N
Height Either ellipsoidal or gravity-related height
Map projection Coordinate conversion from an ellipsoidal coordinate system to a plane
Mean sea level (MSL) Averal level of the surface of the sea over all stages of tide and seasonal variations. Usually defined for a local region from observations of one or more points over a period of time
Meridian Intersection of an ellipsoid by a plane containing the shortest axis of the ellipsoid
Northing Distance in a CRS from an east-west reference line, positive northwards
Prime meridian Meridian from which the longitudes of other meridians are quantified
Projected CRS A CRS derived from a 2D geodetic CRS by applying a map projection
Vertical CRS A 1D CRS based on a vertical datum
Vertical datum A datum describing the relation of gravity-related heights or depths to the Earth, usually related to MSL. Ellipsoidal heights are instead treated as being related to a 3D ellipsoidal CRS 