Most fields will have obstructions of some sort. If these are only a few feet high, they are of no consequence because the touch-down point aimed for will be some way into the field. Once the boundary fence is cleared, it will be usual to open the airbrakes fully and land short of the intended point – with the exception that if the field is very long the brakes might be closed to allow the glider to float along its length.

The same principle applies if there are higher obstructions, except that the higher they are the more the clearance that should be allowed. This is for the fairly obvious reason that obstructions create turbulence which may have an adverse effect on the approach. As a measure of the amount of field cut off by obstructions, eight times the height by which they are cleared is a reliable guide – fifty feet cuts off 400 hundred feet, which may be half the length of an otherwise adequate field.