I'm working on a Scorched Earth / Worms type game.  The map is randomly generated "uneven" terrain.  Checking for collision is easy enough with .getRGB(), however I want some projectiles to bounce around before exploding.  How should I go about finding the angle of the surface that the projectile hit in a map like this? 

The blue background is a separate image from the ground graphic, and is not used for collision detection.  Also, when a projectile explodes, I will be erasing a circle out of the ground image.  I thought about using an array of lines to define the outline of the surface, but trying to figure out how to cut out a segment and adding lines for the hole that was created, sounds like a pain.  The easiest way seems to be scanning pixels around the projectile (after it collides with the ground) to try to figure out the angle, but then how far out should I check, and how would I figure what is too far out?  Has anyone here done something like this before?

It's not so hard, actually. The easiest way to do it is just to count adjacent pixels and have pixels on the right do +1 to the slope and on the left -1, more or less. Depending on how many pixels you sample (3x3 or 4x4 usually) you can get more defined slopes.

Here is some code I have that I used to find angles in a game I made a while ago.