So, first of all, here is a test file:   The column length was added using QGIS (v 3.10 A Coruña), field calculator and adding parameter $length.   If the file is opened with geopandas (v 0.6.1), the property length outputs this:  a.length Out[8]:  0    1581.729679 1     899.177689 2      38.054220 3      47.333848 4     189.591771 5     275.946388 dtype: float64  the values obtained by qgis are:  a['length'] Out[9]:  0    1580.830737 1     898.686067 2      38.033291 3      47.307854 4     189.489982 5     275.796306 Name: length, dtype: float64  As you can see there is a difference of a few centimeters.   Has anyone else experienced this?
I created some linestring features in EPSG:32632 (WGS 84 / UTM zone 32N) exactly 10 000 m long.  QGIS calculates all line lengths (horizontal, vertical, diagonal) as 10 001.9 m. The measure tool explains "Project CRS transformation is turned on and ellipsoidal calculation is selected. The coordinates are transformed to the chosen ellipsoid (WGS84), and the distance is calculated in meters."  When I set the ellipsoid to "None / Planimetric" in project properties, I get the expected results (10 000 m).  I don't understand the phrase "coordinates are transformed to the chosen ellipsoid (WGS84)", is it a transformation to unprojected degrees (4326?)? Since 32632 already is based on WGS84, is there anything to transform? If there is a kind of "great circle" calculation involved, I would have expected lengths can only get smaller.  Is QGIS's calculation correct/meaningful, is it plain wrong or do I see rounding errors?  This are the geometries I tested:  LineString (370000 5615000, 370000 5625000) LineString (366464.46609406732022762 5616464.46609406732022762, 373535.53390593267977238 5623535.53390593267977238) LineString (365000 5620000, 375000 5620000) LineString (373535.53390593267977238 5616464.46609406638890505, 366464.46609406638890505 5623535.5339059317484498)   Edit as addition to MappaGnosis's answer: I forgot that in UTM , this seems to be the simple answer for the title question.
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