Permanency is never guaranteed?ÿ but we can increase the odds.?ÿ
The odds are not in your favor when you set a 2' X 1/2" rebar with a plastic cap sticking 6" out of the ground with no memorials or references.?ÿ
We once had to set a benchmark for some work at a school, and my colleague wanted to use the "A" in the lettering on top of a fire hydrant across the street; something that I had seen done off and on for many years. I've never liked that, because how do you know if that hydrant has ever been replaced? A different model will give you a different elevation! So I demanded we set a more traditional benchmark. Sure enough, later that afternoon a crew came to replace that hydrant. It was not leaking and was not part of the work site, so I have no idea why it was replaced - but it cemented my position that I would never use a hydrant for a benchmark.