Then again, why use 6 minute markers when there may be nothing there to see? Try this in the desert states of the USA or even in East Anglia and you will find that most of such markers are useless. Surely it is better to highlight the most obvious and unique features, at about 6-10 minute intervals, and use the closing angle technique to calculate heading changes. There will, of course, be those who consider it more important to stay on the arbitrary line they have drawn on the map than to reach their destination, but it does take all sorts....