Updating the (1:50,000) Dutch groundwater table class map by statistical methods: an analysis of quality versus cost

Groundwater table (GWT) class maps have been mapped for the whole of The Netherlands at a scale of 1:50,000, and provide useful information for a variety of purposes. Due to strong human impact on the Dutch landscape, water tables have changed and the maps now need updating. Six updating methods have been defined, varying in data requirements and in the smallest spatial unit that can be updated. The performance and cost of each of the methods were measured in a 9228-ha test area. The costs were also extrapolated to larger areas of 46,994 and 75,684 ha using measures of sampling error as criteria. The major cost factor is largely determined by the sample size, which is method dependent. A method that updates GWT maps by evaluating an objective function in locations obtained through stratified random sampling showed the best performance in the test area and a reasonable cost development at larger areas. Validation in the 75,684 ha area supported this conclusion. A kriging-based method performed well, but was expensive. In the near future, elevation data will be available at the national scale. Using these data as ancillary information will decrease cost and increase the accuracy of GWT map updates. This will allow for drawing new map polygons instead of re-labeling existing ones.