This paper presents a mathematical model of a leading line consisting of four beacons situated at the apexes of a rectangle. The ship handler compares angles between appropriate beacons to determine whether the vessel is in the middle of the fairway or not. This article assumes that the observer treats the two beacons on the front as a flat screen onto which rear beacons are projected. In this way he can estimate the quotients of sections created by appropriate beacons, from which he can find the lateral deviation from the waterway axis. This method of navigation is based on the Thales assertion. Described in detail, restrictions are the use of the model, precision assessment, and important in this respect, aspects of the waterway markings on Zalew Szczeciński.