Help! Memory Map and Topo GB disagree!!

[baileyj1976]

Having used my trusty Etrex with the Memory Map software for a couple of years, I have just shelled out for a 60Csx and Garmin's Mapsource Topo GB. My plan was to use MM as usual for route planning, but get a lot more out of the GPS. However, the first proper outing today has revealed a problem: Memory Map and Topo GB disagree!!

The route plotted in MM looked fine as usual, and I am pretty certain that I have setup the 60Csx correctly. The crazy thing is that the route downloaded from MM to my new Garmin is out of position on the unit's map! I am seeing the same thing using Mapsource on my PC: the route is the right shape, but offset from the correct position. I immediately checked that I had got the right map datum throughout, and noted the lat/longs of some waypoints. The actual lat/longs of the waypoints match exactly for all three: the GPS, MM, and Mapsource - they just don't appear in the same places on the Garmin and MM maps.

I would be extremely grateful for any suggestions as my patience and sanity are beginning to falter!

Cheers,
James

[jolly47roger]

I suggest you find some obvious land marks on Google Earth (e.g crossroads, headland) and, using GE's Lat and Long see which of the maps is correct and work from there.

[jolly47roger]

ps don't forget that on Garmins (mine, anyway) you have to set both the British Grid and British Datum to see OS coordinates correctly

[GeoffC]

[philpugh]

If there is a discrepancy and it's consistent (i.e. a fixed distance and direction offset between the two systems then it is almost certainly a datum issue. OSGB and WGS84 tends to show about 100-150 metres offset in an E-W ish direction

The best way to ensure everything is correct is to use WGS84 throught. This may seem a bit counter intuitive when using OSGB mapping - but it doesn't really matter.

GARMIN units tend to use WGS84 for data exchange in any event - independant of the actual map datum in use on the GPS unit.

I have MM and GARMIN set to WGS84 for route planning / uploading / downloading.

I sometimes switch to OSGB grid ref when walking as it makes it easier to look up positions on the map. Luckily on the newer GARMIN units you only have to change the one setting as picking OSGB grid ref automatically sets the correct datum. Of course this doesn't affect the 'location' of routes/waypoints - the unit does the neccessary conversions.
_________________
Phil

[baileyj1976]

Thanks for all the suggestions...I have gone in more circles than a washing machine trying to sort this out. So....with MM, Mapsource, and the 60Csx all set to UTM projection and the WGS84 datum, I only need to apply a minus 25m easterly shift in MM in order to export a route that agrees relatively well on the handheld. I am still not sure why, though. And as I get most of my routes from CountryWalking as .mmo files this could still prove an annoyance. Oh well, nearly there now...

Thanks,
James.

[philpugh]

[baileyj1976]

Okay....I will pretend that no-one got the chance to read the daft paragraph that I have hastily erased.

As suggested, I exported off all my routes and restarted MM afresh. Importing the offending route that has caused all this grief proved enlightening...as it now appears offset in MM too!! I have now manually shifted all the waypoints to where they should be. Given the resolution of my 1:50k map, the exported GPX file now appears good in both the computer's Mapsource and the 60Csx hurrah!!

Looks like the mis-matched datum issue was within MM itself (weird). Phew. Thanks again for all the advice. As a matter of course I will from now on verify my MM routes in Mapsource before downloading to the GPS and setting off!

James.

[philpugh]

Glad you got it working

the fact that you had to correct the waypoints after the export suggests that the datum was wrong somehow. It may be possible to look at the info to see what it was set at - but don't worry too much.

Sudden thought! You mentioned Country Walking as a source of the routes etc. Is this (the one that was causing a problem) one from there? You may want to check what datum they use. Seeing as they use MM to create the downloads (I'm not a member so I can't actually download one) you would expect it to be OK! But perhaps they create and save them in OSGB datum.

Does the 60Csx understand .GPX files? The C300 does and it simplifies things a great deal. I just export routes and waypointns from MM to .GPX and copy them onto the C300 (GARMIN\GPX) and that's it. Everything is ready to go.
_________________
Phil

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