How to split and reproject a polygon shapefile into a grid based on area proportion with QGIS?

Question

I have a shapefile with polygons (municipal.shp) which contains a variable (float numbers) for each municipality of an area. I want to distribute these values into a vector grid (grid.shp) with different projection. So, the splitting should be based on the area proportion of the value. In the image below you can see how the files look like and how the output should be.

How I could do that in QGIS?

Answer 1

If I understand your question correctly you can do the following:

First, both of your layers, the municipalities and the grid, need to be in the same metric projection.

Now you can calculate the value per m² of municipalities using field calculator and store it in a new field, lets say valuem2. Use "value" / $area.

Now perform an intersection of municipalities and grid.

It will look like this (id_2 is the grids id):

When you now calculate the new value using "valuem2" * $area you will get the new proportional value into each new splitted polygon.

Now you can reproject your final layer into whatever you want to have it.

Your final product will look like this. Value per m² is represented in a blue color ramp. Value New is represented in the size of circles at the centroids. Additionally labels with all three values. The only one of interest for you in this final result is valuenew. You can delete the other columns to avoid confusion.

• value is the original absolute value of your municipalities.
• valuem2 is the relative value of value of your municipalities.
• valuenew is the absolute value of your new polygons, the intersection of municipalities and grid. This will be your final product. It was calculated by the new polygons area multiplied with valuem2.