It's a trig point, one of many around the country which were originally used for surveying purposes. They're basically a vestigal structure now as things like GPS have made them redundant for modern use. This design is pretty common but you do find a few variations on the theme - basically all I've seen have the top circles on a post but the support structure varies.

If you check out prominent hills in your area there's a good chance that you'll find some trig points, they were very widespread and large numbers are still around albeit often in a state of disrepair or with vegetation blocking them from view