I do a lot of topographic\geomorphic change detection, mostly with raster digital elevation models but I've also started getting into point cloud differencing too. It's simple subtraction, but the way you do your subtraction can make a big difference in how you interpret the data. I've had several students ask about this, so I thought a short write up was in order. The examples here are topographic (erosion and deposition), but the concepts apply to any raster-based change detection (i.e. land use/land cover change).