USE OF A THREE-DIMENSIONAL LASER SCANNER TO TRACK DUNE MOVEMENT IN WHITE SANDS NATIONAL MONUMENT

A 3-D laser scanner is a ground-based LIDAR (LIght Detection And Ranging) instrument. It is set up on a tripod and sweeps the surrounding environment with optical rays. The rays produce reflections when they encounter solid objects. The instrument determines the XYZ coordinate of each reflection point relative to the scanner position by measuring the azimuth and the two-way travel time of each ray with accuracy better than 1 cm. Reflection points can be spaced within 1 cm of each other on the scanned objects. Using this technology, researchers can scan a solid object from several different directions and later merge the reflection points into a single continuous data set using a common 3-D coordinate system. Spatial interpolation of the reflection points yields a 3-D surface model of the object.

We used this technology to survey a barchan in White Sands National Monument. The dune selected for this study was approximately 50-m in length and 4-m height. The dune was surveyed in January 2003 and the second time in May 2003. Spring is the peak wind season in White Sands with prevailing west-southwest wind. The dune migrated about 6 m downwind during the 4-month period.

In performing the 3-D scanning surveys, we installed 6 stakes along the perimeter of the dune and captured their positions in both sets of scan data. Later, we rotated the coordinate axes of the January data set to conform to the May data set, using the stake positions as the ground control points. We were able to co-register the two scan data sets with 2-cm overall accuracy. Once both data sets were registered in common geographic coordinates, we generated digital elevation model (DEMs) of each using a 2-cm grid spacing. By subtracting the elevation values at each grid cell, we produced a map of elevation change clearly showing the spatial patterns of erosion and deposition, and calculated the volume of the mobilized sand.

During the 4-month interval, the dune changed its form as it migrated downwind. The central axis of the dune became more horizontally compressed in the downwind direction while the horns extended farther downwind. Calculations show that the total volume of sand removed in the 4-month period is 650 m3 and that the volume deposited is 690 m3. There has been little change in the total sand budget while roughly 1/5 of the sand that makes up the dune has been recycled.