UAV-Based Digital Terrain Model Generation under Leaf-Off Conditions to Support Teak Plantations Inventories in Tropical Dry Forests. A Case of the Coastal Region of Ecuador.

José R Rivas,Fernando J Aguilar,Manuel A Aguilar,Alberto Peñalver,Abderrahim Nemmaoui

doi:10.3390/s19081934

Abstract

Remote sensing is revolutionizing the way in which forests studies are conducted, and recent technological advances, such as Structure from Motion (SfM) photogrammetry from Unmanned Aerial Vehicle (UAV), are providing more efficient methods to assist in REDD (Reducing Emissions from Deforestation and forest Degradation) monitoring and forest sustainable management. The aim of this work was to develop and test a methodology based on SfM from UAV to generate high quality Digital Terrain Models (DTMs) on teak plantations (Tectona grandis Linn. F.) situated in the Coastal Region of Ecuador (dry tropical forest). UAV overlapping images were collected using a DJI Phantom 4 Advanced© quadcopter during the dry season (leaf-off phenological stage) over 58 teak square plots of 36 m side belonging to three different plantations located in the province of Guayas (Ecuador). A workflow consisting of SfM absolute image alignment based on field surveyed ground control points, very dense point cloud generation, ground points filtering and outlier removal, and DTM interpolation from labeled ground points, was accomplished. A very accurate Terrestrial Laser Scanning (TLS) derived ground points were employed as ground reference to estimate the UAV-SfM DTM vertical error in each reference plot. The plot-level obtained DTMs presented low vertical bias and random error (−3.1 cm and 11.9 cm on average, respectively), showing statistically significant greater error in those reference plots with basal area and estimated vegetation coverage above 15 m2/ha and 60%, respectively. To the best of the authors’ knowledge, this is the first study aimed at monitoring of teak plantations located in dry tropical forests from UAV images. It provides valuable information that recommends carrying out the UAV image capture during the leaf-off season to obtain UAV-SfM derived DTMs suitable to serve as ground reference in supporting teak plantations inventories.

Highlights

Forests play a very important role in the balance of the terrestrial carbon cycle [1,2]
In order to obtain high quality Digital Terrain Models (DTMs) as ancillary data to support efficient teak plantations inventories in to theobtain tropical dryquality forest located in ancillary the coastal region of Ecuador, and considering that
DTMs as data to support efficient teak plantations most of the teak plantations areforest managed under rainfed conditions, results obtained in this work inventories in the tropical dry located in the coastal region ofthe

Summary

Introduction

Forests play a very important role in the balance of the terrestrial carbon cycle [1,2]. In 2010 the global area of planted teak forests reported from 38 countries was estimated at 4.35 million ha, which 83% grew in Asia, 11% in Africa, 6% in tropical America and less than 1% in Oceania [7], taking into account the data missing from 22 teak growing countries, these figures could underestimate the actual planted teak forests in the world According to this data, planted teak forests in Ecuador were around 45,000 ha in 2010, being this exotic species introduced more than 50 years ago in the province of Los Ríos, where it adapted and grew, becoming the main source of seeds for establishing commercial plantations [8]. Progress in the suggested goals expressed as reforested area has been modest [8], it is difficult to find reliable figures about the dynamic of new teak plantations due to a sub-registration and lack of updated inventories

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