You posted another picture of one of those things before. What is it?
These are US Geological Survey markers. They’re like an official, measured point where the altitude/coordinates are specifically known. They’re used on things like the peak of mountains, the edge of national parks, etc. It’s like a surveying marker for a piece of property, only from the government. I love them because you find them out “in the wild”, and they are very specific and unique to that location.