Let me show you.

Our proper course of march is in the direction of a line drawn from the sycamore to this pine tree.

What we want to do is to prolong that line, and find some tree further on that stands in it.

If I stand on the line, between the sycamore and the pine and turn my face toward the pine, I'll be looking in exactly the right direction, and can pick out the right tree to march to, by sighting on the pine.

The trouble is to get in the right place to take sight from.