In March 2022, the city of Frankfurt enlisted the help of a Cessna 404 aircraft to take aerial photographs of the entire city and neighboring areas for the 23rd year in a row. Late last year, these images were made available to the public, but what exactly goes into collecting these images? Simple Flying decided to find out!

Introducing Frankfurt

Frankfurt is a city in Germany with a population of roughly 760,000, making it Germany's 5th biggest city by population. However, what the city lacks in people, it makes up for in aviation. It surprisingly gives its name to two international airports, with the 'Main' one being the busiest airport in Germany.

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Photo: Oliver Henrich via Stadt Frankfurt am Main, Stadtvermessungsamt

To map the city from above, the plane surveying Frankfurt was required to fly in Frankfurt Airport's (FRA) controlled airspace, including the flight paths. While this does create a challenge, it is one that controllers at one of Europe's busiest airports are well versed in.

The background

The aerial images that you're used to seeing on the likes of Google Maps are taken from satellites high above where aircraft usually fly. But it is also possible to take photos from an airplane, as with Frankfurt's mapping flights.

Mapping South Frankfurt
South Frankfurt was mapped on March 22nd. Photo: FlightRadar24.com

However, it's not as simple as just saying, "let's go flying today". One significant point of consideration is the weather. March is the best month to plan such flights. Typically, snow has departed while trees are yet to sprout their leaves. A cloud-free day is required to complete aerial photography missions. Partly, this ensures that the area being surveyed is well-lit, but it also means that the aircraft can fly in the controlled areas around the airport, with the flights taking place at around 6,000 feet.

Mapping North Frankfurt
The north of the city was mapped on March 28th. Photo: FlightRadar24.com

The aircraft

To complete the imaging flights above Frankfurt, a specially adapted twin-propellor Cessna 404 belonging to Aerowest GmbH was used. The company completes around 60 flights yearly, clocking between 250 and 300 flight hours. The aircraft, registered D-IAPD, is based out of Dortmund Airport, to the country's west.

C404
Photo: Aerowest GmbH

A camera array is lowered through a hole in the base of the fuselage. 8 cameras with 35, 90, and 110mm lenses take photos in varying directions and angles, which are then stitched together to make the final map.

Two flights required

Two flights were necessary on different days to take all the photos required for the finished project. On March 22nd, the city's south was mapped, with the north following on the 28th. Speaking to Simple Flying, Hans Joachim Benfer, the Director of Aerowest GmbH, remarked,

“We can cover up to 800 square kilometers per day. The Frankfurt project covers 2,500 km² with more than 100,000 images required. We are limited by hard disk storage (16 TB), the aircraft’s endurance, and daylight hours.”

Benfer explains that the sun needs to have a minimum angle of 25-30 degrees above the horizon. This means that in February, only two hours a day are suitable for mapping, compared to roughly eight in June. This sun angle restriction is imposed to prevent long shadows on the images captured by the aircraft.

Close coordination with Air Traffic Control

When producing these images, the aircraft must fly right above Frankfurt Airport. In general, this isn't such an issue, as aircraft are typically at or close to ground level at the airport, not flying above it (except for the odd special flight). More of a challenge, however, is passing through the airport's approach path, where the big jets are at similar altitudes to the photography aircraft.

Unsurprisingly, this means that the aircraft can't just rock up and start taking photographs. Benfer added,

"All projects around Frankfurt have to be performed in controlled airspace, mostly Airspace "D". Large Area Projects have to be flown at [6,000 ft] or above to keep distance to approach and departure traffic. For each mission, we apply for a BNL number. This means the detailed flight plan with all flight lines and altitudes will be checked by the ATC Special Use of Airspace division. On the day of the mission, we have to be cleared by the ATC supervisor prior to takeoff with regard to the exact mission as documented in the ATC BNL database."

frankfurt
Photo: Aerowest GmbH

However, long before the BNL number is requested, the gears are already in motion to make everything mesh together on the day. Weeks of planning go into the flights, including working out the required image resolution (flight altitude), image blur (aircraft speed), and image overlap (shutter speed). Considering the area to be scanned and the terrain of that area, the team spent hours calculating all the relevant data for the Air Traffic Control application.

How ATC works with the mappers

As mentioned, ATC plays a crucial role in successfully completing these aerial mapping missions. Simple Flying spoke to Ute Otterbein, a Public Relations Specialist at Deutsche Flugsicherung (The German ATC provider), who remarked that such flights "are indeed a special challenge for ATC".

Otterbein commented,

"In total, DFS receives several thousand such requests each year. A special role is played here by the increasing number of requests for the mentioned aerial photographs from local authorities. These require aerial surveys in specific altitude bands, usually around 4,000 feet (1,200 meters) and thus precisely the altitude bands that are relevant for arrivals and departures to major airports. This poses challenges for air traffic control, especially in the local traffic areas of the major international commercial airports.

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"Such projects can be rejected in advance. In most cases, however, a joint effort is made to find either suitable time windows for the measurement flights. Especially at airports like Frankfurt, this is of course a challenge. It is interesting to note at this point that during the times of the Corona crisis (and significantly reduced air traffic), significantly more measurement and photo flight projects could be carried out. However, since this year at the latest, the situation looks different again."

Compiling the photos takes time

Once the photos have been taken, it is still a while until they are ready for use. They must be stitched together and overlaid onto the existing map before checking that the work has been successful. This meant that the images taken in March 2022 weren't available publicly on the Frankfurt Geoportal until late last year.

2019-141
Photo: Tom Boon - Simple Flying

While the maps are used for various reasons, they also capture history. The 2020 edition of the map flew over Frankfurt Airport on March 30th, 2020. Without realizing it at the time, the photographers captured HS-TUD, a Thai Airways Airbus A380, completing its final flight before being sent for storage at the start of the COVID-19 pandemic. Over the years, the photos also capture the airport's development, including the construction of Terminal 3 and even the northern runway (opened in 2011).

Did you realize that so much work went into taking such aerial images? Let us know what you think and why in the comments below!