Current location readings different ?

Questions and problems regarding map calibration and conversion.

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Joined: Wed Jul 27, 2005 12:26 pm

Current location readings different ?

Postby tez » Tue Aug 09, 2005 7:12 pm

OK, Im new to GPS and Pathaway.

But my (Garmin etrex) GPS is displaying one location as my current location (when static). Pathaway displays a slightly different location as per the configurable display on the map (I chose "location" which I assume to be my current location).

Should not the location on the GPS and that on the pathaway screen be identical (assuming you select the same lat/long display type on both) ?

They're not, and as I change the map datum on the Garmin that difference changes. So its something to do with my datums.

The map Im using is a scanned one, calibrated with 2 grid references from it. I specified "rectungular, AGD66, UTM in map manager. Pathaway also shows the downloaded calibrated map to be ADG66 datum. The GPS is also set to use AGD66.

What am I missing here ! ?? Obviously something .... the discrepancy is of the order of 400 metres which is a fair bit in the world of GPS I think youll agree !

OK - an update : Im still puzzled, but I noticed if I get Pathaway to display (when connected to the GPS) without any map selected, the current location is given as exaclty the same reported on the GPS itself. Select a map for display, and the Pathaway reported location is different from the one reported by the GPS.

I also notice from the Pathaway logs the GPS always outputs the same location as its displaying (no surprise there). ie Pathaway is "interpreting" the current location.

But tha means I can never use coordinates reported by Pathway as waypoints in my GPS - becasue they differ by a few hundred metres.

So this confirms its something I dont understand about the map calibration. To me, the map was calibrated using AGD66, the GPS is using ADG66. shouldnt pathaway always display precisely the same coordinates ?

Im sure someone can educate me on what assumption Ive made thats incorrect !

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Postby Scott » Wed Aug 10, 2005 4:54 pm

Pathaway will display coords in the Datum of the currently selected Map.
If no map is displayed. Pathaway will use WGS84.
Most Garmins report the Datum they are transmitting and Pathaway will convert as required.
If the datum is not transmitted by the GPS, then WGS84 is used.
If you are certain all is correct, let me know, and I'll do some tests.

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Postby tez » Thu Aug 11, 2005 11:01 am


many thanks for the reply.

Im absolutely positive of at least the following :

The Garmin GPS is set to use AGD66 datum.
The map being diaplayed by Pathaway has been calibrated by specifying lat/long(rectangular), Australian Geod66, UTM, then two known points near the top left and bottom right (the outermost grid refs that I can see coordinates from where the grid lines intersect). The coordinates were then specified as grid 56 and the appropriate northing/eastings from the map.

Ive calibrated and downloaded the map twice, and double checked the garmin setup (adg66).

Im then quite sure that :

The map displays quite accurately, needing only some nudge adjustments to coincide with my location.

Given all the above Im also certain that the location displayed by Pathaway is not the same as that displayed by the GPS. To be sure of this I have enabled logging and looked at the traces - the GPS location log records show the same lat/long as the GPS (not too surprisingly), which is not the same as it displays on the map screen.

While Im not too keen on putting my precise location out on the net, I can give you a real example of the GPS displaying coordinates ADG66 xx yy.717, ww zz.820 and Pathaway, while connected successfully, displaying xx yy.812, ww zz.751. The pathaway log shows the GPS is sending xx yy.717 and ww zz.820 - same as on the GPS display.

I know what its like diagnosing and debugging - Im happy to take a couple of digital shots of the GPs vs Pathaway location displays, or send the log, or the maps or all the above, whatever you need. Given there must be thousands of users who dont have this problem, it cant be anything too hard ! ... can it??? !

I have had the garmin connected configured as both NMEA and GARMIN NMEA, it doesnt seem to make any difference.

Im also quite happy to accept it may be something dumb Ive done-- Im just at a loss to know what ...

Im gathering from your reply that my assumption is correct, ie that Pathaway should always display the same location as it receives from the GPS. Thats what I would have thought, of course providing that the GPS is set to the same datum as the map's calibration datum.

Posts: 12
Joined: Wed Jul 27, 2005 12:26 pm

Postby tez » Fri Aug 12, 2005 4:37 pm


OK now Ive thought some more about the info you gave me, and taken a closer look.

If I now set the GPS to WGS84 then what Pathaway displays is effectively the same as the GPS reports when set to ADG66. I say effectively because they're slightly different but by such a small amount its probably the change in readings while I switch the GPS settings.

So it is doing the conversion you mentioned. So armed with your info, I can get it working but only providing I set the GPS to WGS84.

However if the GPS is set to ADG66, then the coordinates displayed in Pathaway are different. Im not sure what its converting to, as I would expect it to do no conversion at all. But after what you said, my guess is its still converting to ADG66 without realising its already in that format - hence the error.

So, it must either be that the GPS isnt outputting the datum its using, or Pathaway isnt interpreting it correctly. I would have thought NMEA specified the datum as part of its fields ......

Im not sure which, theres nothing obvious to me in the Pathaway trace log that says ADG66 - is that what Id expect to see, or is it just a numeric or something ? Have you per chance tested with a Garmin etrex to know if it outputs the datum ?

Anyway my problem was that I was using ADG66 on the GPS thinking that was the right thing to do, whereas it wasnt and in fact it only works by letting Pathaway do the conversion rather than the GPS.

Does that sound right ?

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