Because the contour lines are at 50 foot intervals we can only estimate what the terrain between each contour looks like. An 40 foot cliff could easily hide between contours and not be recorded on the map. With this in mind it is good to remember that while these maps are generally very good, there is still room for the odd surprise. One of the most difficult things to interpret on a contour map is a sense of elevation, ridges, and valleys. Here are a few tips.
Water always flows down through valleys or gullies, never ridges.
Creeks begin at higher elevations and flow down to lower elevations where they join to form larger but fewer rivers. When contours form a bulge that points from a lower elevation to a higher elevation, it is a gully, valley, or bowl. When contours form a bulge from higher elevations to lower elevations it is a ridge. Grid Lines and Grid References On UK Ordinance Survey maps there are two sets of parallel lines which mark the grid lines. These lines are in black and run vertically and horizontally on the map.