I do apologise for the length of this post but I do believe that having a full understanding of the rules when flying off mountains and hills is important.
It appears to me that there are two interpretations of the 400’ AGL rule when flying a drone in hilly or mountainous areas. Basically, it boils down to how AGL should be measured. In the diagram posted by #MertOzkan and which was produced by the CAA, it clearly indicates (to me anyway) that the 400’ is measured between drone and ground parallelly. Thus, the 400’ AGL line follows the terrain below it in the form of a contour line. When contour flying at 400’ AGL, vertical height, but not parallel AGL height, may exceed 400’AGL occasionally on steep ground. I believe this is acceptable and does not contravene the DroneSafe code. However, I am a newbie to drone flying so happy to be challenged but I have had 35 years’ experience in interpreting and applying the law.
The other interpretation suggests that the 400’ AGL height is measured with an imaginary plumb line dangling from your drone. Adhering to this interpretation on steep ground, would bring your drone very close to the hill, cliff or whatever and potentially pose a risk to climbers, walkers etc. who may not be in your line of sight although you still have sight of your drone. This interpretation does not make sense to me.
The ‘plumb line’ interpretation is supported by https://approveddronepilots.co.uk/lets-talk-about-400ft/. But I think they’ve got it wrong (says a mere newbie). They support their case by saying that if you don’t abide by their ‘plumb line’ you could enter the buffer zone between drone and manned AC. They have produced an interesting little diagram which you can find using the above link.
I have amended their diagram to support my argument that if both drone and AC abide by the rules i.e. 400’AGL for drones and minimum of 500’ AGL for manned AC there will not be a problem. Note that military AC may well be below 500’ AGL when flying in the mountains. You need to know the routes they use, but they are generally well publicised .
In my diagram, I have added a another contour line above the 400’ AGL line and this represents the 500’AGL being the minimum height for civil AC (certain exceptions excluded) and hopefully shows that if the 400’ and 500’AGL heights are measured parallelly and airspace users stick to it, there should not be any conflict even though at times drones may well be over 400’ AGL when measured vertically.
Finally, I have decided that to give me plenty of height in reserve when flying off a ridge or peak, generally there is little point going up to 400’ AGL as after all you are pretty high up anyway.
Look forward to your views and any other interpretations!