It's worrying if you can't reply on your maps ... but if the calibration is a bit out all over a map, as Geoff said, then doing a datum shift to get just one place "spot on" is a mixed blessing because it can make everywhere else a bit wronger.

Bertie

Are you saying that doing this does not move the whole map over a bit then? Still can't see a subscribe to topic!!

Bri

Using a datum shift in the 'Edit Map Properties' box will shift the entire map. It may be worth entering a datum shift if your map has an error which doesn't vary by much.

eg. Say your map grid line intersections are always between 12 and 18m east of where they should be, then a permanent datum shift of 15m would definitely be worth entering. That would keep the map to within + or - 3 metres of where it should be, instead of 12 to 18 metres.

If some of your grid line intersections were 10m to the east of where they should be but on another part of the map 10m to the west of where they should be, then a datum shift would not be very helpful.

What I did was to check the grid line intersections at the extremities of my map and near the centre. This gave me some idea of how the error varied across the map.

I discovered that the 'variation in the error' was small and that the map error was always out in one direction. So it was worth entering a datum shift. I ended up with a datum shift of 4 metres North and 13 metres East for my OS25 Lake District map. With this datum shift the map was kept closer to where it should be at any one place. My Garmin 76CSx tracklogs were no longer appearing to the west of where they should be.

Hope this makes sense.

Cheers

Geoff

Brill. I will double check tomorrow. But have you any idea if I do this to the map on the pc, will this then transfer correctly onto my Android?

Bri

Brian,

I expect that if you 'shift' a map on your PC then it will transfer to the phone in it's 'shifted' position. You could use the same trick of putting waypoints on the grid line intersections to check the map's accuracy .....but on the phone this time.

I haven't tried transferring my 'shifted' maps to my iPhone yet, so I can't answer this one with any confidence!

Cheers

Geoff

The USTech guy seemed to feel the same as you. But I shifted my map about the same as you did yours. Then I sent it over to phone. I had already placed a mark on the phone. But they did not quite match up. Don't know what you think.

Bri

The calibration data files for OS mapping is supplied to us by OS themselves. As far as I am aware all we do with the data is to convert it into the .QED file format and associate it with the correct mapping.

However could you please let us know the OS Grid References where the calibration error appear greatest and we can check the OS master files.

Thanks

Oh heck. Kind of tied up now for a few days. But I will try to do it as soon as I can. I hope in the meantime someone else will do it.

Thanks anyway.

Brian

Sorry Jason I have not had time to do this properly. But yesterday and obvious one came up. A mark at 55 5 14,3N 1 32 16,5W seems clearly to be on the track that you will see. But the OS 2012 map puts this mark out into the field! Not much to go on but it might give you and idea.

Brian

Jason,

I wonder if this is a long standing issue that's not limited to the latest 2012 maps.....

Check out grid reference NY 22000 07000. Given the nice round numbers this should be a grid line intersection a few hundred metres ESE of the summit of Scafell Pike in the Lake District. On my 1:25k Lake District map (off a memory Map DVD) a point with this grid reference appears 19 metres west and 7 metres south of where it should be.

Here are 2 images that show what I'm talking about.
https://picasaweb.google.com/geoffmozz/ ... -Ip-mCmwE#

Geoff Moss