Problem with intersecting lines over a polygon layer in QGIS

Question

I have a shapefile with lines (a road network) and I want to intersect it over a polygon layer in order to finally have a polygon layer (each cell has an ID number) with the intersected lines for each ID.

The shapefile with lines has pollutant emission values (CO etc) (in grams). Before the intersection I calculate a new value (value/$length). See the image (for the selected line CO=6662940.61gr):

In the intersected file, in order to calculate the new total value C (in grams) for each grid cell, I sum up the values e.g. CO of each grid cell and multiply with the new length containing in each grid cell. The final result is not correct. I get a higher or lower value in the intersected file comparing to the original one. See the following images For the corresponding line (within two grid cells) I get CO=1568950+4724202=6293153.12gr):

For the whole grid the difference is much higher, I get almost the double values for the intersected file.

I do the following:

Any idea of what could be wrong? Is it right to perform this kind of intersection when working with lines or not?

Answer 1

My approach would be to

1. transform the grid from polygons to lines
2. Split the roads using the newly created line-grid
3. recalculate all relevant data on the split roads-layer
4. use the aggregate-function in the fieldcalculator on the ploygon-layer to sum all relevant data

Or you could

1. transform the grid from polygons to lines
2. split the roads using the newly created line-grid
3. join the attributes, including an ID, from the polygons to the split roads
4. dissolve the split roads using the ID from the polygons
5. recalculate all relevant data
6. join the attributes of the dissolved roads to the polygon-layer.

Answer 2

On the exemple you show us it seems that the original line intersects with 3 grid cells, not only 2. Am I wrong ? It could be the origin of the error. Check if there is another OBJECTID_1=34434 in your INTERSECT layer.

Another thing bothers me. The two new lines we see have the same length. Have you recalculate the length after intersecting ?

Besides be careful when calculating the new value. You have to go for sum(CO_linear*length) and not sum(CO_linear)*sum(length).