Street Tree Crown Detection with Mobile Laser Scanning Data Using a Grid Index and Local Features

In recent years, targeted spraying technology, which was proposed to solve the problems of pesticide waste and environmental pollution caused by traditional spraying methods, has been successfully applied in orchards. In street scenes with a variety of object classes, it is challenging to detect tree crowns, which limits the application of targeted spraying for street trees. Two-dimensional (2D) light detection and ranging (LiDAR) sensors have been widely used in targeted spraying to monitor the presence of tree crowns. Considering a mobile laser scanning (MLS) system with a single 2D LiDAR sensor in push-broom mode, this paper proposes a pointwise method for street tree crown detection from MLS point clouds by using a grid index and local features. First, an efficient two-level neighbourhood search method is proposed to obtain the spherical neighbourhood of a single point by using the grid index of the MLS point clouds. Subsequently, a set of local statistical features, including width features, depth features, elevation features, intensity features, echo number features, dimensionality features and a density feature, are extracted from the spherical neighbourhood. Finally, a supervised learning algorithm called boosting is used to automatically fuse these features and generate a pointwise tree crown detector from a labelled training set. An MLS point cloud with 15,134,000 points is captured from both sides of a 136.5 m street, and the cloud contains buildings, lanes, sidewalks, benches, street lights, bicycles, traffic signs, grids, trees, bushes, turf areas, parterres, and pedestrians. The estimated Bayesian errors of single-feature approaches range from 6.23 to 36.09%, and the error rate of the tree crown detector composed of all features is less than 0.73%, with a recall rate of over 98.30% and a precision of over 99.13%. The experimental results show that the proposed method can provide an online, fine and accurate protocol for targeted spraying.