The Dawn of Aerial Survey

The practice of using aerial photography for mapping and surveying, while perhaps seeming a modern innovation, has roots stretching back further than many might imagine. The advent of aerial survey methods was significantly propelled by the demands of conflict, and its subsequent application in peacetime has provided invaluable insights into our changing world. This article delves into the early days of aerial survey, focusing on its development and the pioneering work undertaken by the Ordnance Survey in post-war Britain.

The Impact of War on Surveying

The First World War acted as a crucial catalyst for the integration of air photography into mapping and survey operations, both on a global scale and specifically for the Ordnance Survey. While aerial survey methods were not a common feature of Ordnance Survey operations in the inter-war years, with only sporadic experimental work conducted in the 1920s, the landscape of surveying was irrevocably altered by the events of the Second World War. During this period, the Royal Air Force (RAF) captured millions of aerial photographs, irrefutably demonstrating their immense value for military reconnaissance and topographic surveying. Coupled with rapid advancements in camera technology, equipment, and techniques, it became evident by the war's conclusion that aerial survey methods would also prove tremendously beneficial for peacetime mapping initiatives.

Ordnance Survey's Air Photo Division

In response to this emerging potential, the Air Photo Division was established within the Ordnance Survey in 1945. This new division capitalised on the surplus aircraft and personnel readily available from the RAF. A significant undertaking during this post-war era was the extensive aerial photographic survey of Great Britain conducted by the RAF between 1944 and 1950, a project known as Operation Revue. This ambitious project was not solely for military purposes; it was also designed to aid in town planning and road construction efforts. The resulting photographs are now a vital part of the National Collection of Aerial Photography, offering a unique historical record.

It wasn't until the 1950s that the Ministry of Transport and Civil Aviation began to increasingly assume responsibility for aerial survey work, gradually taking over from the RAF. From this point forward, aerial survey has remained an indispensable component of the Ordnance Survey's methods for reconnaissance and data capture, underscoring its enduring importance.

Aircraft and Camera Technology

The post-war period was marked by significant budgetary constraints. Consequently, the Ordnance Survey had to utilise high-speed wartime aircraft, such as the iconic Spitfires and Mosquitos, rather than the more sedate Ansons that were better suited for the calmer demands of peacetime aerial survey. These aircraft underwent special modifications to accommodate two cameras beneath each wing, enabling stereoscopic image capture. The cameras were strategically angled to ensure considerable lateral overlap between the two images, a technique that became known as a "split" installation. This method was crucial for creating three-dimensional perspectives from the aerial photographs.

Camera Equipment

The F24 camera was a common choice for the RAF during the Second World War, typically equipped with focal lengths of 5 inches and 3¼ inches. Considerable progress was made during the war in developing mounting systems for cameras that effectively mitigated the disruptive effects of aircraft vibration. Initially, cameras were fitted with heaters to prevent condensation on the lenses and to stop them from freezing in the frigid upper atmosphere. However, these methods were progressively superseded by systems that channelled hot air from the aircraft's engines into enclosed camera compartments, offering a more reliable solution. The development of moving film cameras also played a vital role, as these were designed to compensate for the forward motion of the aircraft during exposure. This innovation allowed for the capture of significantly more detailed photographs, even when taken at larger scales.

The primary cameras employed by the RAF for vertical (directly overhead) photography during this period were the F24 and the Fairchild K17. These cameras were fitted with a range of lenses, offering focal lengths from six to forty inches, allowing for varying levels of detail and coverage.

The Art of Mosaic Creation

The process of creating aerial photo mosaics was a highly skilled and technically demanding endeavour. It involved several critical steps:

Rectification

Original photographs often suffered from distortion, particularly towards the edges of the image. To counteract this, a crucial step was the rectification of these photographs. This process involved geometrically correcting the images to ensure that features were represented in their true planimetric positions, removing the warping caused by the camera's angle and the curvature of the Earth.

Assembly

Once rectified, the overlapping sections of individual photographs were carefully cut and meticulously pieced together. This painstaking process resulted in the formation of a composite mosaic, a seamless representation of the landscape below. The precision required at this stage was paramount to ensure the accuracy and visual coherence of the final product.

Annotation

The final stage involved adding essential contextual information to the mosaics. This included lettering for major towns and cities, providing geographical reference points. Straight borders were also added to frame the mosaic, giving it a finished and organised appearance. These annotations were vital for making the mosaics usable for planning and administrative purposes.

Geographical Coverage, Scale, and Size

The air photo mosaics produced by the Ordnance Survey served as an interim measure, offering a quick and cost-effective solution before comprehensive paper mapping could be completed through traditional surveying methods. In total, the Ordnance Survey published an impressive 6,393 mosaics covering England, Scotland, and Wales.

The primary focus for the creation of these mosaics was generally on the more densely populated and built-up areas. These regions had the most pressing requirements for reconstruction and development, particularly in the aftermath of the widespread bomb damage experienced during the war. The mosaics provided a vital snapshot of the areas most affected and in need of planning.

