Because a sandy coast is a very dynamic coastline, and because stability of the coastline is vital for the safety of the Netherlands against inundation by storm surges (dunes protect the polder-area, which is situated below sea level), already in former centuries it was necessary to get some knowledge about the movements of the coastline. In the middle of the 19th century systematic coastal measurements started. On fixed intervals (approx. 250 m) the position of the dune-foot, the high water line and the low water line was measured every year. In order to have a fixed reference, monuments were placed on the beach. Around 1960 it became clear that more detailed information was necessary. Therefore the system of monuments and base lines was somewhat updated, and a detailed program for coastal measurements was set up. Along the whole North Sea coastline from Cadzand in the south to Rottumeroog a fixed set of measuring lines was defined (see fig. 1). In total approx. 3000 lines are defined this way. The lines have a intermediate distance of 200 m and are perpendicular to the base line. The base line is parallel to the coastline, and is positioned approx. near the high water line. On coastal sections with groins, the measuring lines are placed in the middle between the groins. Profiles are measured every year from 200 m landward of the base line to 800 m seaward of the base line (that means to an average depth of 8 m below mean sea level). Every five years every kilometer a line is measured until 2500 m from the base line. All measurements are made in the period between april and september. The measurements are performed in two parts (see fig. 2): * leveling above the low watter line * sounding below approx. mean sea level line