GPS in 3D or 2D

[starboard]

I use my Furuno gps31 fixed installation on the Yacht in 3D mode (3dimensions,i.e. Lattitude Longtitude and altitude) However having recently purchased a handheld set for emergency use I note in the instructions it states that for marine use greater accuracy can be obtained by using in 2D mode (2 dimensions, Lat and Long) this requires the height of ariel to be input to reciever manually. I realise even in 3D the devise is accurate to within 5 metres so the extra accuraccy is probably irrelavant but it was of interest to understand the system is more accurate in 2D. Is this a well known fact that I have missed in the mist of time???

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[Ric]

It is a fairly simple matter of geometry.

If you imagine your boat on a flat pre Marco-Polo sea, and imagine that it can pick up four satellites (which is typically true) the position of your boat will be most accurately given if the four satellites are 90deg apart in azimuth, and 45 deg above the horizon. This is typically the positions your gps can pick up the four strongest signal satellites in reality, which is why your "2D" position is quite accurate.

Now imagine your boat can float above the Marco Polo sea. The disposition of four satellites that gives the most accurate fix is to have one directly above your boat, and the other three on your horizontal plane spaced at 120deg in azimuth. This disposition happens a lot more rarely, then you have to factor in refraction of the signal etc.

This is a very simplistic way to explain why altitude is more difficult to measure with GPS than position on the surface of the earth.

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[TFJ]

I don't know about the mist of time, its always raining when I go to Troon!
Hope your GPS is working fine when we sail back from Sweden, I don't want to hit any oil rigs

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[tome]

Stick to 3D

2D was useful in the early days when there were few satellites about as it could still give a fix with 3 sats provided you entered the height accurately. With 4 or more sats it can compute a 3D position. Nowadays its rare to have less than 6 in view. Incidentally, the height has to be the height above the spheroid, which if you're using WGS84 is rarely close to sea level. If you enter an erroneous height, it will affect the position in 2D mode.

GPS is weak in the height axis - the error will be more than the horizontal error.

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[Sybarite]

I sometimes get alarmed when I find that I am -10m, sort of out of breath ..

John

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[starboard]

Thanks TFJ, Take it that is some form of hand held gps you have on the foredeck, not a pocket version I guess...looks like its wind driven, that should be ok for the Sgagarak! no shortage there at the moment!!

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[TFJ]

Starboard

It climbed aboard while I was sailing down Loch Ness, it took a bit of wrestling but I finally got it under control, I tried to do all sorts of things with it but in the end cooked it on gas mark 4 for 8 hours, it tasted a bit salty but kept us going until we got to Troon

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[starboard]

Do you have a passport for it to take to Sweden....bet Brigitta would like to get her mouth around it!!!!!!!!

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