Relating GPS co-ordinates to a map

Submitted: Friday, Sep 06, 2002 at 00:00
ThreadID: 1929 Views:1822 Replies:4 FollowUps:1
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At the risk of sounding dumb, how do you relate GPS co-ordinates to a map. I've tried this on a number of occasions to establish my position and have not been very confident with the result, particularly when its a large scale map.
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Reply By: Alex - Friday, Sep 06, 2002 at 00:00

Friday, Sep 06, 2002 at 00:00
G'day Kim, the only dumb questions are the ones you don't ask for fear of looking dumb. First off, the map needs lattitude/longitude gridlines or eastings and northings on it, and the GPS needs to be reading in the same units as the map. After that its a matter of following the gridlines for the GPS position to their intersection on the map to establish your position (although this takes a bit of practice and will still only be approximate). There probably will only be gridlines every so often on the map, so you need to estimate the intermediate point where your GPS tells you you are. There are 60 seconds of lat/long to a minute of lat/long, and 60 minutes to a degree. A minute of lattitide is one nautical mile, or approximately 2km.
Another potential problem is that the map and the GPS are using different datum points. There have been several used in Australia over the years, and on my last Canning Stock Route trip I found that the map was using a different datum to the GPS, and I was up to several km off on survey marker posts I had definite locations for.
Once you get these issues sorted, there should be no problems. Alternatively, a 4wd training commpany or accessories store such a s ARB should be able to point you in the right direction on how to learn this sort of thing. Hope this helps, cheers, Alex.
AnswerID: 6434

Follow Up By: Kim - Monday, Sep 09, 2002 at 00:00

Monday, Sep 09, 2002 at 00:00
Thanks for that Alex.
I guess what I was leading up to, is that I find it difficult (even with a finely graduated ruler) to measure out the minutes,degree and seconds on both the Latitude and longitude sectors,to arrive at an accurate intersecting point.
For example, when I was in the Simpson last year I was parked at a well know feature and decided to test the accuracy of my calculation on a map. I was not able to come close to an accurate fix on the map. Meaning that if I was out in the scrub and wanted to know where I was, I could be kilometres away from where I thught I was.
Hope that makes some sense.
FollowupID: 2896

Reply By: Darian - Friday, Sep 06, 2002 at 00:00

Friday, Sep 06, 2002 at 00:00
If your map has grid refs on it, switch your GPS mode to UTM's which is the same speak. If you know the datum that the map is using, you can select that datum in your GPS library too. That is about my limit ...should learn more about my Lowrance GPS. I think if you bought maps from a pro map shop, they could tell you all you want to know on the GPS tie-ins and that's why we shop at pro shops - to get good advice (while possibly paying a bit more).....dp
AnswerID: 6440

Reply By: Bob - Friday, Sep 06, 2002 at 00:00

Friday, Sep 06, 2002 at 00:00
Kim, OziExplorer make mapping software that links the readout from a GPS to a position on a map on your computer. There are free maps and a trial version of their software at Playing around with this sort of thing will quickly give you experience in map reading and co-ordinate systems (without having to travel too far:-)They are an advertiser on ExplorOz. I use OziExplorer on my laptop connected to a Magellan 315. It makes plane trips more interesting (if you get a window seat that isn't under a wing - the wing screens the satellites :-( On a recent trip to Alice Springs I was able to identify features such as the French Line and the Colson Track from some 30,000 feet.
AnswerID: 6444

Reply By: Darryn- Tuesday, Sep 10, 2002 at 00:00

Tuesday, Sep 10, 2002 at 00:00
Kim, Darian is on the right track with changing the co-ordinate system to UTM but you should also check the map datum is also set to Aust 84 at least. This will then give you grid references which you can locate on Natmap/Auslig maps. Don't try to use maps over 1:100,000 scale as there won't be enough detail/accuracy to do any good. Degrees, minutes and seconds of Lat/Long are only good for aircraft or ships at sea as a Natmap 1:100,000 map only covers 1/2 a degree of Lat/Long which is 2,464 square kilometers. Reg's Darryn
AnswerID: 6523

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