Often hills are indicated on a map by drawing an outline, but when you have forest on top, that outline gets obscured. So how do you draw forested hills? The trick is to use the detail of the forest to indicate the hills, rather than obscure them.
It’s always useful to get some real world reference, even when doing line art maps. Here’s a satellite photo of the Trossachs – a forested region of hills in Scotland. Notice that it’s the density of hard shadows on the trees that tells you where the hills are in the forest. (click the image to go to the map)
1. Draw the outline of the forest and the hills (separate layers)
First, we need to know where everything is supposed to be. Draw the outline of your forest on one layer, and the outlines of your hills on another. If you’re using paper, use pencil for the hills and pen for the forest. We’ll be erasing the hard hill outlines later.
2. Add in the forest detail on the shaded side of the hills (on a new layer)
Here I’m taking the direction of light to be from the top left, so any side of the hill away from that is in the shade. By placing tree details (short curving lines) close together, those sides are darker, and will give a sense of the 3D terrain. The densest lines should be on the steepest edge of the hill, and on the side furthest from the sun (in my case – bottom right). Add similar details around the edge of the forest as well.
3. Erase the solid hill lines, and tidy up
Now remove the solid hill lines. It suddenly looks a lot better – your eye is reading the tree density as hill shadow, rather than relying on the lines!
After removing the lines, you’ll see areas that could take more detail. I’ve added some tree details around the top-left of the hill shapes, to complete the outline, and deepened the detail in the shadow to really make the hills pop out.