Back in 1914, an army surveyor who was carrying out an airborne reconnaissance of potential sites for a new military aerodrome to the west of London is alleged to have held his map upside down – and that, consequently, RAF Northolt was built to the south of what was then called the Great Central Railway Line instead of about a mile to the north-east near where South Ruislip is now. A short apocryphal story perhaps, undoubtedly distorted by the mists of history, but it serves to illustrate the need for aeronautical charts that are designed with the requirements of visual air navigation firmly in mind.