Estimation of the Three-Dimension Green Volume Based on UAV RGB Images: A Case Study in YueYaTan Park in Kunming, China

Abstract

Three-dimension green volume (3DGV) is a quantitative index that measures the crown space occupied by growing plants. It is often used to evaluate the environmental and climatic benefits of urban green space (UGS). We proposed the Mean of neighboring pixels (MNP) algorithm based on unmanned aerial vehicle (UAV) RGB images to estimate the 3DGV in YueYaTan Park in Kunming, China. First, we mapped the vegetated area by the RF algorithm based on visible vegetation indices and texture features, which obtained a producer accuracy (PA) of 98.24% and a user accuracy (UA) of 97.68%. Second, the Canopy Height Mode (CHM) of the vegetated area was built by using the Digital Surface Model (DSM) and Digital Terrain Model (DTM), and the vegetation coverage in specific cells (1.6 m × 1.6 m) was calculated based on the vegetation map. Then, we used the Mean of neighboring pixels (MNP) algorithm to estimate 3DGV based on the cell area, canopy height, and vegetation coverage. Third, the 3DGV based on the MNP algorithm (3DGV_MNP), the Convex hull algorithm (3DGV_Con), and the Voxel algorithm (3DGV_Voxel) were compared with the 3DGV based on the field data (3DGV_FD). Our results indicate that the deviation of 3DGV_MNP for plots (Relative Bias = 15.18%, Relative RMSE = 19.63%) is less than 3DGV_Con (Relative Bias = 24.12%, Relative RMSE = 29.56%) and 3DGV_Voxel (Relative Bias = 30.77%, Relative RMSE = 37.49%). In addition, the deviation of 3DGV_MNP (Relative Bias = 17.31%, Relative RMSE = 19.94%) is also less than 3DGV_Con (Relative Bias = 24.19%, Relative RMSE = 25.77%), and 3DGV_Voxel (Relative Bias = 27.81%, Relative RMSE = 29.57%) for individual trees. Therefore, it is concluded that the 3DGV estimation can be realized by using the Neighboring pixels algorithm. Further, this method performed better than estimation based on tree detection in UGS. There was 377,223.21 m3 of 3DGV in YueYaTan Park. This study provides a rapid and effective method for 3DGV estimation based on UAV RGB images.