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I use the field calculator to measure the length of LineStrings in my shapefile layer, but i get different results when using QGIS 2.18 or QGIS 3.0.

If I am using the local coordinate system the difference between the QGIS 2.18 and QGIS 3.0 is not significant. If I am using WGS84 (I know you should always use the local coordinate system) I get two different results based on the version I am using. In 10 km the difference is almost 2 km..

Does anyone know what might be the reason? Did the algorithm behind the length calculations change in QGIS 3?

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    Measuring in EPSG:4326 results in degrees, not meters or kilometers. Does this difference also exist when using a projected crs?
    – MrXsquared
    Commented Dec 14, 2018 at 9:34

1 Answer 1

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The built-in functions that use a feature’s geometry (e.g. $length) for calculation use the units of the layer’s CRS. Geographic Coordinate Reference System(CRS) such as EPSG:4326 have degrees as units - so the length of the feature would be in degrees and area in square degrees - which is meaningless. You need to use a Projected Coordinate Reference System with units of meters or feet to perform such calculations.

And the algorithm behind the length calculations probably was not changed.

QGIS 2.18

 double QgsGeometry::length() const
 {
   if ( !d->geometry )
   {
     return -1.0;
   }
   QgsGeos g( d->geometry );
   return g.length();
 }

QGIS 3.2

 double QgsGeometry::length() const
 {
   if ( !d->geometry )
   {
     return -1.0;
   }
   QgsGeos g( d->geometry.get() );
   mLastError.clear();
   return g.length( &mLastError );
 }

References:

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