Extracting structural characteristics of herbaceous floodplain vegetation under leaf‐off conditions using airborne laser scanner data

Abstract

Hydrodynamic models of river flow need detailed and accurate friction values as input. Friction values of floodplain vegetation are based on vegetation height and density. To map spatial patterns of floodplain vegetation structure, 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 m² each. Simultaneously, three airborne laser scanning (ALS) surveys were carried out in the same area resulting in three high resolution, first pulse, small‐footprint datasets. The laser data surveys differed in flying height, gain setting and laser diode age. Point density of the laser data varied between 10 and 75 points m⁻². Point heights relative to the DTM derived from the ALS data were used in all analyses. 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 point 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 vegetation‐point distributions, which were subsequently compared with 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 ² = 0.74–0.88). Vegetation density was best predicted using the threshold method for labelling and the PI as a predictor (R ² = 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. The quality of the estimation of vegetation height and density is also affected by point density, for densities lower than 15 points m⁻². We conclude that high resolution ALS data allows to estimate vegetation height and density of herbaceous vegetation in winter condition, but field reference data remains necessary for calibration.

Acknowledgements

This work was funded by NWO, the Netherlands Organization for Scientific Research, within the framework of the LOICZ program under contract number 01427004. We would like to thank the Regional Directorate East Netherlands of the Dutch Ministry of Transport, Public Works and Water Management for the free use of all laser altimetry data. Ardis Bollweg and Regine Brügelmann of the AGI are gratefully acknowledged for timely notification of new laser scanning programs, which enabled simultaneous ground truth data collection. Chiel Kollaard and Andries Knotters are thanked for collecting field reference data of surface elevation and vegetation structure. Luc Amoureus of Fugro was very helpful in understanding the differences between the various datasets.