Note: This map DOES NOT show real air routes, from 1920 or any other year! They are purely imaginary.
The only other information given is that it is from The People's Atlas and produced by the London Geographical Institute.
Now, this is interesting, because it most certainly does NOT show air routes in 1920: there were very, very few, and they certainly didn't criss-cross the world as this map suggests. Many of these routes had not been flown at all, let alone by regularly scheduled services. For example, here's a close-up of the North Atlantic:
There are 8 or 9 international routes leaving Britain, 5 of them out over the Atlantic, which had only first been flown in 1919. And I think the only overseas air routes operating from Britain in 1920 were London-Paris, London-Amsterdam and London-Brussels. It's possible that the date given is wrong, but all evidence points to the early 1920s at the latest, most likely 1919-21. The map shows Petrograd, which was renamed Leningrad in 1924. The 'Bristol Bomber' shown would be the Bristol Braemar, which last flew in 1921, and the 'Handley Page' is the Handley Page V/1500, withdrawn from RAF service the same year. Most significantly, perhaps, the British airship R34 occupies pride of place, and this was written off in 1921 after being damaged in strong winds, therefore the map was probably printed before this embarrassing accident.
So this map does not show actual air routes; it can only be a prediction of future ones.
More than that, I think it's a blueprint for British domination of the world's civil aviation industry. Firstly, note the colours. These show the distance an aircraft could fly from London in one (yellow), two (pink), three (green), four (light green) and five (brown) days, given a (ground?) speed of 100 mph. London is shown as the primary hub for flights to North America and a major European hub.1
Also, as noted above, R34 is featured prominently, and its primary claim to fame was its two flights across the Atlantic in 1919 -- first east-west and then west-east. Although considerably slower than the 100 mph assumed on the map, only an airship could stay aloft for the longest hauls shown, such as the 57 hour flight from Auckland to Valparaiso.2 In 1920, Britain was one of the few countries with the capability to build large rigid airships (even if they were largely copied from captured Zeppelins): the other main contender, Germany, was initially prevented from doing so under the Versailles treaty, and so Short Brothers built R38 for the US Navy in 1921. And although the aircraft of several nations are depicted below the R34, the British ones are the most impressive, particularly the V/1500.3 The next largest are the German Gotha and Zeppelin-Staaken,4 which again were now illegal for Germany to build. It seems to me that by emphasising the size of the aircraft shown -- and by not-always-the-case inference, their long range -- and in noting the long-duration flights needed to fly international air routes, the map maker was suggesting that the future of civil aviation belonged to the nation which possessed the longest ranged aircraft, and further, that this nation was Britain.
This all suggests a more optimistic British take on the aviation age than I generally see. And the time was perhaps right for this. The war was over, the economy was booming and everyone was busy demobilising, reconstructing and beating swords into ploughshares. A country with a near-monopoly of large airship construction, experience with building the largest long-range aircraft then flying and which could lay claim to the first three non-stop flights across the Atlantic would seem to be well placed for the coming era of international air travel.
But it didn't work out this way. For a start, the British 'airship moment' was brief. R38 broke up in mid-air over the Humber before it could be flown to America, and this put an end to military airship development. The 1924 Labour government initiated a civilian airship programme, with the aim of binding the colonies and dominions closer to the mother country. This eventually produced the successful R100, but also the less-fortunate R101, the crash of which put finally an end to Britain's airship ambitions.
Aeroplanes turned out to be a better bet, but not necessarily a safe one. In February 1921, Britain's international services were briefly suspended, because they couldn't compete with subsidised European routes, and only re-opened when they received (smaller) government subsidies of their own. This aside, British civil aviation was generally expected to "fly by itself" and this attitude came in for severe criticism by P. R. C. Groves in 1922, particularly since he saw civil and military aviation as mutually dependent. The RAF's big V/1500s were scrapped and no civil aircraft with comparable range and payload left British factories until the late 1930s (the Armstrong Whitworth A.W.27 and De Havilland D.H.91). Of course, given the tiny potential market for air travel in the 1920s, gargantuan airliners would never have been profitable to operate, but smaller British civil makes generally didn't fare well against German and American competitors either. Commentators continued to bitterly lament the sorry state of civilian aviation into the 1930s, Imperial Airways' lumbering H.P.42 biplanes coming in for particular criticism. Then came the war, the Brabazon, the Britannia and the Comet -- but that's a story for another day.
However, that the colours aren't bounded by ellipses shows that whoever drew the map was not assuming non-stop flights, but ones which would refuel at major hubs where possible. ↩
Of course, seaplanes could rendezvous with ships in mid-ocean to refuel, a technique Lufthansa used on its South American routes. ↩
It's odd that the one aeroplane which had flown the Atlantic non-stop -- the Vickers Vimy -- isn't shown; but maybe this is because it was a specially modified aircraft operating at the very limit of its endurance, and was not suitable for carrying passengers. The Vimy Commercial airliner variant had a much bigger fuselage, and a range of only 450 miles. ↩