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?