Lenart derives boundaries of a Collision Threat Parameters Area (CTPA), which defines the range of velocity and heading which own vessels must not adopt if it wishes to maintain a specified minimum spacing from a given threat. Lenart's treatment of the problem is algebraic, and a geometric demonstration of the validity of the CTPA boundaries may be of interest.

In Fig. 1, O is the position of own vessel. OX and OY are Cartesian axes, fixed in direction as own vessel manoeuvres. T is the position of a threat, whose velocity is represented on the diagram by a line TT´ whose length gives the distance the threat will travel, relative to own vessel's present position, in some time τ, 0·2 hours, say.