16:9 is a crop of the 4:3 frame. It takes height off the image and
leaves width alone. Flying straight down, width runs across the track and height runs along it,
so 16:9 costs 25% of along-track coverage and none of the side coverage.
Ground resolution does not change, because the width is untouched, so nothing in the
imagery looks wrong until processing reports gaps. At mapping speeds the front overlap
can silently drop below a usable floor.
Before launch, check the photo aspect ratio on the controller is 4:3
and the photo size matches the mode chosen here. The exact menu location varies with
the DJI Fly version, so verify it on your own controller.
Vertical shooting must be off. Missions assume landscape
orientation; vertical mode swaps the axes and wrecks side overlap instead.