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 CRS | A 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 |

terminology.txt · Last modified: 2018/10/09 17:29 by allchin09