Structure from Motion (SfM) Photogrammetry with Drone Data: A Low Cost Method for Monitoring Greenhouse Gas Emissions from Forests in Developing Countries

Abstract

Structure from Motion (SfM) photogrammetry applied to photographs captured from Unmanned Aerial Vehicle (UAV) platforms is increasingly being utilised for a wide range of applications including structural characterisation of forests. The aim of this study was to undertake a first evaluation of whether SfM from UAVs has potential as a low cost method for forest monitoring within developing countries in the context of Reducing Emissions from Deforestation and forest Degradation (REDD+). The project evaluated SfM horizontal and vertical accuracy for measuring the height of individual trees. Aerial image data were collected for two test sites; Meshaw (Devon, UK) and Dryden (Scotland, UK) using a Quest QPOD fixed wing UAV and DJI Phantom 2 quadcopter UAV, respectively. Comparisons were made between SfM and airborne LiDAR point clouds and surface models at the Meshaw site, while at Dryden, SfM tree heights were compared to ground measured tree heights. Results obtained showed a strong correlation between SfM and LiDAR digital surface models (R 2 = 0.89) and canopy height models (R 2 = 0.75). However, at Dryden, a poor correlation was observed between SfM tree heights and ground measured heights (R 2 = 0.19). The poor results at Dryden were explained by the fact that the forest plot had a closed canopy structure such that SfM failed to generate enough below-canopy ground points. Finally, an evaluation of UAV surveying methods was also undertaken to determine their usefulness and cost-effectiveness for plot-level forest monitoring. The study concluded that although SfM from UAVs performs poorly in closed canopies, it can still provide a low cost solution in those developing countries where forests have sparse canopy cover (<50%) with individual tree crowns and ground surfaces well-captured by SfM photogrammetry. Since more than half of the forest covered areas of the world have canopy cover <50%, we can conclude that SfM has enormous potential for forest mapping in developing countries.

Estimating Tree Height and Diameter at Breast Height (DBH) from Digital Surface Models and Orthophotos Obtained with an Unmanned Aerial System for a Japanese Cypress (Chamaecyparis obtusa) Forest

“…Digital surface models (DSMs) and ultra-fine resolution orthophotos can be directly utilized to estimate tree heights and canopy size. Usually, these data are difficult to extract correctly because the height of a tree depends on site conditions (higher/lower stem density; [33]), decreased interpretation of surface area at dense forest sites [30], or even exposure to light and wind [34,35]. In addition, directly estimating parameters in lower layers, such as DBH (Diameter at Breast Height), from DSM or orthophotos is also difficult because of tree canopies, particularly in dense forest environments.…”

“…Moreover, this technique can be used to construct 3D models of an area through photogrammetry. This means that there are possibilities for extracting forest inventory data from UAS-sensed information [18,29,30,32]. Digital surface models (DSMs) and ultra-fine resolution orthophotos can be directly utilized to estimate tree heights and canopy size.…”

“…Recent studies have reported the use of UAS in various analyses, including forestry applications [27,28]. Collecting aerial photos by UAS and processing them with the structure from motion (SfM) method is one of the major new approaches to analyzing forest structures [29,30]. SfM is a photogrammetric method of constructing a three-dimensional (3D) model based on multiple two-dimensional images (e.g., photos; [31]), which is receiving much attention in the remote sensing field with the use of airborne platforms.…”

Woodland Mapping at Single-Tree Levels Using Object-Oriented Classification of Unmanned Aerial Vehicle (Uav) Images

“…Progress made in internal sensors, reduction of costs, more flexibility compared to spaceborne and airborne platforms, and precise GPS embedded are some of unique advantages that have enhanced the application of UAVs in a wide range of subjects. Recent studies have exhibited the feasibility of using UAVs in monitoring (Torres-Sanchez et al, 2015;Vega et al, 2015;Mlambo et al, 2017) and measurement of characteristics plant species (Chianucci et al, 2016;McNeil et al, 2016;Ivosevic et al, 2017). In contrary, relatively few investigations have studied the applicability of orthoimages taken by UAVs for mapping biophysical characteristics of trees and shrubs at single-tree levels.…”

Multiseasonal Tree Crown Structure Mapping With Point Clouds From Ots Quadrocopter Systems

“…Various studies showed the full potential of these small and flexible platforms (f.e. Mlambo et al 2017). An overview is given in Colomina and Molina (2014).…”