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Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Wed Nov 08, 2006 12:43 pm Post subject: Bearing, Heading and Track
I believe that the definitions that are given at http://www.pocketgpsworld.com/terminology.php are slightly erroneous. Bearing is the angular distance of one point from another, this is usually referenced to north (either magnetic or True(geographic)) and followed by the word 'To' or 'From'. In the abscence of the direction word, 'To' is assumed.
Track and heading are NOT the same as implied in the description.
Heading is the direction in which the vehicle is pointing, again usually referenced to north.
Track is the direction across the earths surface the vehicle is actually travelling, once again usually referenced to north.
"What's the difference between Track and Heading?" I hear you ask, as GPS World say they are the same.
For land based vehicles, normally none, unless it is temporarily out of control and in a spin or drift.
For aircraft and boats, quite a lot. A simple example will demonstrate the difference. An aicraft is flying at 500 kts with its compass reading 0 Deg (its Heading), there is side wind from the west acting on the aircraft. The aircraft will not travel acros the earth in a due northerly direction as indicated by the compass, but at an angle off to the east of north say 020 deg. This is the Track of the aircraft (some 20 degrees off its Heading)
My guess would be that the track and heading are equivalent from a GPS point of view because the GPS receiver has no knowledge of the attitude of the vehicle, only the direction it is moving in. Hence the heading from a GPS receiver is to be taken with a pinch of salt for very low speeds.
As I understand it the heading (and speed) output in the NMEA string is calculated internally by the GPS receiver comparing current position to last known position.
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