We present an algorithm for overlaying polygonal data with regular grids and calculating the percentage overlap for each cell in the regular grid. Our algorithm is able to support self-intersecting polygons, meaning that some spatial regions may be covered by two or more polygons. Our algorithm is able to identify these cases and eliminate redundant polygons, preventing erroneous results. We also present an optimized version of our algorithm that uses spatial sorting through interval trees, and provide a performance comparison between the optimized and unoptimized versions. Finally, we apply our algorithm to geography data, specifically of bark beetle infestation.