Calculating length for all selected lines

If you need to calculate the total of a given dataset property such as length, the easiest thing to do is just use Python. In this recipe, we'll total the length of all of the railways in a dataset.

Getting ready

You will need to download a zipped shapefile from the following URL:

https://github.com/GeospatialPython/Learn/raw/master/ms_rails_mstm.zip

Unzip it and place it in directory named ms in your qgis_data directory.

How to do it...

We will load the layer, loop through the features while keeping a running total of line lengths, and finally, convert the result to kilometers:

1. First, we'll set up the path to our shapefile:

           pth = "/Users/joellawhead/qgis_data/ms/ms_rails_mstm.shp" 
   

2. Then, we'll load the layer:

           lyr = QgsVectorLayer(pth, "Railroads", "ogr") 
   

3. Next, we need a variable to total the line lengths:

           total = 0 
   

4. Now, we loop through the layer getting the length of each line:

       ...