Timbo:

Probably a semantics question: Your GPS will always show you the shortest distance to the mark; your current speed of the vessel (as it moves in a straight line, not towards the mark): your heading (the direction you're currently going): and the bearing to the next mark (the shortest line to where you want to go). Sorry if I seemed confused, but "velocity is a rate of movement with a definite direction and is used in connection with a current. Speed is a rate of movement in any direction". p.341 Chapman Piloting

Wyatt