remote sensing


How we use 3D height data at OS

Guest blog by OS Surveyor Lee Harvey.

Imagine you’re buying a new house and are worried about the risk of flooding. Or you’re installing a mobile network, such as 5G, and want to know where to place transmitters. You’ll need to know the shape of the ground, buildings blocking line of sight, where will water flow, and a host of other things. OS create a set of 3D height products as well as our 2D maps and data.

Remote Sensing at OS is a big thing. The team spends Spring and Summer (when the weather is better… apparently!) flying up and down Great Britain capturing aerial imagery. These images are used to update our maps quickly and efficiently at head office in Southampton. By taking many overlapping images and using some air triangulation software (that’s the maths of measuring angles from the air), we match these images to their real-world location and work out the height of features on the ground. The software takes these height points and constructs a Digital Surface Model (DSM), which also forms the basis of our orthorectified imagery (a top-down perspective, map-like image). The DSM data includes buildings, trees, bridges and anything which exists at the time the photos are taken. A video game is a good example, as you move your character through any 3D environment, the hard surface that the game graphics display, will be draped over a DSM (although it’s called a Mesh in the 3D graphics industry, the fundamentals are the same). To complement this surface, and to allow lots of clever analysis of the real world, we also create a Digital Terrain Model (DTM). Read More


Farewell to Blackpool Airport

When the last aerial imagery was flown in November it didn’t just mark the end of the 2011 flying programme, but also the end of our tenure at Blackpool Airport. We’ve been flying from the airport for over 50 years, had an office in the area since the mid 1960s, and two members of our Flying Unit have been spending six months of the year there for the last 20 years.

In its early days, the flying programme operated from a number of bases around the country, including Blackpool. Over time, the central position of Blackpool for flying to the far north of Scotland and down to the Isles of Scilly made it the sensible choice to have as a permanent base for the Flying Unit. In addition, the climate in the area meant that it was rarely a fog-bound airport and flying time could be maximised.

The Flying Unit could be in the skies over you anytime that weather allows between March and November, capturing around 50,000 images a year. Five people, some from our field teams and others from head office, work on a rota from our Blackpool airport base during the flying season. The two people on the rota spend around two weeks at a time in Blackpool, flying as often as the weather allows, including weekends.

Read More


The view from above

Our Remote Sensing team have a unique perspective on Great Britain as they work with aerial imagery every day. Whether they are camera operators in our Flying Unit or technicians based in our Southampton head office, they are getting a bird’s eye view of the sights that you and I only see from the ground.

From beautiful shots of remote Scottish islands, to viewing the Houses of Parliament, every day takes the team somewhere different. Although our 2011 flying season has now come to a close (the team usually fly from March to November as that is the best weather window), processing continues on the thousands of images captured for our database. During processing, Remote Sensing see some unusual sights – which won’t be making it into our database and on to our mapping anytime soon.

Read More


Meet our Remote Sensing team


Last week we heard from the Flying Unit in our Remote Sensing team and this week we’re going to find out what happens to the images the Flying Unit send back to head office. As we capture around 50 000 aerial images of Great Britain every year, we need a team ready and waiting to process that information.

Working at a 3D workstation

Working at a 3D workstation

The Remote Sensing team take the imagery sent through by the Flying Unit and use it to update our large scale topographic database as well as populating products such as Land-Form PROFILE Plus and OS MasterMap Imagery Layer. We actually use the aerial photography to provide three key things:

 • grid coordinates of features on the ground;

 • heights of features and topography; and

 • rectified photography.


Remote Sensing are split into sub-teams to manage these tasks. In Aerial Triangulation, simply put, they ‘pin’ the imagery to the National Grid and ensures that it is positionally accurate. Meanwhile Orthorectification use stereo imagery to assess changes in scale across the photography and produce a combined image which is geographically accurate. This becomes the OS MasterMap Imagery Layer and can be overlaid with other elements of OS MasterMap too. Finally, Data Capture use the stereo photography to measure height and location in order to update our topographic database. Imagery is viewed on a monitor using polarised spectacles to view two images virtually simultaneously.

