The net thickness of the aquifer is determined by multiplying the gross thickness with a net-to-gross ratio. Due to the lack of a reliable, consistent net-to-gross database, it was decided to determine for each aquifer a single net-to-gross value, based on geological knowledge and available net-to-gross data. The Delft Sandstone and Alblasserdam members constitute an exception to this rule. Using a reliable database, a net-to-gross map was generated for these aquifers.