The 1974 map showing the depth of the fresh-salt water interface (defined by 1500 mg/l total dissolved solids concentration) in the Belgian coastal plain is a vital document for hydrogeologists. Since its publication, new data have been collected. These comprise water quality analyses and borehole measurements ordered by the Flemish Environmental Agency, the Flemish Land Use Agency or from dissertations and new lithological data synthesised by the HCOV (Hydrogeological Code Flemish Subsurface) mapping. The aim of this paper is to compare the new information with the 1974 map and provide an update for the central part of the Belgian coastal plain. This resulted in only minor differences. The main adjustments are the identification of a number of additional areas where the phreatic aquifer is completely filled with fresh water, incorporation of insights of the groundwater flow system in the dunes, shore and sea and, some small changes in the position of transition zones between fresh-water lenses and adjacent areas where salt water occurs at very shallow depth. The general conclusion is that comparison with new field data obtained in the more than 30 years after its publication, confirms and strengthens the validity of the map.