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Projection systems are always troubling and I cam across and issue yesterday. I created a buffer and did a select by location using that buffer, however it was producing the wrong results. It would select points that were outside the buffer and not select ones that were.

It turned out there was an issue with the projections on one of the initial datasets, with a scale factor in the projection parameters being rounded up.

It got me wondering what is the process of re-projection in QGIS and how accurate is it?

I imagine it is based on GDALwarp?

But particularly projecting to the British National Grid the proj seems very inaccurate: +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs

What are the parameters being used and can I impact them?

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    Did you actually project your data, or were you relying on 'on-the-fly' projection? – Devdatta Tengshe Jul 30 '13 at 9:37
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    It was on the fly, however the two datasets were both in EPSG:27700, however it seemed to be in two version of EPSG:27700. I did get it to work by changing the .prj file, but the reason for there being two different version confused me. – HeikkiVesanto Jul 30 '13 at 11:26
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For the quality of British National Grid reprojection in QGIS, see Raster incorrectly reprojected to OSGB(27700).

Note that the buffer will always be in the layers CRS and units, not the CRS of the project.

  • Thanks, the answer by Andre Joost clarifies it a lot. So it seems that QGIS has reverted to the three parameter conversion. – HeikkiVesanto Jul 30 '13 at 11:18
  • ... only until QGIS 2.0 is out. You can use current GDAL 1.10.0 to reproject the data externally. This has already the 7-parameters transformation re-implemented. – AndreJ Jul 30 '13 at 11:32

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