Map Accuracy and WGS84 Datum Shift

TigerMoff · Occasional Visitor Joined: Oct 22, 2003 Posts: 20

I think you should speak to OS on this one as I believe it is a problem with the 1:50,000 mapping.

jeepers · Occasional Visitor Joined: Aug 25, 2004 Posts: 1

Hi
I've discovered similiar offsets as discovered in this thread in the South West of England. Looking into it I've discovered that it seems to be the actual OS mapping that is to blame - not the software - although am waiting for memory map's development team to get back to me.
I have found that tracks recorded with my gps receiver in wgs84 output track perfectly on 1:25000 maps (aerial and explorer), but are offset on the 1:50000 maps.
To see what's going on I scanned in paper maps of 1:50k and 1:25K of the same region and overlaid one on the other in a graphics prog.
Here's the result.
This accounts for the offset I'm seeing - very noticable in some areas. Very suprising that the 1:50k maps are out. I would have thought all the maps used similiar if not the same base information.
Cheers,
jeepers

DevilsAdvocate · Posted: Fri Nov 05, 2004 2:42 pm Post subject:

I was out walking last week using a 50K map and it was spot on - absolutely.
However when using a 25K (different area) the map is showing position about 50ft further north may be 10-20ft east.
I am up in the Lakes in a cuple of weeks and I intend to test both out, 50K and 25K for the same area.
Regards
_________________
TomTom 720 GO
IPAQ 2210
TTN3
Memory Map
Fortuna Clip-ON
Lexar 256MB CF Card
Arkon Mount

AlunS · Posted: Fri Nov 05, 2004 6:23 pm Post subject:

Just noticed this thread ...

There was a long discussion on something similar to this on Usenet (sci.geo.satellite-nav) a while ago.

It had to do with the rather simplified way most consumer grade GPS receivers do the transformation between WGS84 and OSGB36. They use a technique called a Molodensky transformation which due to local distortions in the OSGB36 TRF (which aren't compensated for) introduces these shift errors. There is a way of correcting these but it would need the implementation of a rather large (10MB or so) lookup table in the GPS unit which GPS manufacturers are probably loath to do just for one datum. On the OS website there's a tool you can use to do more accurate transformations between WGS84 and OSGB36 which does use these tables of offsets.

This would certainly explain the problems people were having taking waypoints from their GPS units in OSGB format and transferring them to paper maps. Try converting the WGS84 version using the tool on the OS site and then see where you end up.

It's possible I suppose that programs like Memory-Map suffer from the same problem, maybe the calibration of the maps was done using this simplified transformation, I don't know.

Hope this helps Smile

AlunS · Posted: Fri Nov 05, 2004 6:28 pm Post subject:

Oh yes, the thread can be found here on Google Groups ... enjoy!

	PocketGPS
	Web