(Modified excerpt from the OpenDroneMap docs)
In a previous post on camera calibration, we discussed the possibility of setting up calibration flights that have the following characteristics: non-parallel (converging) flightlines of 20° with a 5° forward facing camera (assuming 0° is nadir). With OpenDroneMap’s –camera parameter, we can thus import camera models from a calibration flight and use that in a more efficiently flown “traditional” flight plan. Can I use that term for a tradition so young? Well, traditional it is.
The underlying challenge becomes what is the cadence of such calibration flights? If I fly a calibration flight in the AM at one temperature, and the afternoon is substantially warmer or cooler, when do I need to repeat the calibration flight? Equally, what is the longer term cadence for changes such as drift in camera calibration parameters from long-term changes to the camera induced by aging, vibration and other factors.
So, I have been thinking through a thought experiment for the last few days: how with existing flight planners can we plan a flight that is both efficient and contains converging flight lines needed for automated calibration? Here’s what I propose:
Fly with much lower overlap than normal, but using two crossgrid flights (sometimes called crosshatch) separated by 20° with a 5° forward facing camera.
- Crossgrid overlap percentages can be lower than parallel flights. To get good 3D results, you will require 68% overlap and sidelap for an equivalent 83% overlap and sidelap.
- To get good 2D and 2.5D (orthophotos and digital elevation model) results, you will require 42% overlap and sidelap for an equivalent 70% overlap and sidelap.
If the flightplanner doesn’t support crossgrid flights, the bearings needed for each of 4 flights are as follows. If we assume the first flight is 0° then:
- The second flight will be at 20°
- The third flight will be at 0° + 90°
- The fourth flight will be at 20° + 90° or 110°