Unveiling the Past: 2 million aerial photos from 60 Countries offer fresh insights into 20th Century History

1.8 million black-and-white aerial photographs, seven digitisation robots in an industrial building on the outskirts of Edinburgh and an international research team co-led by two economists at Stockholm University. The connection between these can be of great importance for several research areas, thanks to a research project in which a large and previously inaccessible archive is digitised and made freely available by the end of the year.

Photo: National Collection of Aerial Photography / SU

The story begins in 2014, at a research conference in London, where Andreas Madestam - economist at Stockholm University, hears about a huge archive of images. These are aerial photographs taken during the period 1939-1999 and commissioned by the British public authority Directorate of Colonial Surveys, which later became the Directorate of Overseas Surveys.

The aerial surveys initially covered countries that were at the time colonised by Britain and later, post-independence, expanded coverage to more than 60 countries worldwide. The area covered by the aerial photos is thus a significant part of the Earth's land surface, especially in sub-Saharan Africa. There are approximately 1.8 million photos in total.

Rescued from storage in damp conditions

The photos were taken to create maps, but as satellites came to take over the tasks of mapping and imaging the earth's surface, aerial photography became less and less important. Eventually, the Directorate of Overseas Surveys ceased operations, film negatives were dispersed to the countries they covered, and two print collections were archived.

But the museum holding one of the archives ran into financial and legal difficulties. The aerial photos were stored in damp basements, some of them were affected by mould, and the entire archive was nearly destroyed.

The National Collection of Aerial Photography (NCAP) in Edinburgh took custody of the archive and moved it into safe storage conditions. But there was no funding available to digitise the archive and make it accessible for research. And scanning the archive manually could have taken many years.

A gold mine for researchers

This vast archive of aerial photographs, covering parts of the world which often lack other historical records, is a gold mine for researchers. The photos show cities, villages, road networks, forests, and cultivated land, among others. Since they were taken over 60 years ago, it is possible to see how human influence has changed a large part of the earth’s surface.

After hearing about the archive, Andreas Madestam, together with Anna Tompsett, also an economist at Stockholm University, realised the enormous potential to study population density, urbanisation, where and how wealth has grown, and not least for research in their own field of development economics.

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^{National Collection of Aerial Photography / SU}

The extraordinary detail displayed in the aerial photos reveal may also be of interest to geologists, historians, archaeologists, ecologists, cultural geographers and climate scientists:

The project is giving us access to such a vast amount of data. This is a strange situation for a researcher, especially when we study history. Mostly we have tiny amounts of data, and we have to think creatively about what we can learn from them. Here, there are just so many possibilities. We have to ask ourselves what the most important questions are that we should focus on", says Andreas Madestam.

Grants to digitise the archive

But the photographs needed to be digitised in order for researchers to gain widespread access to the archive, and for that significant resources were needed. With an international team of collaborators, Andreas Madestam and Anna Tompsett began to plan how the archive could be digitised and made available for research, and applied for the needed funds.

Thus, in 2017, they were awarded almost SEK 11 million from the Swedish research financier Riksbankens Jubileumsfond for their purpose.

Three years later, they were granted an additional almost SEK 53 million from the same financier to continue digitisation and begin research on historical development based on the aerial photos.

10,000 photos scanned every day

The first research grant made it possible to develop new digitisation techniques. This is where seven robots in Edinburgh enter the story. By adapting technology from the food production industry, robotic arms lift one aerial photo after another onto the scanner to be digitised.

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^{National Collection of Aerial Photography / SU}

At maximum production, almost 10,000 aerial photos were being scanned every single day. In September 2023, the last of the nearly 1.8 million aerial photos was scanned. Researchers from Berkeley, Caltech, Stanford, and MIT are collaborating with the Stockholm team to process the images.

But the individual aerial photos had to be combined into a larger mosaic to make it possible to get an overview of what a city or region looked like. So algorithms and machine learning were important tools in the work of putting the images together. The team of 18 researchers and collaborators involved in the project also includes experts in machine learning.

The team is creating maps of land use, too. Spatial resolution is high, allowing individual buildings and even vehicles to be distinguished:

We can produce examples of places that have seen radical changes caused by humans, such as oil wells, mines, etc. The photos help document what some people call the Anthropocene, the era when the dominant force shaping the environment is the human race”, says Anna Tompsett.

Free access to large amounts of data

While in the first stages of the project, the focus has been on digitising the aerial photos and converting them into data that can be used, the eventual goal is for the researchers to use this data to answer new research questions. Thus, in parallel with digitisation, the researchers are also planning how the data can be used.

A key project’s goal is to increase access to the archive. The mosaicked images will be made freely available for all non-commercial uses under Creative Commons licensing, while other data outputs, such as historical land use maps, will be made available for unrestricted use under Creative Commons licensing.

This autumn, the first study using these aerial photos will be complete.

It focuses on the Caribbean and highlights the long-term interplay between social, economic, and ecological systems after decolonisation. The new data show that urbanisation and restoration of forest ecosystems have occurred simultaneously, illustrating that human development does not necessarily entail negative ecological consequences.

By the end of this year, the research team also plans to release data along with several other research papers that are currently in development.

Discover all the details in the full story, in Swedish.