What's the backsight?

This screen shot reminds me of a story told me by Jim Wheeler. Back in the day using total stations of setting out a proposed centerline for a road. There wasn’t much around so Jim set a temporary back-sight on a boulder near the horizon. Every two or three shots, he’d re-zero on the temp BS and after several rounds of this, noticed that his gun (total station) wasn’t holding it’s angle very well. Four or five more shots later, they began to notice that the staked centerline, which was supposed to be in a tangent (straight as an arrow) was curving off to the left. They kept going for a while until about 25-30 shots in, Jim noticed that the boulder he was back-sighting near the horizon was actually a cow grazing along. The centerline of the road was following the back-sight position of the cow. Oops…