The graphic comes from Archaeology magazine, not sure how popular they are. :’)
One would think that there is still sufficient extant evidence of at one section of the earthwork wall -- and of its position and outline -- to justify the dimensions, rounded corners, etc. as depicted.
Also, one would expect that evidence of tent footprints and tent spacing -- as well as of the fact that they were laid out in sections as shown -- showed up in GPR and/or excavation data.
With that info, you could map the known tent sites, etc., and 'fill in the blanks" using CGI.
If you ever run across more of that actual data, I would really appreciate getting a look at it!
I don't doubt at all that the Roman military were sufficiently organized and disciplined to lay out such a precise camp. What I'd like to see is the data that allowed (probably) sparse GPR readings to be extrapolated to create that detailed graphic. (GPR has never worked quite that well for me...) '-)