Mapping a defended beach with aerial photographs and tidal readings

The Belgian coast lacked accurate soundings, and heavy batteries made a close survey hazardous. Photographs of the waterline at different stages of the tide could instead provide contours for the exposed beach. A submarine stayed submerged near Nieuport for twenty-four hours and measured the water above it every quarter of an hour, establishing the actual tidal curve. Each photograph could then be matched to the water level at the time it was taken. Two mapped points on land fixed the scale despite changes in aircraft height. Planners measured five possible landing places and selected three, calculating that the pontoons needed to be 550 feet long. A separate survey east of Dunkirk tested the method against direct measurements, with agreement reported within six inches.