Gray Crag and Grey Crag

I was curious when I first saw the Wainwright list, Gray Crag and Grey Crag. Both are in the far eastern fells. I hadn’t been on either. On my Wainwright Challenge, Gray Crag was an out and back from Thornthwaite Crag. Grey Crag was the final top on a circuit from Sadgill. An intriguing aspect of the Sadgill walk was the remains of the survey posts relating to the Haweswater Aqueduct. In their day they were obviously fairly substantial structures. A residual relic of the survey post’s inner workings was a cut-out piece of corroded metal plate, leaning against the post. It had amazing sculptural qualities which appealed to me. I would like to know what the function of the survey posts was.