The documentation for the SPEAR Relative Water Depth tool actually states: "For calculating relative water depths, it is typically best to not perform atmospheric correction. Atmospheric correction of littoral or marine areas often alters the data such that calculating water depths may produce anomalous and unsatisfactory results. Unless there is a specific need to perform atmospheric correction and the implications are understood, it is best to skip this step." So, you are right, it is expected that you would get better results when skipping the preprocessing. Peg Exelis VIS