Just a point regarding the 400ft AGL rule in the UK. How are people staying compliant with this in practical terms in wildly undulating environments such as Scottish mountain regions? I am told that no matter what we cannot break this rule when AGL is measured directly vertically down from the aircraft. And that if we take off from a mountain ridge, we cannot fly to the sides of it otherwise we break this rule unless we follow the side contour of the mountain. But since we do not have any real time AGL instrument, and judging it by eye is nigh on impossible, how can this rule be effectively followed in such environments?