Distance & Bearing GPS vs Calculated

[Guest]

I am just writing a little program to help me enter waypoints into my Garmin GPS and create routes from it. (I know you can buy software to do it, but I like a challenge).

To test it, I am comparing the Bearing & Distance from the same waypoints in my GPS to that produced by the program. The bearing is fine in all cases, but the distance is sometimes out a little bit.

It appears to me that the GPS reads a bit higher sometimes (or my program reads a bit lower depending on how you look at it).

If the course is mainly North-South of vice versa then the distance is spot on, if the course is vaguely east or west, then it can be some way out. For Ramsgate to Nab Tower it is underreading by about 3/4mile. For Ramsgate to Singapore it underreads by about 35 miles. It's just strange that its bang on for North & South.

The formula I use goes something like this:


d = Acos(sIn(lat1) * sIn(lat2) + Cos(lat1) * Cos(lat2) * Cos(lon1 - lon2))
dDegrees = d * 180 / PI
Distance = Round(dDegrees * 60, 1)

Which apparently gives the Great Circle distance, and I suppose this is where the error comes in, as if you are going pretty much North-South then you are following a Great Circle whereas if you are going any other way, then you aren't.

Does the GPS give a Great Circle distance?

I don't know, but I'd welcome any input.


Mark

 

[HaraldS]

Think your guess is right. Your formula definitely gives Great Circle Distance.
No idea what your GPS does. My large all in one Simrad allows me to pick a choice when keying in a new route. I usually run it on rumbline, unless I'm going for a long distance, since on short ones I keep just one heading.

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[Jacket]

You can have a great circle route between any two points, and it will be the shortest distance between those two points. If you're going between two points on the same latitude, the shortest route will take you on an arc to the north of that latitude- a shorter distance, but you'll have to keep changing your compass course to follow the arc.

At a guess I'd say your GPS would just set a course that trundles along at the same latitude between the two waypoints. This will give a slightly longer distance, but has the advantage that you can steer a constant compass course between the two waypoints.

So possibly, both your programme and the GPS are correct.

 

[ccscott49]

The formula your using, is to solve rect to polar coords, I think. The gps may be using a more complicated algorythim to work out the distances taken from the earths shape. Not sure, but this may be the reason. I do know the satellites take into consideration the earths flattened sphere (pole to pole.)