I use QGIS Lisboa and need the length of some shapefiles. When I use the field calculator the results are always around 30% higher than in real, also than a measuring in the map.
Does anybody have a similar problem or knows about a solution?
Field calculator uses the CRS of the shapefile, while measurement tool uses the projects CRS, until you uncheck
The two CRS may be the same, or not.
You can right-click on the layer
-> Set Layer CRS for Project, unless you are using openlayers plugin for background layer.