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I have a shapefile with CRS ESPG:32632 where are shown some power lines of a ​​northern Italy area.

I need to calculate the length of the lines and add it, as a new field, in the attribute table. I am also asked to provide the shapefile with with CRS ESPG:4326.

If I save the file with the CRS ESPG:4326, reopen it and then perform the length calculation (function $length of qgis field calculator) the results appear incorrect.

If instead I perform the length calculation operation with the original CRS and only then save the file with CRS ESPG:4326 the result seems ok.

Can someone explain to me why?

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    Welcome to GIS:SE @AntonioMaria! I think in simple terms: ESPG:32632 is a Projected coordinate system which uses metric units to calculate distances (usually metres); ESPG:4326 is a Geographic coordinate system which uses degree units and explains why you would be getting different results. When you save your file into another CRS, you are transforming the geometry of your features (your attributes remain the same). – Joseph Jan 5 '18 at 12:33
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Results of $length are calculated using the units of the projection. Since 32632 is a UTM-projection, hence uses meter as unit, and 4326 uses degree, UTM-based calculations appear correct, or at least are more relatable.

See also: What's the difference between EPSG:4326 and EPSG:36232?

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