Aerial imagery: A bird’s-eye view

OS MasterMap is our flagship product family, but have you ever wondered how a photo taken by a plane makes it onto a computer screen as a piece of data? Photogrammetry is the science of measuring and interpreting objects from photographs to answer questions like how high is that feature?

Remote Sensing is the process of acquiring information without coming into physical contact with the subject under investigation. We use this process, in conjunction with ground-based revision by our field surveyors, to update our large-scale databases

We have a large contract in place with external suppliers to supplement our own flying and photogrammetric production.This gives us the capacity to have to 6 planes flying on our behalf at any one time, allowing us to make best use of good weather conditions and process 60 000 to 70 000 sq km (more than a quarter) of Great Britain each year.

An image taken by one of our digital cameras

An image taken by one of our digital cameras

Our own production starts at Blackpool Airport, the home of our flying unit, which each year captures on average 50 000 aerial images covering 40 000 km² of Britain’s urban and rural terrain. We fly two leased aircraft, both using digital cameras, flying from the last week of February through to the end of November. Targets are captured at flying heights that vary from 3 000 ft to 8 000 ft, depending on the accuracy we need for the mapping.

Digital cameras have revolutionised imagery capture in recent years.As well as providing virtually instant images after download, they allow can provide richer detail in dark areas such as shadows, and this has enabled us to extend the length of the flying season while still providing the specified quality of output. In addition, Sensor technology has advanced with the number of megapixels increasing at a rapid pace. The latest digital camera is 196 megapixels.

You get the picture…

We produce flight planning information from specialist software, which also produces ‘sortie’ reports for the pilots and flight diagrams for air traffic control. This data is transferred to a computer-controlled navigation system, meaning the camera takes snaps at set points. Another system works out the position of the centre of the photograph and the orientation of the aircraft. This provides geodetic control and minimises the need for expensive ground control. Once the imagery has been captured initial quality checks are completed.

Once we’ve got the picture we move on to processing. We ‘stitch’ together the four pan photographs to create one very large image, taking around three to four minutes to process each image. We apply rigorous quality control procedures at all stages of processing. We then make sure everything is in its correct National Grid position and accurately reflects the position of features on the ground, with every point being accurate to an average of 1.1m or better.

Viewing imagery in 3D

We do this by first creating a Digital Terrain Model (DTM), which represents the ground surface as a set of 3D coordinates and is viewed as a series of triangles, with each point giving a ground height measurement. The DTM ignores any man-made features and vegetation and resembles a mesh blanket laid out over the Earth’s surface and, by using special display monitors and glasses, we can view it in 3D.

The imagery is then used to update our large-scale topographic database, and released to customers in the form of OS MasterMap Topography Layer; this is achieved by superimposing the mapping over the 3D imagery. Operators then capture any change directly onto our large-scale map.

The next step is to refine the digital image frames in order to create a more usable imagery product.As the aerial photography is flown in overlapping parallel strips, we can select the best imagery to include in our final product. After being cut into seamless orthorectified kilometre squares, the imagery is quality tested, edge-matched and put into our database, then added to the OS MasterMap Imagery Layer for use by customers.

So that’s how a photo taken by a plane becomes a map!

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5 Responses

  1. That looks lovely.

    Would you consider adding some of your aerial imagery to your (much appreciated) OpenData service?

    One of the real benefits of the aerial imagery from Google Earth et al is that they’ve made it really easy to do cool things with maps. You can find features on the Google imagery that Google’s cartographers (who are just looking for roads) would never have dreamed of charting. So by opening it up, they enable people to do exciting stuff.

    It would be great if Ordnance Survey could do the same – and with the OpenData service, you’re already going in the right direction. Doesn’t have to be “here’s a load of free aerial imagery”, just allowing people to view it (via OpenSpace) and trace from it would be enough.

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