It is very much about trees. First of all, there will be fewer branches to the north. This is usually easiest to see if you look up along the trunk of the tree. On the image, you can also see that ants like to build their nests on the south side of the tree. Take a map.
Lets look at a map made for orienteering.
The edge you must be using, is the edge that is parallel to the direction of travel arrow. And then, put B somewhere along the same edge, like it is on the drawing. If you do this wrong, you'll walk off in the exact opposite direction of what you want. So take a second look. Keep the compass steady on the map.