Drone-based structure-from-motion photogrammetry captures grassland sward height variability

1. Grasslands deliver a range of ecosystem services, including the provision of food and biodiversity, and regulation of soil carbon storage and hydrology. Monitoring schemes are needed to quantify spatial changes in these multiple functions alongside ecosystem degradation. Sward height is widely recognised as a key spatial variable in the provision of these services. Current manual monitoring approaches are labour intensive, and often fail to capture spatial patterns of important features, including sward height. 2. Proximal sensing from small aerial drones carrying lightweight cameras can be transformed into surface height models using image-based structure-from-motion and Multi-View Stereo-based approaches; this presents a new opportunity for monitoring the spatial structure of grassland sward height. We combined aerial photographs with field survey data and an open-source image-based modelling-processing workflow to generate sward height measurements for a field comprising mainly Lolium perenne (perennial ryegrass) and Trifolium pratense (red clover). We compared the derived measurements with in situ data captured on the same day using traditional agronomic sward height techniques to determine the quality of the drone-derived surface model product for sward characterisation. 3. The SfM and Multi-View Stereo-based surface model had a mean absolute sward height measurement error of between 3.7 and 4.2 cm. To produce field observations with equivalent quality would require up to 550 sward height measurements for the study site (area: 8,059 m(2)), which is not feasible over larger extents required for conservation of key species or agronomic purposes. 4. Synthesis and applications. We demonstrate how the collection of precise and detailed information on the spatial structure of grasslands can be made over management-relevant extents. Aerial digital photographs can be transformed into surface models using an image-based modelling approach: structure-from-motion and Multi-View Stereo techniques. Image-based measurements of sward heights were compared with manual sward height data captured on the same day. This novel source of vegetation spatial information could improve sward management for conservation and agronomy applications. The approach supports frequent surveys, at user-controlled revisit times, and delivers data for spatial monitoring of key grassland functions and services.