GPS direct track distances

[huntsman]

if i were on the ground in perth, australia and selected a direct track to sydney, would the track distance be that of the above ground distance or the (shorter) direct (thru the ground) distance?

[Squealing Pig]

Tried it on a Garmin GPS III Pilot from N000 00.000 W090 00.000 to N000 00.000 E090 00.000. The GPS gives 10819 NM. This sounds about right as it would be 10800 NM if you assumed the world is a perfect sphere (It is not, it is slightly longer around the equator than from pole to pole) and the world is 21600 NM in circumference. (based on 360 degrees at 60 NM per degree) Straight through the earth would be 6875 NM.
So in answer to your question Huntsman it measures point to point distance at the surface as far as the GPS III is concerned.

Anybody else like to try the same experiment with another type of GPS ?


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Push forward - Pigs get bigger
Pull back - Pigs get smaller
Pull back some more - Pigs get bigger again ??

[scroggs]

I think you'll find that all GPSs sold use a model of the globe called 'OGS84' to calculate distances and tracks. This global model is the same one used in most aviation navigation equipment. Assuming your inputs don't state an elevation above or below the surface, the calculated track will be a great circle at mean sea level.

[EchoTango]

Did same test with Magellan. Same conclusion.

[BIK_116.80]

WGS84 perhaps, scroggs?

[scroggs]

BIK, yes, that's the one. W and O are a bit close together on my keyboard!! (Or: didn't look at the book before bursting into print!).

[BIK_116.80]

I do believe it stands for World Geodetic System, or something like that. The 84 bit refers to the year 1984, which is when the current version of the WGS was ratified.