Which geographic coordinate system /projection would you recommend to measure the length of hundreds of lines which are globally distributed (southern and northern hemisphere)? The lines have a length up to 200km.

Lines generated from start- and endpoint


If you are going to measure the planimetric length of the lines (length( $geometry)), I can not recommend any reference system. The deformation produced in the distances when transforming a curved surface into a flat one, can only be worse in some systems than in others, for the points that are far from the center of the projection.

On the other hand, if you are going to measure the ellipsoidal length of the lines ($length), all the reference systems are equally good and the result obtained after projecting the coordinates of the vertices to any of them will be the same. All you need is to choose the right ellipsoid in the properties of the project.

  • Thank you! Then my attempt was right. I started calculating the length in kilometers by using the expression '$length / 1000' and used WGS84 as coordinate system. – remotesatellite Mar 14 at 10:41
  • Your are welcome, @remotesatellite! The ellipsoid and units settings in the General tab of the project properties determine the output of the $length function. Make sure that the ellipsoid is WGS 84 and the distance units are meters in that case. – Gabriel De Luca Mar 14 at 10:47

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