Crown-CAM: Interpretable Visual Explanations for Tree Crown Detection in Aerial Images

Visual explanation of “black-box” models allows researchers in explainable artificial intelligence (XAI) to interpret the model’s decisions in a human-understandable manner. In this letter, we propose interpretable class activation mapping for tree crown detection (Crown-CAM) that overcomes inaccurate localization and computational complexity of previous methods while generating reliable visual explanations for the challenging and dynamic problem of tree crown detection in aerial images. It consists of an unsupervised selection of activation maps, computation of local score maps, and noncontextual background suppression to efficiently provide fine-grain localization of tree crowns in scenarios with dense forest trees or scenes without tree crowns. In addition, two intersection over union (IoU)-based metrics are introduced to effectively quantify both the accuracy and inaccuracy of generated explanations with respect to regions with or even without tree crowns in the image. Empirical evaluations demonstrate that the proposed Crown-CAM outperforms the Score-CAM, Augmented Score-CAM, and Eigen-CAM methods by an average IoU margin of 8.7, 5.3, and 21.7 (and 3.3, 9.8, and 16.5), respectively, in improving the accuracy (and decreasing inaccuracy) of visual explanations on the challenging NEON tree crown dataset.