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If "enable on the fly" is on and the CRS changes at all during the lifetime of a project then seemingly, it refuses to measure (using the measure tool I just get 0.00000). So new project with all the layers added in the same CRS (OSGB British National Grid EPSG:27700 which we used one QGIS 2.18) it measures, but it gets it wrong. When compared with Google it's about 0.8m rather than 1m.

If the project is all saved as and only ever goes into EPSG:3857 then it seems to be right, but if you accidentally add a layer with a different CRS, it doesn't measure at all.

Am I doing something wrong?

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    EPSG:27700 will be least wrong, Google will be about sin(52) % wrong – Ian Turton Aug 7 at 15:48
  • I think 27700 is about 20% out...I think to get a 50m buffer I need to request an 80m one using CRS 27700... So the area results need to be divided by 0.8? But why? Does anyone have a more accurate number or CRS for use in the UK? – Belinda Aug 7 at 15:52
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    I just zoomed into a running track on a Google Satellite image in QGIS and under EPSG 27700 the distance from the 100m start line to the finish is... 99.827metres. Not much different with EPSG:3857 set. A 100m buffer round a point on the finish line goes right along the 100m start line. Can you show us images of examples where its failing? Perhaps overlaid on a running track for scale? (Note the straight is >100m because there's a 110m hurdles event...). – Spacedman Aug 7 at 16:10

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