Contour mapping in a sandbox

We are at the British Geological Survey in Keyworth today. One of their displays in the foyer is a sandbox (yes, a child's sandpit where you can build sandcastles). However, the BGS boffins have hooked up a distance sensor over the top of it, linked to a computer that then projects a coloured contour map of whatever is in the sandbox over the top of it. 

So, needless to say, many minutes of fun were had by all building islands and volcanoes in the sandbox, and watching the computer update the contours as you worked. Of course, it didn't take long to work out that it would also contour your hand, your face or anything else you stuck in the sandpit, but I won't go any further into that.