The LiDAR data was provided in two forms: a digital terrain model (DTM) which has had surface artefacts removed via 'cleaning' of non-ground laser retums (hence does not include buildings), and a digital surface model (DSM) data set which contains original laser returns, The DSM can misrepresent the ground surface and bias flood modelling (e.g. by showing the canopy of trees rather than just the trunk, and missing or rounding the edges of buildings and defences).