Best Practices for Obtaining Imagery for Stitching
- Include unique objects in the photos
Stitching software likes to see odd things in your photo pile. Those unique inconsistencies can mean the difference between a good stitch and leaving your pickup with an empty tank of gas at the end of the day with nothing to show for it. Be it cloud-based or desktop, stitching software picks up on those unique things in your photo pile.
Stitches tend to fall like a house of cards if variability isn’t dotted across a field (bet you never thought all those red ant hills at the site could be a good thing, huh?). Is your field bounded by a road? Include it in the flight. Don’t really need the corners around the center pivot? Capture them anyway as that variation could make all difference toward a successful stitch. - Fly Higher
Fly as high as the law allows and get as much area as you can into every shot. The more tie points stitching software can find between overlapping images, the more likely it is you’ll have a useable product. Flying higher will lower the resolution. Offer the lowest resolution that is required to do the job. - Fly Smart
There comes a point of diminishing returns with respect to acquiring too much data. The goal is to maximize the efficacy of flights. Flying higher also gives us the added bonus of being able to cover more ground (a lot) faster; but short of the regulations changing, what we can do is dial in image overlap. Most mission planning software has options to modify both sidelap (cross-track) and frontlap (along-track). In the context of a typical multirotor aircraft equipped with a stock camera, shutter frequency (and in turn frontlap density) ultimately falls back to ground speed. Flying slow really chews through battery on a multi-rotor; and it goes without saying that where sidelap is concerned, less rows means less flight time. Multi-rotors typically draw similar current when hovering or flying forward at about 60% of max or about 30 knots.
Take for example, two flights on the same field: Front and sidelap both at 80% gives us a successful stich to be sure, but tweaking the side and frontlap to 65% and 75% respectively gives us almost four and a half additional minutes of flight time that could be better put to use elsewhere. More than sufficient coverage comes at a cost of battery life. This field can be covered with about 1/3 less overlap and still offer similar results. Extra photos don’t help after a certain point. You and your processor need to work out these finer points. - Everybody Freeze
Stitching software has a love/hate relationship with change. Point cloud works because of difference, but when those differences are temporal and not spatial, “GIGO” rears its ugly head. Stitching software can get quite confused if the similarity isn’t there between images that should be; and unexpected temporal change is the number-one cause of failed image stitch when nothing seems wrong with a flight.
Temporal change offenders include:- shift in wind direction mid-flight that blows vegetation canopy in another direction.
- Vehicles or livestock on the move.
- And, the no. 1 bane of aerial image mosaics: Slowly moving, dense, spotty cloud cover.
- No Two Maps Are the Same
The longer a drone is in the air, the larger the dataset. Larger datasets are inherently more difficult to process, so it goes to figure that those big acreage flights open yourself up to error. It may take more time, but smaller acreage missions may make more sense to cordon-off problem areas so that the rest of your data isn’t “poisoned” by a handful of bad images.