This is a quick tip for drone pilots shooting boats.
Rather than looking at the screen on your controller to see what altitude your drone is at, look at the horizon.
If you're photographing boats with a drone it is very important to know how high you are above the wet stuff. Too low and you risk clipping a wave, not high enough and you could run into the boat you're shooting.
If you can see the horizon in the frame then where the horizon bisects the boat is the same level above the water as the Drone.
In this shot the Drone is just above the height of the Radar but below the top of the Sat Com.
In this shot the Drone is at the height of the Anchor, maybe 2 Metres above the water.
In this shot the Drone is at the height of the top of the mast.
In this shot we're pretty low, maybe 1.5 Metres.
In this shot the Drone is at the height of the base of the Sat Com dome.
In this shot the Drone is ... Oooops.
Nah just kidding.
When over water it can be difficult to judge your exact altitude. You could use the reported altitude from your controller but that is measured using a barometer built into the drone. The base / take off measurement is taken when the Drone is started on board the boat. When you fly out over cold water that barometric pressure will lose it's relevance to the initial measurement as the air density is different from the take off point.
Also a number on a screen has limited usefulness. Relative height is the important measure.
I hope this little tip will help my fellow Drone pilots who may not have already been aware of this relationship between the horizon and altitude.