| | | 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.
GPSMan User Manual
GPSMan User Manual and the GPSMan logo images by Miguel Filgueiras are licensed under a Creative
Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
| | | Datums and ellipsoids |