So that was Remote Sensing 101 and you should have a rough idea of what the team do. I caught up with Jon, Peter and Nicholas to get their perspectives on their roles and discover how things have changed during their time in the team.

Jon has spent two periods of time in Remote Sensing and seen a number of changes between those two times. Part of Jon’s role is working on ‘knitting’ images together. If you think that the cameras on the aircraft are taking hundreds and thousands of images, then you need to find a way to stitch those images together to create one vast seamless image of Great Britain. The images need to be joined in a sensible means – you wouldn’t want a join across a feature such as a house or bridge for example. So Jon works with images, creating the joins and avoiding features, cutting around buildings and so on. Jon told me, “It’s known as seam lining and is especially challenging in built-up areas with large numbers of features and tall buildings creating shadows. It’s ideal to be working on somewhere like Norfolk – which is flat and relatively easy to do!”  

Using the glasses to view in 3D

Using the glasses to view in 3D

Watching Jon and his colleagues work, my first observation, was apparently a very common one “doesn’t it strain your eyes working in 3D?” Jon assures me this isn’t the case, it’s more about relaxing your eyes and looking into the distance – much like with those 3D hidden image posters that were popular in the late 1990s.


I also wondered whether it had been more challenging for the team to move to our new head office. Jon said, “Now that we all work on PCs, we’re not that different to the rest of the building, so the move was quite simple. We just needed blinds up at the windows to make sure we could complete our screen work without any glare. Before PCs, when we had specialist equipment we needed reinforced floors to hold it all – and they had to be sprung to remove the vibrations from all of the equipment.


“We do have a different set up to most of the business as we work from two screens, with one in 3D. But other than the desks, we’re really not that different.”


Peter and Nicholas work on stereo plotting – making sure that things are in the right place in the simplest terms. They can be working on a variety of mapping scales ranging from urban sweep to mountain and moorland. They also undertake areas where it would be dangerous or access would be denied to our field surveyors, such as airports, refineries, railways, new motorways and so on.


Although my initial thought was that it must be great to have a rural area to check as there’s less change to identify, Peter pointed out that if you have a 5km by 5km area of Ben Nevis, you still have to check every section of it and be able to hunt out the smallest change.


Nicholas told me that he has seen many changes during his time in the team – and later this year the editing system will be changing again. “I undertook two months of training outside Remote Sensing before moving into the team. The analogue stereo plotting machines like the A8 and A10 took a long time to set up as you needed to recreate the position of the aircraft as it took each image. The current set up is more of a ‘trick of the brain’ using the dark glasses to create a 3D image on the screen rather than setting equipment up and looking through optics.”


Peter remembered the introduction of digital workstations around 2002. The digital photogrammetrical workstation (DPW) allowed for an on-screen display by which you could view imagery in 3D using special glasses. Peter also recalled the changeover to colour photography around five years ago and the switch to digital cameras. These have allowed the flying season to be extended, as he said “There have been many changes in Remote Sensing as it constantly evolves…we are now moving forward with a new system and I’m sure we will move on to greater and better things.”

Having a quick look through the specs that these guys work on, I have great admiration for the level of concentration required on a daily basis. I only wish I could show you the world as they see it – maybe once we have 3D computer screens as standard, I’ll do just that.


Up in the air

I’m sure most of us have been more than happy with the lovely sunny, dry weather this spring, but how many can say that it’s helped them complete their work too? Personally, I’ve given more than one resentful glance out of the window at the glorious weather – not while writing for the blog of course! – but my colleague John has been embracing it whole heartedly. 

One of the planes our Flying Unit use

One of the planes our Flying Unit use

John is part of the Ordnance Survey Flying Unit. Working as part of our Remote Sensing department, the Flying Unit could be in the skies anytime that weather allows between early March and November.

Five people, some from our field teams and others from head office, work on a rota from our Blackpool airport base during the flying season. The two people on the rota spend around two weeks at a time in Blackpool, flying as often as the weather permits, including weekends.

  Read More