An algebraic method our CEPS scientists developed for estimating the amount of erosion that took place during crater modification. A profile of a modified impact crater (red) is compared to a slightly smaller profile of a fresh crater (blue). The diameter of the fresh impact crater (as well as the corresponding parameters) is adjusted until the volume of eroded material (yellow) equals the volume of infilling (green). Volumes are estimated using the Thereom of Pappus. The results are calculated in cylindrical coordinates, which is why the area of infilling (green) looks larger than the area of erosion (yellow).