Question about Magellan GPS
Altitude can be referenced to any of:
1. The WGS84 ellipsoid (a mathematically defined surface which forms the basis for GPS position calculations)
2. A selected datum's ellipsoid (another mathematically defined surface - shifted with respect to the WGS84 ellipsoid, but usually defined to closely match the local surface of the Geoid for a geographic region).
3. A Geoid (a surface of equal gravitational potential - caused by density variations within the earths crust)
4. Mean Sea Level (slightly different from the Geoid surface, because of permanent ocean currents, winds, temperature and salinity differences)
Coordinate data transmitted using the Garmin protocol is transmitted in WGS84. If there is no height information, then the height is assumed to be zero on the WGS84 ellipsoid. If there is height information from the GPS, then this is assumed to be referenced to the ellipsoid of the datum which is currently set in GPSU. If you now display this data using some other datum, the altitude is referenced to that datum and you will see a value which represents the difference in height between the two datums (or ellipsoid surfaces).
Coordinate data tranmitted using the NMEA interface, is referenced to a user selected datum. GPS Utility ASSUMES that you have set the same datum as the GPS unit and displays the altitude as received over the interface without change.
GPS Utility displays the altitude as an 'ellipsoid' height. It does not have a 'geoid' height model which would account for local variations of the gravitational field. Mean Sea Level may differ from the the ellipsoid surface by a number of metres. Over a local area, this offset will be fairly constant.
As GPSU does not have a geoid model, you should set GPSU to the local datum before you download data from a GPS receiver. The geoid height and local datum height are normally very similar. If you need to re-reference or adjust the height information then you can use the Calibrate Altitude tool in GPSU.
Check download page in this link: gpsu.co.uk/download.html
Keep in touch.
Posted on Mar 22, 2011
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Posted on Jan 02, 2017
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