Ordnance Survey Air Photo Mosaics of Scotland, 1944-1950
These air photo mosaics provide key information on the landscape of post-war Scotland, including good detail of urban topography and land-use. They complement paper mapping, and represent the first widespread use of aerial survey methods by Ordnance Survey.
The sections below describe:
- Background to aerial survey methods
- Aircraft and cameras
- Creating the mosaics
- Geographical coverage, scale and size
- Publication, censorship and security concerns
Background to aerial survey methods
The First World War was an important stimulus to the use of air photography in mapping and survey work, both internationally and for Ordnance Survey. Aerial survey methods were not generally employed by Ordnance Survey in the inter-war period, however, apart from experimental work in the 1920s.
However, during the Second World War, millions of air photos were captured by the Royal Air Force. Their value for military reconnaissance and topographic surveying was proved beyond all doubt.
With rapid improvements to cameras, equipment and techniques, it was also clear by the end of the war that aerial survey methods would have a great value too in peacetime mapping work. In 1945 the Air Photo Division was established in Ordnance Survey, utilising the surplus aircraft and personnel from the RAF.
The extensive RAF aerial photographic survey of Great Britain (1944-1950), called Operation Revue, was also intended to assist town planning and road building. Around 500 sorties were flown in Scotland resulting in the collection of over 280,000 photographs. These are held in the Royal Commission on the Ancient and Historical Monuments for Scotland.
It was not until the 1950s, that the Ministry of Transport and Civil Aviation increasingly took over aerial survey work from the RAF. From this time, aerial survey has been an essential part of Ordnance Survey's reconnaissance and data capture methods.
Aircraft and cameras
cameras under wings.
Due to post-war budgetary constraints, high-speed wartime aeroplanes such as Spitfires and Mosquitos had to be used, rather than the slower Ansons that were more suited to peacetime aerial survey. The aircraft were specially modified to take two cameras – for stereoscopic capture – beneath each wing.
During the Second World War, great progress had been made in mounting cameras to avoid aircraft vibration, and ducting hot air from the engine to the cameras to prevent them from freezing up.
The F24 and Fairchild K17 cameras were mainly used by the RAF at this time for vertical (i.e. overhead) photography, with a range of lenses from six- to 40-inch focal lengths.
Creating the mosaics
Creating the mosaics was a technically skilled job, involving:
- Rectifying the original photographs to compensate for distortion at the edges of the image
- Carefully cutting overlapping sections of photographs and pasting them together to form a composite mosaic
- Adding lettering for major towns and straight borders to frame the mosaic.
Geographical coverage, scale and size
The mosaics were produced as an interim measure by OS, as a quick and cheap expedient before proper paper mapping could be surveyed. In total 221 mosaics were published of Scotland, focusing on the more settled areas that had greatest requirements for reconstruction and development.
Each mosaic was drafted at a scale of six-inch to the mile or 1:10,560, and followed standard National Grid sheet lines. Each sheet has a mosaic area of approximately 52 x 61 cm (height x width). Each sheet covers an area on the ground of 25 square kilometres. In total some 2,130 square miles, or 5,525 square kilometres in Scotland are covered by the mosaics.
Publication, censorship and security concerns
The mosaics were originally intended for official use only, but were offered for sale to the public from 1945 - 1947 in an effort to recoup costs. Unfortunately, sales were poor and in addition, there were security concerns that the mosaics might fall into the wrong hands.
From 1950, newly doctored mosaics were re-issued for a few key locations, including airfields and military installations. For these sensitive locations, a false landscape of fields and hedgerows was carefully drawn in, or the site obscured by clouds!
Security concerns grew. In March 1951, libraries were warned that the original 'true' mosaics should be withdrawn from public use, and in 1954 the mosaics were withdrawn from sale completely.