Map makers

Mark Broadbent finds out about the specialist work of the Ordnance Survey Flying Unit, which maps Britain from the air

Cessna 404 Titan G-FIFA is one of two aircraft used by the Ordnance Survey Flying Unit to photograph the UK.
All photos Ordnance Survey

Appearances can be deceptive. Cessna 404 Titans G-FIFA and G-TASK look nondescript, arguably boring, to a casual observer.

However, these two aircraft carry out some of the most unusual civil aviation work in the UK.

Flying from their Nottingham East Midlands Airport base, the Titans are used by the Ordnance Survey (OS), the UK’s national mapping authority, to capture up-to-date and accurate high-resolution aerial images of every part of Britain.

Between February and November each year these aircraft criss-cross the UK, with specialist air camera operators from the OS Flying Unit on board capturing on average around 50,000 aerial images of urban, rural, moorland and mountain terrain, from the Shetland Islands to the Isles of Scilly.

Together with the images taken by the OS’ team of around 200 ground observers of views aircraft cannot obtain (for example, views beneath trees or structural canopies), the images captured by the OS Flying Unit provide continuous topographical updates to the master map of the UK, the 2D ortho-rectified overview of every piece of land in the country.

Flying along lines

The OS aims to complete a total survey of the entire country every three years, but obviously as landscapes change over time the master map continually needs updating. This need provides the bulk of the OS Flying Unit’s work. Each year, the OS identifies target areas across the UK of which it needs new aerial images to update the master map. Those areas are then given to the OS Flying Unit, which organises targets into blocks of airspace of around 154 square miles (400km2).

The Titans fly along lines in a predetermined grid pattern within these blocks, with the camera operator in the cabin taking the photos as the pilot flies the aircraft along a line.

In practice this sounds straightforward; making it happen is less so.

Although the UK is relatively small (the country’s area would fit easily into the territory of many US states, including Alaska, Nevada and Texas), the UK’s airspace system is complex. There are four classes of controlled airspace (A, C, D and E), one class (G) of uncontrolled airspace and four types of airspace (control zones, control areas, airways and air routes). Terminal Control Areas are established at the junction of airways around the major airports and the airways connecting Terminal Control Areas are normally eight nautical miles (16km) wide, between 5,000ft (1,524m) and 7,000ft (2,133m).

As well as the complexities of the different classes and types of airspace, the UK’s airspace is busy; air navigation services provider NATS on average handles 6,000 flights every day. Detailed planning and close liaison with NATS is required if the OS Flying Unit is to do what it needs to do safely and efficiently, so at the start of each flying season OS Flying Unit and NATS officials meet at the air navigation services provider’s Swanwick control centre in Hampshire to work out a programme detailing the areas the OS Flying Unit will operate in and when.

Roger Nock is one of seven air camera operators with the OS Flying Unit. He told AIR International: “We’re aware we can impact on operations quite a lot, especially if we’ve got large blocks [of airspace]. It’s not the heights as much as the north/south/east/west size of the blocks. If you’ve got a 1,000ft block and you’re talking about 400km2 for a survey area, that’s a lot of airspace for air traffic to vector airliners around you.”

Nock said NATS is happy with the Titans operating at an altitude of between 8,000ft (2,438m) and 10,000ft (3,048m), which sometimes means the aircraft are operating above airliner traffic, but slotting the Titans into that system of Terminal Control Areas and airways is not the only planning consideration.

Sometimes, Nock pointed out, it isn’t always possible to complete survey work in a certain block of airspace in one go, because part of the block may encroach on a restricted area set aside for military training or weapons testing. For instance, only eight lines from 12 in a block might be completed on one flight, with the remainder having to be completed at another time when the restricted area is deactivated. The planning process has to account for when the land in restricted areas will be available.

The nature of the grid pattern the Titans fly in a survey area, with the aircraft reversing direction and flying along another line to complete the grid, is a further major planning consideration, especially for operations in uncontrolled airspace. NATS issues Airspace Co-ordination Notices informing users about the OS Flying Unit’s activities.

Nock said: “On the morning of a flight we try and get in contact with as many parties as possible to make them aware of our presence.” The OS Flying Unit also looks at the NOTAMs (Notices to Airmen) to see if there are any activities (such as hot air balloon flying or parachuting) that might affect flying through an area of uncontrolled airspace.

Other airspace users are a further consideration. Nock said: “When you’re planning, you’re looking at how you’re going to impact on the operations and we try and make it so it minimises the impact.” Plans are sent to airports for feedback before flights.

The Titans’ interiors are dominated by a computer operating system, housed in the silver box, which is used to operate the camera and control aperture, shutter speed and focal length.
This view of the Titan turning away from the camera shows one of the hatches in the underside of the aircraft where the camera sits.

Nock emphasised the focus of the OS Flying Unit’s planning activities is on minimising issues for air traffic control: “We have a wash-up meeting [with NATS] at the end of a flying season to see how things have gone and how we can help in the future.”

Changing weather

Aside from making its operations happen in a busy airspace environment, the changeable British weather is inevitably a key factor in the OS Flying Unit’s day-to-day operations. The most needs to be made of any bright, clear days to collect data and Nock explained if a weather window allows a different block, or part of a block, to be covered rather than the one initially planned, they will fly to where the conditions are more favourable.

This flexibility to change plans quickly underlines the close working relationship between the OS Flying Unit and NATS, and it also means the distance and duration of a survey flight varies. Nock said: “Our average sortie is around two or three hours, but you can be up there for as long as nine; the only limitation is the pilot’s duty hours. If you find a clear spot, you’ve just got to go for it. You’re there for as long as weather and air traffic control will allow. It’s not until you get there that things can change.

