Is anyone aware of a calculator that allows you to figure out the distance you can fly at 400 feet altitude before going below the tree line? I think thay would he a really handy thing to have. You should be able to calculate given some approximate knowns (such as tree height and distance from those trees). Just wondering if anyone has found a simple way to calculate that?

I think Pythagoras could be at help here. Let's use this pythagoras calculator If a tree (or a building) is 15 meter tall and you are standing 100 meter away from it, it will give you an angle of ca. 8.5 degrees. Let's fill in 15 and 100. edge a = 15 (50 feet) edge b = 100 (330 feet) Gives you angle A = 8.53 degrees So how fare can you travel before you LOS is blocked/interfered by the tree? You will fly at 400 feet (122 meter ca.) and you LOS will be blocked when angle a < 8.5. So let's try the value edge a = 122 (400 feet) angle A = 8.53 degrees and it gives you edge b = 813 (2 667 feet) so you may travel 2667 feet with an attitude of 400 feet before the obstacle will start to interfere with you LOS. I probably find some app for this calculation to. With the proviso that my theory and math is correct

Nice solution Jsun, explained nicely here Line of Sight | Line of Sight Distance | Math@TutorVista.com

I just do it in my head and I'm not particularly smart... Highest tree 140 ft Distance 1800 ft So I can fly just short if 5400 feet before signal is blocked by a tree.

I just found 2 apps that make this calculation incredibly easy!! No pencils/paper needed! Using 1. Clinometer (measures the angle to the top of tree line), and 2. Right Angle Triangle Solver (does the trig calcs). The free version of Clinometer does not come with the camera ability, but for $1.50 you can purchase the camera version and relative angle. You simply find the angle by pointing the clinometer to the ground and then top of the tree line, and then take that angle and insert it into the Right Angle Triangle Solver. In my example below, a is always 400', and you just insert the angle in A. b will then show the distance in feet you can go before the bird goes below the tree line. In my example below, I can go out 2,741' before falling below the particular tree I aimed the clinometer at.

I likes pictures and stuff "The image below in no way represents actual measurements and is used to pictorially represent the need for measurements to avoid loss of LOS."

Well... yes and no. You've got your basic LoS which indicates that you can see the aircraft without obstruction. Then you have what I call LoST/Rx whereby your altitude as previously discussed, allows transmission between RC/AC to be unhindered at greater distances depending of course on the transmission capabilities and other environmental properties. This is all probably a given for the most part. But I thought it worth putting out there.