Terrestrial Laser Scan Error in the Presence of Dense Ground Vegetation

Terrestrial laser scanning (TLS) data-sets are seeing increasing use in geology, geomorphology, forestry and urban mapping. The ease of use, affordability and operational flexibility of TLS suggest that demand for it is likely to increase in large-scale mapping studies. However, its advantages may remain restricted to specific environments, because of difficulties in defining bare-ground level in the presence of ground-level vegetation. This paper seeks to clarify the component contributions to TLS elevation error deriving from vegetation occlusion, scan co-registration error, point cloud georeferencing error and target position definition in TLS point cloud data. A multi-scan single-returns TLS point cloud data-set of very high resolution (∼250 points/m2) was acquired for an 11 hectare area of open, substantially flat and 100% vegetated coastal saltmarsh, providing data for the empirical quantification of TLS error. Errors deriving from the sources discussed are quantified, clarifying the potential proportional contribution of vegetation to other error sources. Initial data validation is applied to the TLS point cloud data after application of a local-lowest-point selection process, and repeat validation tests are applied to the resulting filtered point cloud after application of a kriging-based error adjustment and data fusion with GPS. The final results highlight the problem of representing bare ground effectively within TLS data captured in the presence of dense ground vegetation and clarify the component contributions of elevation error deriving from surveying and data processing.