“Sometimes aircraft come into the area which curtail your visit, or the weather changes. If there’s a front coming in and your target is on the eastern side of your block, you finish the line just ahead of the weather. It’s quite a satisfying feeling when you finish a line, you turn around and the whole block has gone [into the weather]. If you’d timed it minutes later, you wouldn’t have been able to complete the block. That’s the sort of tight margins we work to.”

The Cessna 404s are ideal, because they offer ten hours’ endurance and a 400kts (740km/h) cruise speed, meaning the aircraft can speed to where the weather is clear. Nock added: “We’re constantly looking at our intelligence, webcams, Met Office forecasts. If you see a chance, you’ve got to go for it. It’s in the lap of the gods really, but it’s very rare we come back with nothing.”


On board each Titan is a 196-megapixel Vexcel high-resolution camera with eight lenses. There are two camera hatches, fore and aft. Generally the camera sits in the aft hatch behind the operator’s seat on a gyrostabilised mount.

Nock described the workstation in the cabin. A crucial piece of equipment is a laptop loaded with software that details the flight lines planned for the sortie. This software is GPS linked and allows the camera operator to check the accuracy of the line being flown relative to the planned track, and presents data on the variables affecting aerial photography, such as sun angles, altitude and speed.

The operator also uses a scope to measure drift. Nock said if the aircraft is high enough and it’s a clear day the scope shows the view up to 32 nautical miles (60km) ahead, meaning any upcoming weather can be seen and a decision made about whether it is worth attempting to fly the line.

The laptop and its software are linked to a computer operating system (COS), which is at the operator’s workstation in the cabin.

The COS is used to operate the camera, with the operator using buttons on an adjoining control panel and keyboard to control aperture, shutter speed and focal length.

The COS screen gives a continuous view of the image through the camera, enables an operator to confirm the lenses fired and check the images exposed correctly. A black and white thumbnail is produced, allowing a check to be made for haze levels and cloud (or smoke from fires or chimneys). There are only three seconds to quality control each image before the next image appears.

Nock said the Vexcel cameras are due for replacement: “Newer cameras are more integrated, but even though the system is a little older than some, the way we use it with the software and the programming enables us to do a lot more with the photographs. We push it to its very limit.”

The photos taken during a flight are stored on two 3Tb data drives (one primary, one back-up) which each weigh 55lb (25kg) and are linked to the COS. At the end of each flight, the drives are unlocked from their position on the cabin floor and taken to the office where the images captured during a sortie (each one is 500Mb in quality) are downloaded and transferred online to the OS head office in Southampton. If the file size is too large, they are put on to a portable drive and posted.

Significant planning is required to fit the survey flights in controlled airspace. Sometimes this involves the Titans flying above airliner traffic.
Software linked to GPS allows the camera operator to check the accuracy of the line being flown relative to the planned track. This view shows Portsmouth Harbour, with the new HMS Queen Elizabeth aircraft carrier visible.

Towards a digital twin

The images collected by the OS Flying Unit and put into the OS master map have numerous applications. Combined with the other OS mapping products, they provide crucial information for government agencies, emergency services, local councils and surveyors in roles ranging from building planning applications to flood risk management.

The flying unit additionally undertakes ad hoc survey flights in response to specific requests to the OS for bespoke information. There were around 130 of these requests last year, Nock said. For example, the Land Registry, councils and emergency services might want images of new road junctions or bridges, or surveyors who can’t obtain access to a specific area of land might want aerial pictures to be taken.

The OS Flying Unit has become busier over the last couple of years. Nock said: “We’re producing a lot more than we ever have and we’re busier each year; 2016 was a record for us in getting 50,000km2 of imagery.”

A key factor in the increased workload is the advancement in software used to analyse raw data. Nock explained: “Information is being used in ways that ten years ago didn’t exist. It isn’t just the image, it’s the height and altitude data that can be taken from the image.

There’s a massive need for detailed, constantly updated maps.”

A good example is work the unit undertook recently for the Rural Payments Agency (RPA) to photograph every single hedgerow in England and Wales. Nock said: “The infrared channel allowed us to pick up the extent of the vegetation and the [RPA] were amazed how quickly we could produce a bespoke dataset for them. It’s technology we have in these cameras, the way we operate them and what we can produce.”

There is likely to be more demand from within the OS itself for aerial images as the agency targets producing a digital twin: a fully 3D simulation of the real UK.

Unmanned operations?

Unmanned aircraft, from large and mediumsized systems through to hand-launched, micro/nano-sized platforms, have in recent years found greater use for survey and photography work ranging from offshore energy and emergency services to utilities management and historic buildings conservation.

Nock acknowledged UAVs could in the future play a role in the way the OS collects data, but insisted there are no plans to stop using manned aircraft. Current UK legislation mandates visual line of sight operations for civil UAVs, which limits their suitability for the OS as they would only be able to operate over a small geographical footprint.

Nock said: “There’s no way you could do what we do, producing 400km2 of imagery within 50 minutes. It may initially seem costeffective [to use UAVs], but when you find you’ll need thousands of them the costs will increase. If you’ve got sunny conditions you could quite easily put a UAV up there, but the way we produce imagery means we can be so flexible. In relative terms the aircraft are fast; there’s not a like-for-like replacement.”

The Titans’ relatively unheralded role in helping the OS map Britain looks set to continue. AI