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Problem with GPS and Maps


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Bought my new boat that was fitted with a navman Trackfish 6500. When the guy showed me the GPS functions it all looked OK out in the middle of the water, and he said that he didn't use the maps / GPA very much. Now I've come to use it I've found that there is an error between where the GPS is saying the boat is, and what the map is displaying. I found a marker on the Map and put the curser over it.

The curser said the marker was at 33 deg 38.915min S 151 deg 11.830 min E

The GPS was sayign teh boat was currently at 33deg 37.113min 151deg 12.020m

The distance difference was 1.83nm 173deg to the marker.

Looked through the manual and can't find any fixes. Also tried referencing against Google maps but they don't use the same decimal system for measurement.

Any ideas on what I could do?????

Thanks, Garry

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Yes, I have a downloaded one off the Navman Web site. I'll do some searching on the "datum" used. Can't seem to find anything on calibration or similar. Thanks

is there a reset function, maybe resetting, formatting and then adding software back on may help.

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The datum should be on WGS 84 (World Geocentic System 1984). most probably a large off set in the system that needs correcting

Edited by Whaler 255
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GPS is a great source for problems... your unit may be set to display WGS84 co-ordinates but is your mapping software set to another co-ord system? for reference WGS84 is a dynamic datum.. i.e it is not fixed at any particular time and moves. however, if your mappiing software is fixed it may be say GDA94 datum which is a static (fixed) datum and you will get differences between the two. further, some users set the parameters to UTM which is a projection that mathematically attempts to imitate the earth's surface. without actually seeing what settings your unit has in it, it is difficult to explain how to fix the issue. i use high accuracy DGPS in my work which consists of a base and rover system and the units differentiate between each other to give a very accurate location (within 10mm) but it costs almost $100k, your average sounder combo/plotter is a stand alone single frequency based system that unless you can get DGPS corrections, these may only be accurate to say about 15 metres (radially). Sure, some will say they get better than that and i dont wish to argue about that but getting back to your issue, you need to sourse that the maps (in particular you are utilising) are correct in relation to the co-ords your unit is displaying. after all, the GPS co-ords would be technically correct within themselves as long as a datum shift has not been applied previously within the unit to have this occur. other than a factory reset if you are unclear as to whether this may be the case, i cant really provide any direction you should look at without firstly checking out your gear.



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Hi Raiders, thanks for the replies. Bisso, all I can say is I know know how my wife felt when I tried to explain to her how a Rolling Mill works. I checked the GPS system and it was set to WBS84 thingamy, and I couldn't find an offset as being set to anything. Basically I knew it was time to hit the big button, ie I found the "factory reset" menu option. Pressed, it, waited, and it came back on! Had to make a couple of guesses at some defaults but,,,,,, hooray! I checked a couple of reference points that I got off google maps and it all looks good. Even the way points that I had set are now in the water, and no longer on land! All is good. There is even a stack of extra information on the screen that wasn't there before. Even some photos of landmarks have been stored, just hover over the little camera sigal, and press enter. A-mazing technology.

Next thing to do is to get it down to the ramp and head out into the wide open yonder. The ocean. Can't wait. A few more weeks on the sidelines, then I'm out there......

Thanks again, Garry

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