It would not easily be practicable, however, to obtain proof of flying above 2000 ft. in the area between the yellow and red lines. It is considered that no adequate return would be received for the expenditure involved in the establishment of the observation posts referred to above, and indeed that no useful end would be served by establishing prohibited areas –whether complete or partial – in such a case as this. If you concur in he above and inform War Office accordingly, it is understood that the question will be referred to the Overseas Defence Committee with a view to obtaining a ruling which would cover any similar cases arising in the future. CGCA 23.6.20 Further Response of Air Ministry: 1. The question of espionage from aircraft was discussed between the Services in 1919. It was generally agreed that, while it is difficult or even impossible to ensure that civil aviation shall not prejudice the secrets of fortifications to some degree, other methods of espionage were surer and more accurate. Prohibited areas in the absolute sense were consequently abolished though certain areas were not to be flown over at a less height than 6,000 feet. This provision would make observation slightly less easy but at the same time it was drafted with a view to obviating danger from forced landings. 2. G.O.C. Hong Kong is no doubt unaware of these decisions. When informed of them he will probably model his policy on the lines of that adopted at Home. Possibly the O.D. Committee could advise local authorities on this subject. 3. As regards the particular measures proposed for Hong Kong, the Air Staff offer the following comments: a) The regulations are somewhat complicated and the distance between the yellow and red lines is insufficient to account for the differentiation made between them. b) The rules are unduly strict in that within the red line there are any number of ground observation posts at greater heights than those to which seaplanes are restricted. This is inconsistent, since telescopes and other instruments can be used more readily from the ground than from aircraft. c) Not only are aircraft flying at a height of 150 feet over a crowded harbour dangerous to shipping, but such extreme low flying is impossible under any conditions. d) Wind currents at Hong Kong are likely to be such that low flying will be very undesirable; lighter than air craft in particular would be most unduly tossed about.
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