I'm curious if anyone knows specifications on the overall length and width of the video frame (or image frame) of the Mavic Air at different fixed heights. I would like to use this UAV to measure body length in different cetaceans.
I'm curious if anyone knows specifications on the overall length and width of the video frame (or image frame) of the Mavic Air at different fixed heights. I would like to use this UAV to measure body length in different cetaceans.
Another approach is to use the angle of view which, according to DJI, is 85deg diagonally or 50deg horizontally. After that it's basic trigometry - you have a right-angled triangle with the height provided by your altitude, the base provided by half the width of the frame, and the angle at the top provided by half the AoV.
Assuming we want to calculate the distance along the horizontal (long) edge of the frame to use as our scale for measuring the size of the cetacean:
Length = 2 x Altitude x Tan(25)
Tan 25 is 0.4663...., so as a rough rule of thumb, the length of the horizontal axis will always be around 90% of your altitude, or 93.2% if you want to be more precise.
Note that all that assumes that the camera is pointed straight down, if not then it's stil doable if you know the angle it's at, but there's a lot more trig involved.
And it should be as simple as that. But I've proposed that in a number of threads and in at least one case it's been reported that it gives significant (> 25%) errors. I haven't yet got around to direct measurement, but it's pretty easy to do and it will avoid any ambiguity.
The quoted AoV for a lens is often an approximation, but a margin of error of 25% is a lot and I suspect is more likely to be indicative of accuracy problems with the altimeter, or the camera wasn't actually pointing straight down. Direct measurement would correct for the approximate AoV figures, but if you can't reliably set the camera angle or the altimeter isn't up to scratch then your accuracy will suffer.
Another possible wrinkle could arise from the small variances between actual altitude AGL and GPS-reported AGL due to the Earth's uneven magnetic field and equatorial bulge. A quick Google seems to indicate DJI's altitude reporting is somewhat less than perfect in this regard, so any discrepency between the two would throw off the scaling calculation as well.
For OP's use, I'd definitely be looking to test against an object of known length at a number of altitudes and locations to get a feel for what the margin of error mght be, and whether that is acceptable or not.
Or maybe think out of the box... The drone is over water here, a flat terrain, so the operating altitude could be constant. That means you could take off, ascend to your preferred operating altitude, orientate the camera, then take a picture of the boat or some other surface object of known size. You now have an image of something you can use as a rule as long as the drone remains at the same altitude for the duration of the time its surveying the whales.
Maybe make a segmented 50' chalk line in a parking lot and photograph it at 20', 30', 40', etc, elevation.