Do geographic and satellite coordinates differ?

anthonyk · Occasional Visitor Joined: Dec 09, 2004 Posts: 4

I am a real newbie -- haven't even seen a GPS device working yet -- but have been experimenting with GPS track maker and some maps available online before trying to decide what kind of system I will buy.

My first problem is that when using the map adjust tool to recalibrate Track Maker, I discovered that the geographical coordinates for Dominica are very different from the GPS trackpoints that others have posted on the web. If I calibrate the standard gif map available on the web using the coordinates which are 15.38 N and 61.28 W for the northernmost tip, when I merge GPS tracking data for waypoints etc the northern tip is registered as 15.638 N.

This makes the geographic map sit in the Atlantic Ocean well to the East of the outline afforded by GPS data.

Sorry for my ignorance, but why should this be? I have checked the bearings in a number of atlases, all of which give the same geographic data. But the satellite images of Dominica from the web seem to agree with the GPS data.

I would welcome any enlightenment!

Thanks

Tony K

lbendlin · Posted: Thu Dec 09, 2004 2:54 pm Post subject:

Oh, there are tons of different datums (?) around, with different geoid models, from differnt organisations etc. The GPS receivers nowadays use mostly the WGS84 datum.. Your paper maps may (make that will) have a different datum.

http://www.maptools.com/UsingUTM/mapdatum.html
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Diplo · Posted: Thu Dec 09, 2004 3:32 pm Post subject:

StreetMap have a handy concrsion tool you can use for the different formats :
http://www.streetmap.co.uk/streetmap.dll?GridConvert

Diplo · Posted: Thu Dec 09, 2004 3:36 pm Post subject:

Ooops, you need to post to the URL... Embarassed

Try http://www.streetmap.co.uk/streetmap.dll?GridConvert?name=1,1&type=OSGrid

Then just change values once in there.

anthonyk · Occasional Visitor Joined: Dec 09, 2004 Posts: 4

Thanks, Ibendlin. I was wondering what was happening. And thanks for the hyperlink too, which I looked at. Does this mean to say that when I look at printed world atlases they might all be based on different datum sets?

I am used to the world of using map and compass and factoring in movement of the magnetic north. Were maps based on that system also using different methods? I can understand the differences of 200 yards quoted in the web site you referred to, but the differences between my 1985 geographic printed maps and the GPS version amount to approximately 20 miles!

Ah well, at least I now know how to go about this.

Thanks again

Tony

anthonyk · Occasional Visitor Joined: Dec 09, 2004 Posts: 4

And thanks Diplo! This is another really good bit of info. I'll try the conversions.

Thank goodness for this group!

Tony

anthonyk · Occasional Visitor Joined: Dec 09, 2004 Posts: 4

Actually, for whoever reads this thread and has been making the same mistakes, I ought to point out that I am an even bigger fool than I first thought. I was looking at datum conversion tools, and when I used the one at

http://kgsweb.uky.edu/CoordConversionTool.asp

I realised that I, oh idiot, had not done the basic job of converting degrees minutes and seconds to decimal format, and was instead punching in 15.30 for 15 degrees and 30 seconds.

I hope other idiots like myself will avoid this!

Publicly shamed

Tony

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