This is the concept of geometry, to get the expression for horizontal distance h as a function of d, we shall use the Pythagorean theorem;
c^2=a^2+b^2
thus;
hypotenuse=d
horizontal=h
slanting=8600
thus;
d^2=h^2+(8600)^2
This implies that:
d=sqrt(h^2+(8600)^2)
thus;
d(h)=sqrt(h^2+(8600)^2)