Earth's surface, our primary habitat, provides essential ecosystem and social services, such as carbon sequestration and food production. Numerous studies reveal that global changes are destabilizing the Earth's surface, as evidenced by extreme events' increasing frequency and complexity. These events trigger substantial losses across various sectors, including the economy and public health, necessitating accurate detection. Currently, large-scale monitoring of phenomena like vegetation disturbances and wildfires is achieved using remote sensing, with detection accuracy expected to improve through advanced machine learning techniques. However, these approaches primarily provide specific, event-based information, detecting only predefined types of events. Macroscopic mapping, which involves identifying these instabilities without relying on specific event types, remains unresolved despite its value for comprehensive detection and broader understanding.
To fill this gap, we propose a novel method for detecting unstable surfaces, termed Surface Anomalies (SAs). We hypothesize that a surface's evolution is influenced by its initial state and environmental factors within a given geographical region, including climate and land use. Consequently, homogeneous surfaces with similar initial states under comparable environmental conditions are expected to follow similar evolutionary trajectories. Building upon this hypothesis, SAs are defined as surfaces with evolutionary trajectories that deviate from their homogeneous counterparts. Compared to event-based definitions, our conceptual framework accommodates instabilities that are not predefined yet are nonetheless important, providing more comprehensive detection. Compared to algebraic change or anomaly detection methods, our definition offers a more precise characterization of SAs and has the potential to reduce irrelevant detections.
We operationalize this conceptual framework using remote sensing imagery in a two-stage process. In the first phase, we model the normal evolutionary patterns of a region. This involves acquiring a pair of baseline images where each pixel represents a surface, spectral values represent surface states, and differences between images represent evolutionary trajectories. We apply K-Means clustering with a sufficiently large number of cluster centers to segment the imagery, with each cluster corresponding to a type of homogeneous surface. For each homogeneous surface, we fit a Gaussian Mixture Model to the distribution of evolutionary trajectories, representing normalcy. In the detection phase, we acquire new image pairs from nearby locations and calculate the probability that their evolutionary trajectories fit within the GMM of the corresponding homogeneous surface model. Lower probabilities indicate higher instability. This probabilistic approach allows us to detect surface anomalies by identifying deviations from normal evolutionary patterns.
We evaluated our method's effectiveness by comparing it with traditional non-event-based approaches such as algebraic change detection and change vector analysis. This comparison was performed on a dataset encompassing various types of SAs, including wildfires, floods, volcanic activities, deforestation, and bark beetle infestations. Our method's results indicate significant improvements, substantially reducing false alarms and omissions. In summary, our method for detecting SAs from a macroscopic perspective has the potential to enhance our understanding of how Earth's surface responds to global change.