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I've collected a bunch of GPS polylines (WGS84).

What is the easiest way to measure the length of these lines in KM using QGIS or ArcGIS Desktop?

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If these lines cover a significant portion of the globe, make sure that you are using a projection/algorithm that supports geodesic measurements. – DavidF Sep 21 '10 at 19:24
up vote 7 down vote accepted

With ArcGIS, project them to a projected coordinate system then use the field calculator to calculate their length.

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  1. open the GPS polylines in WGS84
  2. use "Save as ..." and choose a CRS in meters for export
  3. load the new shapefile
  4. go to attribute table, enable editing and start field calculator
  5. use "length" operator to calculate line length and write into a new attribute
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If you need step by step directions, this tutorial will help

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