lengthlineqgis
My goal is to find the total length of roads per county for a particular state. You would think there would be a simple government spreadsheet with this information in it….
I cannot figure out how to find the summed lengths of a polyline layer(roads) in each county (polygon) in QGIS. I have tried using Vector-> Analysis -> Sum line lengths but it just reproduces the county shapefile with a length column, of which I have no idea what unit it is in.
Give the Sum Line Lengths tool a try, from the Vector > Analysis Tools menu:
This will create a new shapefile that summarizes each input polygon vector layer feature by the length of input line vector layer.
The output shapefile should have an attribute field called LENGTH as you can see in the above screenshot, but you can change that to SUM_LENGTH or something similar.
Then you can visualize your map on that field, if that is your purpose:
NOTE the units are important to your inputs, so in my example decimal degrees are not too meaningful...but if your input coordinate system is in metres you should be good.
Use the GroupStats plugin, which works like an Excel pivot table and allows you to calculate statistics, such as for length in your case, for various combinations of groups.
Best Answer
"Sum line lengths" calculates length in the units of the input layer's CRS. So if the input layer is in WGS84, the results will be in degrees. If the input layer is in some UTM projection, it will be meters or feet.
NOTE: Reprojecting on-the file does NOT change the results of length calculations. You have to reproject the layers permanently using "Save as ..." and use the newly generated files.