I want to make a geographical statistics application that would show a map and draw some colors on it depending on the density of each section of the map.

What I supose I would do is divide that map in smaller sectors (40 Squares X 40 Squares for example) and for each entry of a statistic, a coordinate would be assigned to it. Then when I would draw it, I would set the transparency of ,let's say, red to represent the density of the statistic for that coordinate on the map.

I know that I could use BitBlt or DDraw (maybe something else, let me know)... But I have no idea if this is in fact possible and how I could alter an image.

Any suggestions or exemples?