Question on IP's and terrain

[CarnageINC]

I've read through the User Manual, on pg 20 it covers IP and Trenches.

My question is; are the defensive bonuses of terrain and IP's combine together or is the highest value of either the IP or terrain used when determining defensive values?

IP gives you 20% bonus, and woods give 30% bonus, so is this a total of 50% or just 30% for the woods?

I'm not understanding this whole thing correctly?!?