British National Grid Co-ordinates appear out of position in BaseCamp

Former Member over 11 years ago

I've been having an issue when inserting British National Grid co-ordinates into BaseCamp (4.1.2) and my eTrex30. If I pick a known grid intersection from the Ordnance Survey paper Map (eg TQ 46000 94000) the point appears where I expect to se it in the eTrex, however in basecamp the point is around 375 metres further south. BaseCamp is referenceing the same overlay map.

Basecamp tells me the point has world co-ordinates of N51 37.294 E0 06.545

Ordnance Survey tells me the point is at N51 37.553 E0 06.471

Regards

Rob
:confused:

0 SUSSAMB over 11 years ago

Strange ... Basecamp gives me N51 37.550 E0 06.471

Did you also select WGS84 as the datum in Edit, Options, Measurement?

0 Former Member over 11 years ago

Hi

Thanks for the tip. My datum was correct, however the heading wasn't set to grid. I've reset my BaseCamp options to Position = BNG and Heading = Grid and all seems to work fine.

Many thanks

Rob

0 SUSSAMB over 11 years ago

Don't understand why heading would make any difference to the grid ref but glad it's working for you now.

0 Former Member over 11 years ago

My guess is the setting was on True North, so the deviation from OS Grid North threw up the shift in the point I was trying locate.

Regards

Rob

0 SUSSAMB over 11 years ago

Well a grid is a grid is a grid, it has nothing to do with heading. I have my Basecamp set on true north and haven't seen any problems :confused:

0 Former Member over 11 years ago

On the OS map I'm using there is a note regarding grids, basically it states that at the centre of the map sheet True North is 1degree 35 minutes west of Grid North. The OS website also states "On OS Landranger Maps, the difference between True North and Grid North is given for each corner of the map and is shown in the legend". If two grids are rotated around a common point then the only place they will be co-incident is at the point of rotation. I can only assume this had the effect of relocating the grid point I was trying to reference.

Regards

Rob

0 SUSSAMB over 11 years ago

Well that would be true if there were 2 'grids' being rotated around a common point, but grid 'references' (which is what we're talking about) don't work like that, so I'm afraid your assumption is incorrect. True north, 'grid north' and magnetic north come into play when you are considering things like compass bearings, not points on the map. That is determined by the map datum and position format selected. This is a useful discussion that may help your understanding:

http://bhgcinfo.wikidot.com/tutorials:gps-position-formats-and-map-datums

0 stuartmw over 11 years ago

True north, 'grid north' and magnetic north come into play when you are considering things like compass bearings, not points on the map.

That is my understanding too. Magnetic north varies according to region of the world.

0 Former Member over 11 years ago

Interesting article which seems to have answered my question.

When I first put in the OS co-ordinates in Basecamp the grid option was set to lat/long, datum WGS84 via the options dialogue box in the program.

The point location dialogue box allowed me to enter the co-ordinate I required with the option to input in OS format. What it didn't show was the datum is still locked on WGS84 and not Ord Survey Great Britian, which seems to have produced the shift I experienced. Once I went back to options and locked both the grid and datum to OS the point entry reflected what I could see on the paper map.

Thanks very much for the advice.

Regards

Rob

0 SUSSAMB over 11 years ago

That would explain it. Glad you got to the bottom of it.