Two novel methods for field measurements of hydrodynamic density of floodplain vegetation using terrestrial laser scanning and digital parallel photography

Hydrodynamic vegetation density, the sum of the projected plant area per unit volume, is an important parameter for floodplain flow models. This paper compares two novel techniques to quantify this parameter in the field using Terrestrial Laser Scanning (TLS) and digital Parallel Photography (PP). Field reference data consisted of (1) a stem map, describing the position and diameter of 650 trees in a single forest patch (2) 17 manually measured plots distributed over forest and herbaceous floodplain vegetation. PP consists of a series of digital photographic images of vegetation against a contrasting background. The centre columns of all images were merged into a single composite parallel image. This mosaic was thresholded to determine the fractional coverage of the vegetation, which is converted to vegetation density using the optical point quadrate method. TLS was carried out using a Leica HDS3000 time of flight laser scanner. Data processing of TLS data consisted of slicing the points around breast height. In a polar grid the vegetation density was predicted, using the optical point quadrate method corrected for missing points. Both methods were compared to the field reference data. PP (R² = 0.996, RSE = 0.0037) performed better than the TLS (R² = 0.77, RSE = 0.022). Advantage of the TLS method is the ability to provide a detailed 2D or even 3D distribution of the vegetation density. PP is cheaper, faster, and data processing is limited. We conclude that TLS and PP are two complementary techniques that show high accuracies for field measurements of vegetation density.