As I mentioned before, this thread started out as a post about shadows and how they can be used to create a feeling of space. Then, as I was writing the post about shadows, I felt remiss that I had never talked about getting a feeling of space and depth into line drawings...so then I thought I would (quickly) jot down the basics of getting depth into drawings without tone. But, the more I jot down the more I realize I want to say about it - plus, that post on shadows is becoming bigger and more complicated every time I work on it, so this helps me put off figuring that one out for a while.
Anyway, these are some more ideas about how to get the illusion of depth, space and form into a drawing, specifically drawings without color or tone. Both color and tone are very useful tools when it comes to describing space and form, so we have to work a little bit harder to do it with just lines. Maybe all of this stuff seems obvious. Anyway, he's a few more stray thoughts on the topic at hand...
Having a form lay flat against the ground plane (or come into contact with it) can definitely describe the space of your drawing without much else being necessary. A figure laying against the flat ground will tell you a lot about the space by the way the parts of the body overlap one another and move away from the viewer in perspective. Here, the way the body hits the flat ground and the way it squashes against the flat form of the floor gives a solid feeling to the space and feels like there's definitely some depth to the scene. Plus, having the Dad overlap Dennis is helpful to sell what's in front of what (see the last post to read more about overlapping objects to create space).
Similarly to this, you can achieve a nice feeling of space by simply having your character's feet planted firmly in perspective. This can convey a very convincing sense of space when it is done right.
Obviously the same thing applies to anything which is firmly planted on the ground plane and is drawn with perspective that looks convincing. Or even an object that's up in the air.
Just planting different areas in your picture at different heights can be effective. Putting the planes that are further away higher up in the frame is the simplest way to get a sense of space in your drawing.
We all think of having forms shrink as they head off towards the horizon, but don't forget you can give your drawing perspective that recedes in the vertical plane, instead of the horizontal, when appropriate.
Alternating areas of texture against empty spaces also helps convey a sense of space because it makes it seem that there are several different areas within the same frame. This is what we see in real life. When you look in any direction, you see many, many different areas stretching out towards the horizon: grass, asphalt, sidewalk, etc. As artists we have to come up with ways to simplify so we don't have to draw everything while still making it feel like our pictures have a lot of depth. So when you alternate complex areas with blank areas it gives that kind of feel, and it's pleasing to the eye to linger on the textured areas and then rest while glancing at the empty spaces.
The blank areas don't always have to be white; they can be black silhouettes instead, of course. They could even be areas of grey tone as well.
Obviously this is a useful design principle: to balance complex areas against empty areas. It would be meaningless to put one complicated pattern next to another, or to put an empty space next to another. The two types of areas only mean something when balanced by each other for contrast.
Also you can get this same sense by having alternating areas of dark and light in your composition. Seeing several areas of light and shadow on the ground plane helps define the ground plane and make it seems as if it is receding farther back in space than if you drew your ground plane as just a blank space. It can be a better way to get depth to a ground plane than drawing a lot of textures on the ground to show the form of the ground (like if you drew all the individual blades of grass on a lawn or all the cobblestones of a street).
Lastly, never forget the benefits of aerial perspective to sell the idea of space. When you look towards the horizon, all the particles of dust, moisture and pollution between you and the objects on the horizon make things look hazy or indistinct. The farther away you look, the more of those particles there are between you and the object you're looking at. The cumulative effect makes things look hazier the farther away they are.
If you look at some of the illustrations in the previous post, the artist Cosey used thicker lines on objects in the foreground and thinner lines for objects that are far away on the horizon line, a very common solution to achieve the effect of aerial perspective.
Remember that the areas closest to our eye usually has the most contrast and will generally be darker. The areas further away will have less contrast and will generally be lighter in tone.