Flat water surfaces were used for many years to estimate the curvature of the earth and to transfer heights above sea level from point to point. During calm conditions and with minimal flows along the River Murray it has been possible to transfer heights above sea level to within a few centimetres. A 65 kilometre stretch of the Murray was used for a detailed study. Readings from “tide boards” were compared with those obtained from adjacent run of Bench Marks with adjusted heights above sea level.