D4 Precision Geomorphology and Topography

Topic

High-precision geomorphological surveying aims to detect changes in terrain elevation at scales ranging from millimeters to centimeters, in order to quantify the erosion, deposition, and mass-movement processes described elsewhere in this series of documents.

Airborne laser scanning emits laser pulses from an aircraft or helicopter and measures the time it takes for each pulse to return; based on this travel time, a digital terrain model can be generated with a resolution of twenty-five centimeters to two meters and a vertical precision of five to fifteen centimeters. Furthermore, this scanning method can penetrate vegetation to reveal the bare ground beneath.

Terrestrial laser scanning, performed from fixed points on the ground, achieves a much finer resolution—on the order of a millimeter—at distances of up to one kilometer; it is precisely this proximity to the terrain that allows the method to capture detail impossible to obtain from the air.

Drone-based photogrammetry, meanwhile, processes photographs that overlap by more than eighty percent; this high degree of overlap enables a 3D reconstruction technique—based on the apparent movement between successive images—to generate a point cloud comparable in quality to that produced by laser scanning.

Comparing digital terrain models acquired at different times makes it possible to quantify the sediment budget of an entire watershed or hillslope: areas where elevation increased between measurements indicate sediment deposition, while areas where elevation decreased indicate erosion.

Uncertainty in the comparison of digital terrain models propagates statistically, and it is precisely this propagation of uncertainty that allows for distinguishing truly significant elevation changes from those that are merely measurement noise. Thus, from the millimeters and centimeters with which this document begins to the measurement noise with which it concludes—encompassing airborne laser scanning, terrestrial laser scanning, and drone-based photogrammetry—geomorphological precision surveying ensures that erosion, deposition, and mass movement all leave a measurable imprint on the very elevation of the terrain.

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