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I have two layers both containing points - each point in the first layer has a corresponding point in the second layer (same primary key). One layer has CRS EPSG:27700 and the other layer has EPSG:4326. Project CRS is EPSG:27700.

I want to calculate the distance between each pair of points. I tried the Distance Matrix, however the distances returned were nothing like those returned by Measure Line (which I used as a check).

I then tried using the field calculator, with the expression:

distance($geometry,geometry(getFeature('LAYER2','KEY', "KEY" )))

However again the values are much too big, e.g. for one pair of points:

Measure Line give 99.654m; distance gives 849950.964330714.

I wondered if the problem related to the different coordinate systems, so I saved the EPSG:4326 layer as a shapefile with CRS EPSG:27700. This time distance() returns sensible results, e.g. for same pair of points:

Measure Line give 99.654m; distance gives 99.6801982600587.

For a given project CRS, shouldn't distance() give the same results irrespective of the coordinate systems of the individual layers?

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    Your intuition was right. You cannot calculate distances between features in different coordinate systems. – Micha May 8 '15 at 9:40

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