Topographic position index (TPI) is an algorithm increasingly used to measure topographic slope positions and to automate landform classifications. We applied TPI to a geoarchaeological research project in northwestern Belgium but its use led to erroneous landform classifications in this heterogeneous landscape. We asked whether deviation from mean elevation (DEV) was a better method for landform classification than TPI. We found that it enabled more accurate geomorphological assessment when using northwestern Belgium as a case study.