Have you ever seen one of these? They're all over the place - most likely there are some right in your town, in plain view. It's a benchmark.
What is a benchmark?A benchmark is a point whose position is known to a high degree of accuracy and is normally marked in some way. The marker is often a metal disk made for this purpose, but it can also be a church spire, a radio tower, a mark chiseled into stone, or a metal rod driven into the ground. Over two centuries or so, many other objects of greater or lesser permanence have been used. Benchmarks can be found at various locations all over the United States. They are used by land surveyors, builders and engineers, map makers, and other professionals who need an accurate answer to the question, "Where?" Many of these markers are part of the geodetic control network (technically known as the National Spatial Reference System, or NSRS) created and maintained by NOAA's National Geodetic Survey (NGS).
Some Geocachers enjoy hunting for benchmarks as sort of a side hobby, since often times in our treasure hunting we bump into them anyway. You don't actually even need a GPS in order to find benchmarks. You can go to the Geocaching.com page on benchmarks HERE and on the right hand side at the top, plug in your zip code to find benchmarks close to you. You can click on a particular one and read the datasheet for it, which gives coordinates, as well as written directions for finding the benchmark. The datasheets themselves are very interesting - they document attempts over the years to check on the benchmark. Some benchmarks are very old and the written directions might consist of something like, "10ft east from large oak tree, 100 yards south from Mr. John Smith's stable, and 18 paces north of a large rock outcropping" (but there should be more modern directions documented as well). Even if you don't hunt for any, the datasheets can be fun to read! Go take a look and see where there are benchmarks that you never noticed!
There is much more interesting info about benchmarks - if you want to learn more, be sure to check out Geocaching.com's page on Benchmark Hunting. If you can find any benchmarks, Geocaching.com will even let you log them.
Yes, we've found lots of benchmarks. :-D