A Method for Correcting the Closest Point of Approach Index During Vessel Encounters Based on Dimension Data From AIS

Abstract

In the field of maritime safety, ship encounters are a core component of traffic management and analysis. The calculation of the closest point of approach (CPA) is one of the most important methods for collision risk assessment during an encounter. Due to the factors influencing collision risk in water transportation, the existing CPA index exhibits uncertainties in the awareness of dangerous situations. In this paper, to overcome the shortcoming of the CPA index, which does not consider the vessel dimension, we utilize a polygon to represent the ship position based on related information from automatic identification system (AIS) and propose a method to obtain corrected values of the distance at the CPA (DCPA) and time to the CPA (TCPA) through the geometric relationship of the relative motion between involved vessels. Case analysis is conducted through practical encounter cases in a traffic-intensive area outside a port. The comparison results verify that the proposed method effectively reduces the uncertainties of the original point-based CPA index to better assess the collision risk. The advantage of our correction method is that the dimension-related influencing factors can be integrated without manual intervention, and the improved index retains the objectivity and clear physical meaning of the original CPA index. Therefore, the proposed method has high practical value, especially in busy water areas where a high-risk assessment accuracy is required.