Aerial mosaic photography

Say one wants a current, 100%-coverage view of a large area of land from the air. Maybe some of you can retask a satellite quickly, but that is not an option available in most cases. However, you can do it by hand with a standard DSLR camera (and large memory cards). It just takes some additional equipment and, more importantly, some careful calculations before you take off. A reliable GPS attached to the camera for geotagging each shot is one essential. Another is a programmable timer to flip the shutter every X seconds. You need an airplane and pilot, of course; preferably, the A/C should have a photo window and autopilot programmable to fly repetitive tracks. If you have your photo window open for a long time, you want some gloves.

Having assembled those items, the rest comes down to altitude and field-of-view calculations. A consumer-grade Nikon DSLR has the following fields of view, in degrees, at these focal lengths:

42mm focal length – 30.755x 20.779y
28mm focal length – 44.836x 30.755y

I highlight those focal lengths because either of them is acceptable for full-coverage aerial photography at 2000′ AGL with one-mile track spacing. 28mm is appropriate when taking horizontal pictures, and 42mm is appropriate for taking vertical pictures. It is assumed each track (of any length — that part is immaterial) is flown twice, once in each direction, or that two cameras are used simultaneously, one from each side of the aircraft.

Coverage between tracks

The 30.755-degree field of view common to both focal lengths means that when the camera is held at 35 degrees below horizontal the photo will cover level ground from 1655′ to 5494′ from the flight track. The overlapping shot from the next leg will then cover, in relation to the first leg, the area from 3625′ to -214′. The two legs together cover -214′ to 5494′, the entire mile of track spacing plus 214′ of overlap on each side of the track.

The area from 1655′ to 3625′ is covered by both legs. This is a significant and largely unnecessary overlap in the area centered between the two legs. The only alternative, though, is to shoot higher than 35 degrees below horizontal, making these shots even more oblique. Quality aside (for now), this option does cover the entire land surface.

Shot spacing

Shot spacing must be calculated at the bottom of each photo, the area of coverage closest to the flight track and thus the narrowest. At 28mm when shooting horizontally, the 44.836-degree focal width means each shot covers 2098 horizontal feet; with a fifty-percent overlap, 6 shots are required per nautical mile. At a speed of 90 knots, each mile is covered in 40 seconds, so a shot is required every 6.7 seconds.

Similarly, at 42mm when shooting vertically, the 20.779-degree focal width means each shot covers 952′ horizontal feet on flat ground; with a fifty-percent overlap, 13 shots are required per nautical mile, or one every 3 seconds.

Coverage of 1-mi tracks at 2000 AGL

The oblique angles that result from trying to cover an entire mile between tracks of a parallel line search with one-mile spacing make for less than detailed photography. Each line of photography must cover, at minimum, the area underneath the next track over, which means shooting over a mile away. Results from testing this suggest that it would be a significant improvement to do 0.5- or 0.6-mile spacing between tracks. That would allow higher camera zoom and steeper camera angle, thus greater photo detail and more vertical perspective; the drawbacks include a 67%-100% increase in time spent aloft and processing photos as well as photo storage.

It comes down to the customer requirements, of course. If the requirement is full coverage of an expansive area and oblique angles are acceptable, one-mile track spacing will work. If greater detail and sharper perspective are required, shorter track spacing and another camera configuration are necessary. In the case of a lengthy track like a flooding river, the questions for the customer are how far inland needs to be covered and at what level of detail — and can the photos be taken vertically (at one-mile track spacing, horizontal photos result in excessive overlap parallel to the flight path while eliminating the possibility of covering more ground perpendicular to the flight path and at a higher level of detail). The altitude and camera setting can be calculated appropriately in response to the identified requirements.

Calculating other options

Here is the camera-specific field-of-view calculator I used. That’s the easy part if you know what camera you want to use.

The rest of the calculations are simple trig. Having neglected that since high school, I relied on an online calculator like this one. I will not explain it, except to point out that you have to plug in your chosen altitude AGL  (side a). It’s necessary to run this a couple of times using the camera angle plus/minus half of the vertical field of view of the camera from the previous calculator to calculate the near and distant edges of the photos. Camera angle is a bit difficult to maintain at any given angle given the bouncing and shifting of the A/C, so it’s necessary to calculate a focal length that will allow a little extra on either side of the photo to ensure overlap (between tracks) or full coverage (of the customer’s requirement for perpendicular coverage).

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