You can make any size image map (16 bit for much less stepping) and relocate and resize in TG by transform shader. Resolution must be taken into account of course. The ditch around the camp is a 1x1K greyscale (which is too big, but who cares) 8-bits, because it's only a meter deep. 256 grey tones means 1m divided by 256 = 4mm per step, you don't see that. Even it it were 10m deep you wouldn't see the stepping in 8-bit if you add some fractal.
Ditch is ditched anyway, it's a prickly hedge now.