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Using QGIS's "Measure Line" tool I get a distance of 1.22km between two points.

But if I use the scale indicated at the bottom of the screen, 1:50000, and measure with a ruler, the distance is 2.5km (as if the scale were 1:25000). The project is in UTM.

The same thing happens in Print Composer: the scale bar is 1:25000 compatible, although the scale is set to 1:50000.

What is happening?

example

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    Measures on the screen is affected by the screen DPI (more or less the number of pixels per inch) and by your screen resolution, while the computed display scale considers only the DPI (I couldn't find in the code if it uses a constant or if retrieves the current setting) --> don't measure on screen. Read this post for a similar question (on paper, not on screen)
    – JGH
    Commented Dec 6, 2019 at 13:18
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    You can add a scalebar to your map canvas (View menu > Decorations > Scale Bar). It won't fix the discrepancy between the numerical scale and what you get when you measure with a ruler, but it maybe helpful as a reminder not to trust the numerical scale.
    – csk
    Commented Dec 6, 2019 at 20:27

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