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eTrex Legend basemap accuracy

 
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jingernut
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Joined: 26/06/2003 13:25:36
Posts: 6
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

Can anyone tell me how accurate the basemap is in the eTrex Legend?



I have one myself and when looking at my position on the basemap, it shows my position and an accuracy of 25 feet, but, i'm not displayed on the map where I should be. It's showing me somewhere about 1/4 mile out.



I got an Ordnance Survey map out and put in a location as a waypoint (my work). The waypoint showed on the map, but well out from where it really is. Upon navigating to this point on the map, i end up at work as i should but the map on the Legend is quite wrong.



Sorry for the long post... just trying to explain.



I will be getting R&R maps soon, hopefully these will be a lot more accurate.



Any info is greatly appreciated by a complete newbie here!
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Privateer
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Joined: 30/12/2002 17:36:20
Posts: 4899
Location: Oxfordshire, England, UK

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

Hi Jingernut,



I don't have an eTrex Legend so I can't comment on the accuracy of its basemap. However, have you checked which Map Datum you're using? The eTrex may be using WGS84, whereas the Ordnance Survey maps use "ORD SRVY GB" this might explain why the waypoint is off.



Regards,
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jingernut
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Joined: 26/06/2003 13:25:36
Posts: 6
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

The waypoint isn't off though.



The test I did was to input a waypoint into the Legend. The waypoint being the building where I work.



I then drove to my work and on the Legend I was displayed as being at the waypoint. This is all fine.



It's just the position of my location/the waypoint on the basemap is wrong. Only by 1/4 mile or so.



Some of the basemap seems to be fine and is quite accurate... it's just this little thing is niggling me at the moment.



Am I missing something here?



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Oldie
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Joined: 22/11/2002 13:33:48
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PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

Quote: Originally posted by jingernut on 26June2003

Can anyone tell me how accurate the basemap is in the eTrex Legend?






Recently I asked Garmin a question about the size of the circle that indicates the accuracy of the position shown on the map and I received this follow up message



"My apologies, I got it slightly wrong: The accuracy circle is a combination of the GPS accuracy and map data accuracy - basemap data is 'guideline' data, so the circle is from ~200m (UK) to over 1km (central Africa); where as MetroGuide data is to within a few meters.



Because you're not receiving a good satellite fix (usually you should get 45 ft or less) this accounts for the huge accuracy circle.



Regards,



Neil Stevens



Service Support Engineer

Garmin (Europe) Ltd"



Thus it would seem that the UK basemap accuracy is about 200 metres. If you have the Garmin MapSource program then you should load the map segments appropriate to your area of interest.



Also Robert is right. If you are making comparisons with OS maps then set the Legend Map Datum to Ord Srvy GB.



Richard

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jingernut
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Joined: 26/06/2003 13:25:36
Posts: 6
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

Thanks for the help guys. Nice and quick replys!



The map datum and position formats have been changed appropriately already.



I should be able to try a MapSource Roads and Recreation UK package tonight so I will see how more accurate it becomes when I use this.



Cheers again!



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MikeB
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Joined: 20/08/2002 11:51:57
Posts: 3859
Location: Essex, UK

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

I think you will find that the big difference is down to basemap inaccuracies. When I reviewed the GPSMap76 I was a bit preturbed by the error in position shown on the basemap. The position was correct as proven by the output to TomTom and other applications.



When I installed some more detailed Mapsource data the roads magically became perfect. I suspect that this is due to the ammount of data required for the basemap of Europe as opposed to a detailed small area of the UK. The main roads for me were almost 1/2 a mile out with a differential fix.
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jingernut
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Joined: 26/06/2003 13:25:36
Posts: 6
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: eTrex Legend basemap accuracy Reply with quote

Cheers for all your input.



I have now uploaded some of the MapSource Roads and Recreation data to my Legend and it now shows my position near perfect. Excellent!



Cheers
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