I have a grid of 10 km x 10 km polygons, and am planning survey transect lines across the area. I am trying to use the Sum Line Lengths tool to calculate the total transect length in each polygon. The transect line is a single, continuous line, which zig-zags across the grid. The CRS is UTM, units metres.
The issue I have is that the summed length is incorrect - it is less than it should be.
I think this is due to the transect line repeating its track across the grid cells - might this be the case?
So the transect crosses over precisely the same ground a few times.
Another minor issue, but perhaps a clue to what is happening, is the line count is also incorrect: it's either 0 or 1, when in some cells it should be 2 or 3.
I've done various tests with line layers on the same grid of polygons, and Sum Line Length gives the correct output if the lines, or single line with multiple passes through the cell, do(es) not repeat its track precisely as I want it to in reality... The number of lines / cell is always wrong though.
I am using QGIS v2.18.16.