FOOTPRINTS IN THE SAND: THE USE OF LOW ALTITUDE AERIAL PHOTOGRAPHS TO DETERMINE THE PATTERN OF HUMAN TRAFFIC IN A LAKE MICHIGAN COASTAL DUNE COMPLEX

Human trampling is a mechanism for opening and maintaining regions of bare sand on sand dunes. Our goal is to examine patterns of trampling in Lake Michigan coastal dune complexes near Saugatuck, Michigan. Raindrop patterns on sand are recognizable in low altitude aerial photographs, whether acquired using drones or pole aerial photography, and are erased by footprints made since the last rain event. Measuring areas of disturbed and undisturbed sand therefore provides a way to monitor human use of the landscape. The technique is limited by saturation, where new footprints erase old footprints, so a first step is to develop saturation curves. We used two experiments to evaluate saturation over an area and along a trail. In the area experiments, people made 15 passes of 15 steps over a 9 m² area of evenly raked sand for a total of 225 steps. The trail test entails a person making 3 passes at a normal stride back and forth over an area that is 0.62 m by 2.6 m. For both tests, we used a camera on a 4.8 m painter’s pole to take pictures between each pass and compared the number of footprints on the photographs to the number of actual footsteps, to create saturation curves. Initial data for the square area test show that the curves are linear to approximately 10 steps per meter square, then level off to saturation at approximately 14 steps/m². Initial data for the trail test show that saturation varies but was reached more rapidly, at approximately 8 steps/m².

Preliminary observations indicate that more trampling in dunes occurs on trails, with less trampling in areas of mixed vegetation and open sand patches adjacent to trails. Furthermore, larger open areas in sand dunes accessed by trails experience high amounts of trampling, in some areas reaching saturation, while comparable open areas not accessed by trails experience low trampling.