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Measuring forests with dual wavelength lidar: A simulation study over topography

Hancock, S; Lewis, P; Foster, M; Disney, M; Muller, J-P; (2012) Measuring forests with dual wavelength lidar: A simulation study over topography. AGRICULTURAL AND FOREST METEOROLOGY , 161 123 - 133. 10.1016/j.agrformet.2012.03.014. Green open access

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Abstract

Accurate measurements of biophysical parameters are essential for understanding the distribution and dynamics of global vegetation, which exerts an influence on the carbon cycle and atmospheric circulation. Spaceborne, large footprint lidar has been shown to be a valuable tool. It is capable of measuring denser forests than other existing remote methods. However large-footprint lidar struggles to separate ground and canopy signals over topography and in the presence of short vegetation. This prevents the physically-based measurement of forest properties (such as canopy height and cover) at an acceptable accuracy (sub 10 m root mean square error for height) without the use of external data. The necessary external datasets are not yet available at a global scale at high accuracy. In this paper the issues of measuring forests with large-footprint, monochromatic lidar are presented. A number of subtle effects, such as shadows beneath crowns, can hamper the reliable measurement of forests. It is proposed that a dual wavelength lidar will allow the separation of canopy from ground returns in these situations and so allow the physically-based measurement of forests. An initial algorithm is developed and tested with Monte-Carlo ray tracer simulations as a proof of concept. Some refinements are needed to make the method more robust, but the initial form was found to determine the start of the ground return over steep slopes and a range of forest densities, canopy heights and vertical structures with a root mean square error (RMSE) of 2.7 m and mean bias of 67 cm for canopies with covers below 99%. This resulted in canopy height RMSE of 2.88 m with a bias of −23 cm. Such a system will allow measurement of a much broader range of forests than is possible with monochromatic lidar and could form a second generation spaceborne lidar mission.

Type: Article
Title: Measuring forests with dual wavelength lidar: A simulation study over topography
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.agrformet.2012.03.014
Publisher version: http://dx.doi.org/10.1016/j.agrformet.2012.03.014
Language: English
Additional information: This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Keywords: Lidar, Topography, Tree height, Dual-wavelength, Understory, Large-footprint
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
UCL > Provost and Vice Provost Offices > UCL SLASH
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS
UCL > Provost and Vice Provost Offices > UCL SLASH > Faculty of S&HS > Dept of Geography
URI: https://discovery.ucl.ac.uk/id/eprint/1352228
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