Down in the marsh land, it's pretty flat.
Unless natural features can be used, like water,bayous etc.,
it is largely D-R. Pace beads help, or most of the time, just
estimated distance walked.
The real task is to find something to use for triangulation,
most often radio towers (antennas >500 FT tall) which can be seen for miles.
That being said, the worst case is the desert flat lands.
Almost feature-less. Miles and miles of the same.
Not a chance of any feature like stream, road.

I really wish we had hills even (mountains would be great).
You know it's bad when the contour interval is 2 FT! on a topo map.
There is a reason a lot of people get lost in the marsh or swamp lands, both have the tendency to look like every other place in marsh or swamp. If you loose track of where you are, it can be a challenge to triangulate and back track. Been there done that, spent the night, also not fun. Didn't have a GPS then, now I would never be without one, but also wouldn't be without MAP & Compass either.