Survival skill FINDs terrain for Ambush; Tactics skill to best utilize it?

[Refplace]

Originally Posted by thom

Well, I was thinking that the PCs would follow the backtrail and, along the way, see what might be good terrain, and then have to make a Tactics roll. It seems a little incongruous (to me) to make a Tactics roll to "find" the best general terrain for an ambush. The hobbits have already volunteered to send their best to help the PCs conceal themselves when they set the ambush, so that's not a problem...

thom

P.S. To clarify, the baddies don't know about the PCs, the group arrived the day after the baddies first visited...

Tracking to follow the backtrail.
Survival, Geography, and Area Knowledge I would not let help here as the first is really irrelevant and the latter two would be more used to find good areas of possible ambush in an area. However your area is limited to the trail.
Tactics is going to be the best choice to figure out the best of the possible options for setting the ambush. Then Camouflage and Stealth to hide.
Survival might be useful to help if they could use the terrain or nearby animals to help the ambush, including making some traps like deadfalls or pit traps.
Survival might help in avoiding a bad spot for an ambush. Examples would be a dry river bed if a storm is incoming, where animals might alert people of your presence, etc. I could see Survival as a safety in the first case and a complimentary to camouflage in the latter.