Chapter 9 treats two topics. First it gives us detailed analyses of how to position the army in various terrains. Then it shows how to read the opposing army through the physical evidence of its movements. For example:

Many trees move.
He is approaching.

Birds rise up.
He is concealing himself.

Or:

Dust is high and sharp.
Chariots are approaching.

It is low and wide.
The infantry is approaching.