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 in peacetime mapping work. The extensive RAF aerial photographic survey of Great Britain (1944-1950), called Operation Revue, was intended to assist OS in revising its maps of the UK following the great changes that had taken place during World War II. From this imagery, aerial mosaics along National Grid sheet lines 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.
Creating the mosaics was a technically skilled job, involving:
1. Rectifying the original RAF photographs to compensate for distortion at the edges of the image
2. Carefully cutting overlapping sections of photographs and pasting them together to form a composite mosaic
3. Adding lettering for major towns and straight borders to frame the mosaic.
Each mosaic was drafted at a scale of six-inches 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.
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. Sales were poor and 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 and in March 1951, libraries were warned that the original 'true' mosaics should be withdrawn from public use; in 1954 the mosaics were withdrawn from sale completely.