OK, I have the proof. I'll be using an image I drew myself for reference, as Tom's Image didn't highlight the correct areas to solve the problem.
The goal here is to find the length
a, which is the vertical difference between
A and
B.
Let's use the above model, where
O is the centre of the Earth,
A is our standing position, and
B is our viewing position. The diagram shows us that (assuming that the earth is a perfect sphere for this example, otherwise it becomes far too complicated,) the radius
r is the same along the line
OA as
OB. Since we know the length of the arc
AB (which I just realised I forgot to label, let's just call it
l) we can find the angle
θ, (in degrees, it would be
l/
2πr x 360.)
Now, let's assume the position of the Earth is at (0,0). (Note that for this example we do not need to worry about moving in the
z direction, because the example assumes no movement in this direction, and thus can be ignored.) Now using simple geometry, We can define the point A (0,
r) and B (
rsin
θ,
rcos
θ).
Now we know that the length
a is the difference in height between
A and
B. So by defining the coordinates, we get,
a =
r -
rcos
θ.
And this is the difference in height between any 2 points on an arc of known length.
(Please note that I have not yet tried the numbers given in the example so I don't yet know if this will help my argument or not. I did this deliberately so my proof would not be biased in any way, thus the use of unassigned letters to show the answer.)
I will apply the numbers from your 30 mile example and post it in a new post.