Multispectral Canopy Lidar
University of Edinburgh with Selex Galileo
Fast Track Project
This project demonstrated the feasibility of a multispectral canopy LIDAR (patent pending) for detailed structure and physiology measurements in forest ecosystems through the design and construction of a bread-board instrument.
The basic principle is to combine in one instrument the proven strengths of passive multispectral sensing to measure plant physiology (through the NDVI and PRI indices) with the ability of LIDAR systems to measure vertical structure information. Such an instrument is potentially superior to the use of single wavelength LIDAR systems combined with passive multispectral data since it generates “hot spot” reflectance data independent of solar illumination and is able to penetrate to otherwise shaded regions of the canopy understory. In addition to offering information on the vertical distribution of physiological processes, it also has value in separating canopy from ground returns, and in the calibration of passive multi- and hyper-spectral instruments. Such measurements will allow better mapping of forest structure and processes directly related to photosynthesis, significantly improve our ability to measure and map the terrestrial biosphere and our understanding of the carbon cycle, land cover use and biodiversity.
The breadboard instrument was built by Selex-Galileo. It was constructed with a tuneable laser that offered a degree of flexibility to investigate different wavelengths. In this project, four wavelengths were selected – 531, 550, 660 and 780 nm. These were selected to allow measurement of two indices commonly used in passive sensing, namely, the Normalised Difference Vegetation Index (NDVI) (660 and 780nm) and the Photochemical Reflectance Index (PRI) (531 and 550 nm). Since health and safety constraints hindered the use of the instrument from a forest tower, laboratory-based measurement were conducted for two trees and pseudo-vertical profiles were obtained for the two indices. Although not fully optimised, the instrument did allow realistic values of the indices to be measured. A follow-on PhD studentship is planned to expand on these measurements. In parallel to the laboratory measurements a model-based analysis was also conducted. The concept for a multi-spectral canopy LIDAR (MSCL) instrument was tested by simulating return waveforms using models providing tree structure (TREEGROW) and leaf reflectance (PROSPECT). The modelling was used to assess the structural and physiological information content that such a device could provide, especially if the normally structure-dominated return waveform would pick up small changes in reflectance at the leaf level. Multi-spectral waveforms were simulated for models of single Scots pine trees of different ages and at different stages of the growing season. It was shown that the LIDAR waveforms would not only capture the tree height information, but would also pick up the seasonal and vertical variation of NDVI computed from two of the four MSCL wavelengths inside the tree canopy. It could be demonstrated that a new multi-wavelength LIDAR predictor variable could significantly improve the retrieval accuracy of photosynthetically active biomass as opposed to using a single wavelength LIDAR alone. It remains unclear, however, if these findings would persist for forest stands; thus such experiments simulating more complex scenarios will be the next task in this modelling framework. The work is led by Dr Iain H Woodhouse, University of Edinburgh with Selex Galileo Ltd