[Qgis-user] Re: GPX, OS Grid References and Google Earth

[M.E.Dodd]

If you are interested in accuracy of 10m or better then just check how the data are getting in, if its all coming from gps as wgs84 lat long then hopefully the conversion and plotting will be ok but if you are relying on the gps's version of the os grid then its liable to have errors often in the region of 7m.  This error is on top of any location error on the gps unit itself, the error is simply due to conversion between wgs84 and os.
http://onlinelibrary.wiley.com/doi/10.1111/j.2041-210X.2011.00118.x/abstract  I wrote a little about this but it's a generally known issue with consumer grade gps units and os grid.


-----Original Message-----
From: Des Callaghan [mailto:des.callaghan at ecostudy.co.uk] 
Sent: 23 April 2012 14:58
To: qgis-user at lists.osgeo.org
Subject: RE: [Qgis-user] Re: GPX, OS Grid References and Google Earth

Many thanks Lene and Nick, this worked a treat. Combined with the Google Satellite basemap, imported using the fantastic 'OpenLayers' plugin, this is just what I needed. Many thanks for your help. Best wishes, Des 

-----Original Message-----
From: qgis-user-bounces at lists.osgeo.org
[mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Nick Hopton
Sent: 23 April 2012 11:04
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Re: GPX, OS Grid References and Google Earth

Hi Des, just to add a lttle to what Lene wrote. As mentioned, import the GPX file to QGIS, then save it as a EPSG:27700 projected shapefile. To do this right-click on the GPX file in the layers panel, go 'Save As...' and set 'Format' to 'ESRI Shapefile'. Set the CRS for the new shapefile by clicking to the right of 'CRS', and picking 'Selected CRS', then press the 'Browse'
button. In the window that appears enter '27700' (without the quotes) in the 'Filter' field. Highlight 'OSGB 1936 / British National Grid' in the list below and press 'OK'. Then give the new shapefile a name and click 'OK'.

Next, load the new shapefile into QGIS, right-click on it in the layers panel and go 'Set Project CRS from Layer'. You can now add OS grid coordinates to the shapefile's attribute table. To do this, highlight the layer in the layers panel and (from the top menu bar) go Vector -> Geometry Tools -> Export/Add geometry columns. All you need to do here is tick 'Save to new shapefile', give the new file a name and press 'OK'.

Now load the new shapefile and have a look at the attribute table (right-click on the layer in the layers panel and go 'Open Attribute Table'). You should find that it contains two new columns, one for OS eastings (x-ordinates) and the other for northings (y-ordinates). The ordinates form OS grid coordinates, not grid references (ie. SD4568798345 will be shown as 345687 498345, for example). QGIS doesn't know about Ordnance Survey grid references I'm afraid, but if this is a problem then come back.

Nick.

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