Extracting Structural Characteristics of Dormant Herbaceous Floodplain Vegetation from Airborne Laser Scanner Data

To map spatial patterns of floodplain vegetation structure for hydrodynamic modelling, airborne laser scanning is a promising tool. In a test for the lower Rhine floodplain, vegetation height and density of herbaceous vegetation were measured in the field at 42 georeferenced plots of 200 m each. Simultaneously, three airborne laser scanning (ALS) surveys were carried out in the same areas resulting in three high resolution, first pulse, small-footprint datasets. The laser data surveys differed in flying height, gain setting and laser diode age. Laser points were labelled as either vegetation or ground using three different methods: (1) a fixed threshold value, (2) a flexible threshold value based on the inflection point in the normalised height distribution, and (3) using a Gaussian distribution to separate noise in the ground surface points from vegetation. Twenty-one statistics were computed for each of the resulting point distributions, which were subsequently compared to field observations of vegetation height. Additionally, the Percentage Index (PI) was computed to relate density of vegetation points to hydrodynamic vegetation density. The vegetation height was best predicted by using the inflection method for labelling and the 95 percentile as a regressor (R = 0.74 – 0.88). Vegetation density was best predicted using the threshold method for labelling and the PI as a predictor (R = 0.51). The results of vegetation height prediction were found to depend on the combined effect of flying height, gain setting or laser diode age. We conclude that high resolution ALS data can be used to estimate vegetation height and density of herbaceous vegetation in winter condition, but field reference data remains necessary for calibration until a standard measure of sensitivity is supplied together with the laser data.