It does help to have a good map, especially if you're on the side of a hill, which I am. But as the crow flies (like an aerial map) is very different on a hillside than as the foot walks. I have made what I call triangle maps that plot the distances on the side of a hill with one side of the triangle being the sloping ground (A), the second side is a tall pine tree at the bottom of A, (B), and the third side is where the high point of A intersects an imaginary level line to the pine tree, (C). It's kind of like doing an MRI of your property, looking sideways (across a hill) and gauging distances, rather than only using the bird's eye view.
these triangles let you know the real distance, even though the distance of C (as the crow flies) might only be two-thirds of A if it's really steep. I have one steep slope on my place where A is 30 feet long and C is only 20 feet, so when I made a set of stairs there, using the 20-foot regular bird's-eye-view map measurement would have been a mistake. I would have been short 10 feet of stairs, plus with the triangle measurements I had the drop in elevation correct, which is part of the math of stairs.
I stand at a distance and picture how many lengths of PVC pipe it would take to cover the distances. I think in terms of 10' pieces of PVC pipe. I lay out several lengths of 1/2" PVC pipe on a flat surface nearby to check back and forth what I'm eyeballing. Or if I've got access to the hillside I connect the PVC pipes down the length of it, then I can walk in any direction and still see how the PVC line relates to the slope and the other land markers, like fences, trees, boulders, structures, water tanks. All of the lines connecting those things can become triangles that can be noted on a hand-drawn map. I always do hand-drawn maps and judge things by this is half of that, or this is two thirds of that, or this is twice as high as that, so the general proportions can be written down.
Have you seen Google Terrain?
"Life flows on within you and without you"...George Harrison
Coastal California, USA, Mediterranean climate - no summer rain, a little frost mid-winter