To measure 100 miles, assuming your 120 foot measurement and assuming that game mechanics = in-world logic, you'd need to cast your spell 4400 times per 100 miles, moving at a glacial pace as you measured. You'd also need to ensure that your path was perfectly straight, you accounted for elevation changes, that you precisely marked each endpoint, etc etc etc.
If you're seeking accuracy, measuring the outside of a 200mi x200mi square would require doing all those calculations across 35,200 castings. If you were measuring the internal dimensions to about a one mile accuracy, you'd be in the millions of castings and you'd still need to ensure you accounted for all of the above.
Regardless, we know that it doesn't work that way because FR sourcebooks tell us that maps are inaccurate, rare, and expensive. We know that the maps are inaccurate because they've told us so!
It is when it does not seem to solve the issue. We will try it again.
I may try that, but the old one being critically broken for years is a terrrivld look.