Retrieving Mangrove Biophysical Properties using Drone-Based Structure-from-Motion (SfM) Photogrammetry

Abstract:

Determination of biophysical properties of mangroves, e.g. tree height and diameter at breast height, is necessary for assessing mangrove ecosystem dynamics and growth. However, traditional surveying methods (e.g. diameter at breast height, location with GPS, and tree’s height with hypsometer) are time consuming and expensive. In this research we aim to assess mangrove properties by Unmanned Aerial Vehicles (or: UAV/drone) based photogrammetry. The additional benefit of this methodology is that mangrove environments which are often difficult to access can be reached. We focus on Porong Delta, Indonesia. In this area rapid delta progradation has taken place since 2006 due to an extreme mud volcano eruption. The regional climate conditions and added nutrient flux to the new delta have provided a suitable environment for growth of different mangrove species, such as Avicennia spp., Rhizophora spp., and Sonneratia spp. We used an off-the-shelve drone to generate a high-resolution spatial canopy height model (CHM) for the mangroves by using structure-from-motion (SfM) photogrammetry with an achieved DEM resolution of 5.2cm/pixel. This approach is used to determine the structural characteristics of mangrove stands and is validated with ground-truthing on two deltas. The dense point cloud derived from SfM photogrammetry is processed in LAStools and bare earth extraction with a Cloth Simulation Filter method. The CHM is generated from the processed point clouds. The structural information of mangroves is retrieved by using the lidR package in R. Our analysis lead to a median tree height on north and south delta of 4.2m and 3.5m, respectively. Analysis of photogrammetry data shows that drone-based observations of the mangrove canopy height are a useful tool to provide trustworthy data of mangroves characteristics.