Geographic Coordinate Systems

A Geographic Coordinate System describes the way that coordinates on the earth's surface (which are generally described in degrees of longitude, degrees of latitude, and elevation above the surface) are transformed to a cartesian coordinate system that can be represented on an inherently planar medium such as a sheet of paper or a computer screen. More...

## Classes | |

class | BaseGCS |

Geographic Coordinate System class. More... | |

class | Group |

Geographic coordinate systems Group class. More... | |

class | GroupEnumerator |

Group Enumerator class. More... | |

class | Ellipsoid |

Definition of the globe as elliptical distortion of a sphere. More... | |

class | EllipsoidEnumerator |

Ellipsoid enumeration class. More... | |

class | Datum |

Position and orientation relative to a WGS84 Datum. More... | |

class | DatumEnumerator |

Datum enumeration class. More... | |

A Geographic Coordinate System describes the way that coordinates on the earth's surface (which are generally described in degrees of longitude, degrees of latitude, and elevation above the surface) are transformed to a cartesian coordinate system that can be represented on an inherently planar medium such as a sheet of paper or a computer screen.

In general, a Geographic Coordinate System (referred to below as a GeoCoordinate System or GCS) is fully described by a projection type, the mathematical parameters that customize thae projection, and a datum or ellipsoid.

The projection type describes the mathematical formula for taking a position in longitude, latitude, and elevation on (or near) the earth's surface and converting it x, y, and z cartesian coordinates.

The datum describes the assumptions in force when a particular latitude, longitude measurement was made, including the shape of the earth and the reference points to which the measurements was relative. There are a number of well-known datum, almost all of which were originally published by one governmental body or another.

In some cases, measurements are relative to an ellipsoid rather than a datum. In that case, the latitude and longitude are assumed to be mathematically correct on the ellipsoid's surface. Like datums, there are a number of well-known ellipsoid definitions.

There are many different Geographic Coordinate Systems in use, and it is always possible to create new variations, either by inventing a new projection type, varying projection parameters for existing projection types, or by varying the datum. Often the best choice is to use a GCS that has been established as a standard for the relevant geographic area, usually by a governmental body. For example, one or more "State Plane" geographic coordinate systems have been defined by state governments in the United States. An extensive Coordinate System Library is supplied with MicroStation and related products, and Geographic Coordinate Systems from that library can be used by looking them up by name - see the appropriate GCS constructors.

A full discussion of Geographic Coordinate Systems can be found in "Elements of Cartography" by Arthur H. Robinson, et al. ISBN-10: 0471728055, and in many other books on the subject.

The GeoCoordinate API makes use of the "CS_MAP" library published by Mentor Software, Inc. for all the projection calculations and datum conversion algorithms. Mentor also supplies the Coordinate System Library.

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