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I read a lot of posts concerning that subject, nevertheless I cannot solve to problem of getting $length in the field calculator in meters rather than in decimal degress in QGIS.

The CRS of original shapefile is WGS84 (EPSG:4326). I reprojected the shape to World Euidistance Cylindrical (Sphere) (EPSG:3786). I did the same in project settings (with on the fly transformation enabled).

However I am still getting the length of the polylines in decimal degrees. Any ideas?

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If your shapefile has been correctly reprojected, and matches the project CRS, then on-the-fly projection shouldn't be necessary. – L_Holcombe Dec 5 '12 at 7:00
We can't help you unless we know what software are you using? – RyanDalton Dec 5 '12 at 7:01
It's QGIS I am guessing because QGIS has the $length variable. – Nathan W Dec 5 '12 at 7:13
Yes, it is QGIS – user13337 Dec 5 '12 at 8:13

I don't think you really reprojected the Shapefile to EPSG:3786.

To really reproject the data, use "Save as ..." and select EPSG:3786 (or better even a fitting UTM CRS as Andre mentioned) as a target CRS. You can calculate length in meters in this new file.

On-the-fly reprojection in project properties ONLY affects how data is rendered. It does NOT affect calculations and geometry operations.

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Furthermore, EPSG 3786 is not really suitable for measuring. It defines all meridians of longitude with equal distance. In reality, the further you come to the poles, they get nearer to each other. You better take a projected CRS like UTM for real measuring. – AndreJ Dec 5 '12 at 8:41

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