Abstract:
Quantifying and monitoring of many natural hazards requires repeated measurements of a topographic surface whose change reflects a geological or geophysical process. Topography and topographic change measurements are routinely made using techniques such as Interferometric Synthetic Aperture Radar and GPS, but both of these techniques have limitations for these purposes. A technique attracting increasing attention for its ability to perform accurate high-resolution topographic mapping (including sub-canopy) is laser altimetry, or lidar. Here, we evaluate the feasibility of a new method for using laser altimeter return echoes, or waveforms, to detect relative elevation change. The method, dubbed the return pulse correlation method, maximizes the shape similarity of coincident laser return waveforms from two observation epochs by shifting them vertically. We evaluate the accuracy of the pulse correlation method using laser altimeter data acquired over the NASA Wallops Flight Facility, VA, a region where no elevation change is expected within the time period of the surveys, and at Assateague Island, MD, a highly dynamic barrier island where several meters of erosion and deposition have been observed. Results show that use of the pulse correlation method generates elevation change estimates similar in magnitude to those obtained by simply differencing coincident laser altimeter elevation measurements (dubbed the spot comparison method). Along the beach at Assateague Island, MD, similar patterns of accretion and deposition are detected using both the pulse correlation and spot comparison methods, although some horizontal resolution is lost using the pulse correlation method because of the wide footprint spacing of the waveform-recording laser altimeter used in the study. In the test case presented here, increasing the size of the laser footprint from 25 to 60 m caused the magnitude of the vertical change signal to be underestimated, confirming that the resolution of the measurement technique and the scale of the deformation features should be considered when planning survey missions. The use of this method can improve the accuracy of surface change estimates made using laser altimeter waveforms, especially beneath vegetation, by eliminating the subjective interpretation of waveforms used to extract a single elevation measurement.