It's because theta is so small. Considering a right triangle of height h and a much longer base, h must be quite large for the hypotenuse to be 6 m longer than

It's because theta is so small. Considering a right triangle of height h and a much longer base, h must be quite large for the hypotenuse to be 6 m longer than the base. Below is a simple example: base 1000 hypotenuse 1006 height 109.7087052 Right triangle here is of course an approximation to the original problem, just to demonstrate what is going on.