GPS Manager User Manual -- Datums and ellipsoids

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## Datums and ellipsoids

A horizontal (or geodetic) datum defines the form and the position relative to the Earth axis of the geometric reference surface of the Earth used for locating points and in projections. The form is an ellipsoid which is usually defined by giving its semi-major axis `a`, and its flattening `f` (or its inverse), i.e., the quotient of the difference between its semi-major and semi-minor axes by its semi-major axis. Its relative position is described by the shift in Cartesian coordinates (`dx`, `dy`, `dz`) with respect to a reference datum, usually the "WGS 84".

GPSMan contains comprehensive sets of datums and ellipsoids. Their definitions have a remark field used for documenting them whenever possible, as well as fields for the error estimate in metres (`ex`, `ey`, `ez`; the value `-1` stands for unknown), the number of satellite measurement stations, and the zone of validity given by S-N latitudes and W-E longitudes. All these fields are for information only and may be empty. The definitions can be inspected (but not changed) from the corresponding entries under the `Definitions` menu-button. It should be noted that some datums have variants for different regions. For instance, the "European Datum 1950" has at least 15 such variants and it has been observed that Garmin receivers do use the local variant for Portugal/Spain when this datum is selected and a waypoint in Portugal is created. This means that using the average "European Datum 1950" in such a situation may lead to large position errors. Probably the same will happen with other datums having variants.

Users may define their own datums and ellipsoids from the entries under the `Definitions` menu-button. These definitions, that cannot override those in GPSMan, are automatically saved in a file in the GPSMan user directory, and will be loaded when GPSMan is started. Currently GPSMan does not prevent changing or forgetting a datum or ellipsoid that is in use: it is the user's responsability to avoid inconsistencies due to such operations. When sharing files having data depending on user-defined datums with other users, the definitions of the relevant datums and ellipsoids should also be shared.

|  |  | Datums and ellipsoids