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).
Why search for benchmarks?
The interesting thing about benchmarks is that a majority of them are located in plain sight (though largely ignored by the general public). Searching out these locations and documenting them allows others to share pictures of the various areas where they are placed. There's a certain excitement to be the first to find and document a control point, as well as seeing what others have found through photos on the website's benchmark gallery. Some of these points haven't been visited and documented in a very long time, so you may also be rediscovering long neglected objects of American history as well!
For the Beginner
The first step in any benchmark hunt is to choose some benchmarks to search for and get their datasheets. Do not set out with coordinates only; you must have the verbal to-reach description in the datasheet with you when you go. A good first step is to search around your own Zip Code using the search window at the top right corner of this FAQ page. For your very first trip, select some benchmarks that have recently been found so that you will be sure to see one in person on your first trip. The easiest to find are the ones that have been found by someone else using Geocaching.com and have uploaded pictures showing the mark's location. If none near you have been found by people using Geocaching.com, then choose some that have recovery reports from the last 10 years, since the probability is greater that they are still there, and findable. You should select some that are location-scaled and some that are location-adjusted so that you see what that difference means in the field.
The Hunting Process
When you arrive at the area of a benchmark's location but before you get out of your car, read the datasheet carefully. The first thing to note is whether the mark is location-adjusted or location-scaled. Next, read all the benchmark's recovery notes from the most recent to the oldest. In each recovery note, the most important item of information is the distance and direction of the landmark that is the the shortest distance from the station. Start with the closest landmark of all the recovery notes and see if you can find it. The most accurate instrument you have is your tape measure, not your GPSr, so use the tape measure first. If the mark can't easily be found, continue with the next closest landmark and measure from it to an intersection area with the distance you measured from the first landmark. If you don't see the mark yet, probe the ground in the area with a probe, (benchmarks often get buried) searching for the disk plus its monumentation. If you haven't yet found the mark, read older recovery notes again to try to get more information, since many recovery reports don't bother repeating older recovery information that's still good.
For more information please visit: http://www.geocaching.com/mark/
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