I am using QGIS and have a vector grid covering part of South Europe. The grid is made of rectangles each with equal size 0.01 degree. So there are thousands of features defined by four values : left, right, top, bottom, which resulted from a standard 'Create grid' task. Also there is a field Count which shows some integer value for each grid cell.

Then I clipped my polygons - about 100 which represent species occurrence in small areas from the big grid. Each polygon I received now has some rectangles (1 to several hundred) defined by coordinates and a value in the count field.

How can I unite all the polygons and sum any overlapping values in each cell? I mean if three polygons occupy the upper left grid then it receives the value 3, etc.

This should be probably better with rasterization but I don't know how to make the pixels the same as these grids which were given to me. CRS is the same for every polygon.

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