The surveyed lands were divided into square townships that are six miles on a side, 36 square miles in area. Each square mile, or "section" is identified by number, 1 to 36. In some areas of the far west (such as here) the GLO placed brass survey markers, like the one to the left of the cairn in the photo, to mark section corners.
Back in the day before hand-held global positioning systems, we navigated our field surveys with USGS topo maps and compasses and these sorts of markers were welcome friends that helped us find our way. Optical surveyors using transits also depended on these and most we see here are marked with cairns, wooden masts or reflective markers like you can see in the picture at top.
Highly accurate GPS systems eliminate the need for these. Each of my survey crews carries a hand-held GPS unit with sub-meter accuracy and we are totally spoiled in finding our way around. We typically check the work of the old surveyors, and my crew chief tells me that the marker you see above is about 10 feet west of where it should be.
I think that's darn fine shooting for 1912 technology, don't you?