By my place in Florda the USGS Installed white 2 inch PVC markers about 5 feet tall and around a 1/4 mile apart.
They are marking Section lines and enable them to be viewed from the air, 95% of the ones here are in saltwater marsh type areas.
One piece of property I own happens to be on a section line along it's southern boundry, it's also one they marked with the white PVC posts.
