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I've been working with loads of shapefiles projected according to OSGB 1936 (CRS 27700). The project CRS is set to EPSG:27700 and "On the fly reprojection" to EPSG:27700 is set (I keep this as a default as I work with OSGB 1936 95% of my time).

I decided to download OSM layers from the OpenLayers plugin. When I load this, the EPSG in the bottom right of the screen goes from EPSG:27700 to EPSG:3857

Question 1: should this be happening anyway as it would suggest that the whole project has been re-projected to EPSG:3857?

As soon as the OSM layer is loaded it's way out of alignment but a quick pan seems to bring it altogether quiet nicely.

What I have noted is that (Question 2), if I try to re-specify the CRS to 27700, then the shapefiles "move" by a few tens of meters with respect to the OSM. Does this suggest that QGIS is doing a second re-projection (from what to what?)?

Also (Question 3), does it mean that if I import another shapefile that would be originally a dataset digitised with EPSG:27700 it would be re-projected to EPSG:3857 because the CRS in the bottom right of the screen was set to EPSG:3857 after the OSM import. This is all a bit confusing and logic would suggest that the import of a non-ESPG:27700 (project default CRS) should not go and change the EPSG setting in the bottom right of the screen. Anita Graser's book says on page 34 "Your project CRS will automatically be changed to Pseudo Mercator when you load a background map using the OpenLayers plugin option".

Does that mean there is no way of keeping ESPG:27700 active in the project when importing basemaps from OpenLayers? If so, what alternatives are there?

What I am trying to bottom out is to understand the correct way of importing basemaps (usually Google, Bing or OSM through the OpenLayers plugin) into my projects (always EPSG:27700) without losing alignment and getting layers to correctly re-project. I'm trying to support greater use of QGIS in my company but don't want to lose credibility by promoting the use of a tool that is misaligning datasets. Don't get me wrong. QGIS is a great tool and I am very fond of it but importing layers with different CRS's is quite common these days and getting this right from the start is half the job done.

I did quite an extensive Google search before posting and didn't come up with a recent, clear cut answer to my questions above. There is a bit of content out there on problems with older versions of QGIS, PROJ4, etc. but nothing recent for QGIS 2.0 so worth an update.

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The requirements of the Openlayerrs plugin have not changed with QGIS 2.0.

The osm tiles are in EPSG:3857, and to draw them nicely, it is required that the project has that CRS too.

If you want a OSM background in CRS27700, you can select a zoom level that fits the area of your interest, make a screen copy with File -> Save as Image, remove the OSM layer, change CRS to EPSG:27700, then add the picture you just saved, and make sure that layer has EPSG:3857.

  • Interesting ... very combersom method but from your reply it seems to me that this is a problem only if I need to create a map where I need to show the OSGB1936 grid. Is that correct? In fact, let assume that: a) i load 4 shapefile in default CRS; b) I upload OSM through OpenLayers; c) I create a new vector layer while the CRS is set to 3857; d) I start digitising on the new layer, save; e) I remove OSM layer; f) I set CRS back to 27700. QUESTION: Would my new shapefile represent features correctly considering I digitised them under 3857 and now I am showing in 27700? – geotux Jan 20 '14 at 23:45
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    BTW, I think it would be fair to say that the developers of the plugin(?) or QGIS(?) should consider overcoming this limitation. ArcGIS has no problem showing Google/OS/Bing as a basemap on an EPSG 27700 grid. Or did ESRi push the boundaries to achieve this? – geotux Jan 20 '14 at 23:52
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    The vector layer can be any CRS you want. Reprojection of vector layers is quite easy and fast, but a reprojected 256x256px tile will have nasty black holes at the edges. It would be possible if you download all tiles, stitch them together, then reproject the single picture. But that might take too much time until you see the result. Not sure how ESRI do it though. – AndreJ Jan 21 '14 at 5:14
  • @geotux This is close to my heart. In ARC it's possible to display Ordnance Survey OpenSpace layers (including 1:50,000 Landranger mapping) in EPSG:27700. I'm sure that this could also be done in QGIS, which would be very useful. N. – nhopton Jan 21 '14 at 11:31
  • nhopton, this is esxactly the point I'm trying to make. Surely its not a problem that is confined to EPSG 27700 but quite likely to all CRSs .... just to say that other people in oether countries shoudl be facing the same problems and I do nto believe this is an occasional issue that I and a few oether in the UK are facing. Is it on the ToDo list and if not how can it make its way there? DO you think that it would be worth raising this in the QGIS support/developers lists? – geotux Jan 21 '14 at 19:37

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