For what it is GPS is great it's just not accurate enough to do what he wants it to do.
He doesn't need global accuracy for a field, he needs something simpler, faster, cheaper and more accurate over a relatively small area.
I was involved in the early days of using satellites for global positioning and was a geodetic surveyor, did 1st order work and even did some work that came to be a part of WGS84. Heck I even moved Guam a mile. What we did is where GPS came from.
I know what GPS is good for and what it isn't and this ain't one of em. For what he's doing a string with knots in it is more accurate and repeatable than the GPS gear he has access to. Global Positioning System, it says what it is and everything is in its name and for what it is, it is wonderful, it just isn't so great for everything.
An original member of DMA here, which later morphed to NIMA and today is NGA or the National Geospatial Intelligence Agency so I pretty much know this kinda stuff cold.
Defense Mapping Agency
However with all that said, if you don't wanna get lost, don't leave home without it because, it's great for that.