To locate and map features on the ground we use aerial photographs or LIDAR images.
The latter are created from millions of laser pulses sent from an aeroplane.
Then an enormous amount of computing power is used to produce very detailed 3D models of the ground surface and at the same time it can strip off most of the trees.
Then features like dykes, roads and ditches can be followed from open ground to beneath the trees.
This of course is not possible in photographs.