​how do you find the measure of a distance that you cant measure directly?

Probably with trigonometry.  :/

There is a formula to measure the distance between you and the Horizon:

Distance to the Horizon: 3.57155 X sqrt(H/M)=Km, where H/M=your height in meters above sea-level. Example: If you are standing on top of a building that is 100m high, then the distance to Horizon is: 3.57144 X sqrt(100)=35.71Km.