Scale and Coverage Details

Each mosaic adhered to the standard National Grid sheet lines, ensuring consistency and compatibility with existing mapping systems. Typically, each sheet featured a mosaic area of approximately 52 x 61 cm (height x width). The scale of the mosaics varied, with different sheets covering different ground areas:

Collectively, these mosaics covered an estimated 18,960 square miles, or 49,100 square kilometres of Great Britain. This represented just over 20% of the total land area, providing a significant, albeit not exhaustive, aerial overview of the nation.

Publication, Censorship, and Enduring Value

Initially, the mosaics were intended strictly for official use, distributed only to local authorities, public utility services, and government departments. They were marked with a "Circulation Restricted" stamp, meaning they were not made available to the general public. The Town and Country Planning Act of 1947 placed an obligation on Local Authorities to produce plans every three years. However, given the Ordnance Survey's limited capacity to survey and publish large-scale maps promptly, the air photo mosaics presented the only viable means of fulfilling these planning requirements.

From 1948 onwards, an effort was made to recoup costs by offering the mosaics for sale to the public. Unfortunately, sales figures were disappointingly low. Furthermore, growing security concerns emerged, with fears that the mosaics might fall into the wrong hands and reveal sensitive information.

Security Concerns and Withdrawal

From 1950, newly revised mosaics were reissued for a select few key locations, including airfields and military installations. For these sensitive areas, later editions sometimes bear the edition letter 'B' in the lower left margin. These revised editions often featured obscured installations, with false fields or painted clouds used to conceal strategic sites. It is important to note that some mosaics first published in the late 1940s also contained doctored information. Security concerns escalated, and by March 1951, libraries were formally warned to withdraw the original, unedited 'true' mosaics from public access. The situation culminated in 1954 when the mosaics were completely withdrawn from sale. The rationale behind this decision was that changing security regulations would necessitate the meticulous checking of every single mosaic. It was deemed that the considerable effort involved would not be justified by the low sales volume.

Legacy and Usefulness

While the Ordnance Survey did not utilise these air photo mosaics to revise its large and medium-scale mapping due to perceived inaccuracies, their value for historical research is undeniable. They provide a unique, contemporary snapshot of the landscape, offering invaluable insights for those studying landscape history. For the Ordnance Survey, these mosaics represented a pragmatic, albeit temporary, solution to provide some form of landscape coverage before up-to-date regular mapping could be produced. In comparison to conventional Ordnance Survey maps, the mosaics often reveal greater detail due to their less generalised nature. They also showcase a rich variety of land uses, even though they typically contain fewer place names and descriptive annotations.

Additional information, often absent from regular maps, can be found on the mosaics. This includes details on land use and field boundaries on the 1:10,560 scale mosaics, and even insights into traffic flow on the 1:1,250 scale versions. The exact date of the aerial photography is also clearly indicated on each mosaic in the lower margin. Despite the challenges posed by 'doctored' information, falsified mosaics of this nature can usually be readily identified by researchers.

The Evolution of Aerial Survey

The journey from the rudimentary aerial observations of the early 20th century to the sophisticated satellite imagery of today is a testament to human ingenuity and technological advancement. The Ordnance Survey's air photo mosaics from the mid-20th century represent a significant chapter in this evolution. They highlight how necessity, particularly during times of war and its aftermath, can drive innovation in surveying and mapping techniques. The legacy of Operation Revue and the subsequent work of the Air Photo Division continues to inform our understanding of Britain's post-war development and the changing patterns of its landscape.

Frequently Asked Questions

Q1: When did aerial survey methods first begin to be widely used by Ordnance Survey?
Aerial survey methods were not generally employed by Ordnance Survey in the inter-war period, but their widespread use began after the Second World War, with the establishment of the Air Photo Division in 1945 and the subsequent Operation Revue survey (1944-1950).

Q2: What was the primary purpose of the Ordnance Survey Air Photo Mosaics?
They were produced as an interim measure to provide quick and cheap landscape coverage before proper paper mapping could be surveyed, particularly focusing on areas affected by bomb damage and requiring reconstruction.

Q3: Why were Spitfires and Mosquitos used for aerial surveys instead of slower aircraft?
Due to post-war budgetary constraints, these high-speed wartime aircraft had to be used, even though slower aircraft like Ansons were more suited to peacetime survey work.

Q4: Were the air photo mosaics available to the public from the start?
No, initially they were intended for official use only and were marked 'Circulation Restricted'. They only began to be offered for sale to the public from 1948, but were later withdrawn from sale completely in 1954 due to security concerns and low sales.

Q5: What kind of additional information can be found on these mosaics compared to regular maps?
The mosaics often show more detail because they are less generalised and can include information on land use, field boundaries, and even traffic flow, which are not typically found on conventional Ordnance Survey maps.

Further Reading

For those interested in delving deeper into the history of aerial photography and surveying, further resources can be found through national archives and historical mapping societies. Exploring collections of aerial photography covering the United Kingdom can provide a visual journey through Britain's past.