This crossed with
@Mesquite but budgeted if I’ll delete it all now
You may be getting a bit too much advice right now but.
Sorry if the what follows is known to you already. I also have an excellent egg sucking technique if you’re interested
The area is wooded so whatever you use you will have to work in steps. Map out a series of rough straight lines that take the closest route to your project. Each end point to give a straight line view to the next.
if you are going to fell trees then obviously do that first to get the straightest line. If not, work around them as
@Mesquite suggests.
Start with a datum point, preferably in the ground. It doesn’t have to be an OS datum and it doesn’t matter what it’s actual height is as long as its position is permanent for the duration of the project and is recorded. You could just drive in a post to nearly ground level and use that. Work the longest useful straight line that is available, one that takes you to the next straight line. Establish your new level and sight up the next one until you reach your end point. We did this around the perimeter of a forest plantation in Delamere Forest - nearer to 2KM than 200M and in a circle.
I’ve also brought a level from the roadside to half way down Belle View Park in Newport (Gwent) full of ornamental trees.
It takes time so give yourself time.