Cut the reachable area where the glide slope meets the ground, instead of
assuming flat terrain at the launch elevation.
Range a glider can reach by gliding down from the given altitude at the
entered glide ratio (L/D). Without terrain it is a
true geodesic circle over ground assumed flat at the launch elevation.
Switch terrain on and the circle becomes a dashed reference, with the
shape showing what the ground actually allows — cut short by ridges,
carried further down valleys. Everything shaded
red is out of reach. Click or drag the pin to
move it.
The air mass panel adds an average vertical air movement: rising air
stretches every shape, sinking air shrinks it, and air rising faster than
the glider sinks means an endless glide, so nothing is drawn.
Fly to draws where you can glide to from the pin, keeping the arrival
height above terrain the whole way. Descending cone reverses the
question: the ground from which you could still reach the pin at this
altitude — there the arrival height is what you must have left
over the pin, and en route the glide need only clear the terrain.