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Changing layers changes position on map - Datum problem?

 
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Guy



Joined: 16 Nov 2007
Posts: 10
Location: Kent, UK

PostPosted: Mon May 05, 2008 2:14 pm    Post subject: Changing layers changes position on map - Datum problem? Reply with quote

When I change layers the position on the new layer is out by many metres. If I have the GPS connected then once there is an update from the GPS the position corrects itself.

It looks like the magnitude of error you might get if initially using the wrong datum with the new map then using the right one as soon as the GPS info comes in.

I'm using OSGB36 with the UK national grid for my maps.

Example:

I point the map in one layer at UK grid reference TV 58001 05000

If I switch layers in TrekBuddy to another map using the UK national grid, the centre of the map is positioned at TV 57886 05058

Now...

TV 58001 05000 is latitude 49.92434366446 N, longitude 0.1998907250821 E in WGS84 (using a "6" parameter Helmert transform from the result of unprojecting the grid reference back to OSGB36)

If you project this latitude and longitude back to the grid using the BNG projection, without transforming it to back to OSGB36 first then it comes out as grid reference TV 57885 05074

This is really close to the TV 57886 05058 that TrekBuddy thinks the map centre is located at. (16.03 metres)

Much closer than the 128.8 metres that TrekBuddy is out by to start with.

The error when changing maps is in the same direction whether you increase or decrease the scale of the map - ie it's a consistent error.

Without the GPS running it means that you have to always fix the centre of the map after changing layers.

Thanks

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Guy
HTC Desire HD, Android 4.0.4 (ICS)
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