This work forms part of a study addressing volume monitoring over submarine sandbanks through a temporal sequence of DTM models computed from repeat MBES surveys. To perform reliable temporal geomorphometry, the main issue is to get an estimate of the systematic errors of each MBES survey. To this end, while supposing that the evolving shapes still exhibit some partial redundancy, this paper proposes to apply a new trend surface extractor to the first-order time difference between two successive DTMs.