The word "benchmark" normally refers to a test or comparison. For example, in the information technology field a benchmark would reveal the speed of an IBM mainframe computer performing certain tasks compared to another type of computer doing those same things.
There is another type of benchmark that may be less familiar to most of us which is based on "geodetic control points". These benchmarks are typically brass or aluminum disks which are permanently affixed to a rock or monument — in other words to something that is not supposed to ever move. Surveyors and civil engineers use the benchmarks as reference points to enable them to design roads, buildings, bridges, or to make maps. The benchmarks are created and maintained by the National Geodetic Survey (NGS) — as of this writing there are 736,425 benchmarks in their database. The benchmarks are also in the geocaching.com database and provide a great extension to the sport of geocache hide and seek.
The interesting thing about benchmarks and horizontal control points is that a majority of them are located in the open (though largely ignored by the general public). Like geocaching, searching out the locations of benchmarks and documenting them allows the enthusiast a sense of accomplishment — it is fun. Some "benchmarkers" share pictures of the various areas where the benchmarks are placed. I have found this really helpful. Some benchmarks are really hard to find even though they are "in plain view". Most of them were placed at least fifty years ago and many have been covered over with dirt, leaves, moss, or in some cases have been destroyed by construction projects. They are also hard to find because the latitude and longitude were established using maps — there were no handheld GPS’s back then. I have found one that was 300 feet from where it was supposed to be.
In the third week of August 2005, 901 benchmarks were logged by 371 users. Overall, 68,083 benchmarks have been found. Impressive but it is less than ten percent of the marks in the database. There is still plenty of time to find an undiscovered piece of American history!
While riding in the mountains on a hot August day I was able to discover benchmarks LY1199 and LY1200 near Hamlin, Pennsylvania. As you can see, each benchmark has a web page that shows geodetic information and a log of any visits to the mark by anyone. When my GPS indicated I was within 500 feet of the waypoint, I pulled the trike off to the side of the road. At first the sign was not apparent because it was overgrown with weeds. A few feet away I found the metal disc buried in a rock in the ground. The elderly gentleman across the street near a farm house was watching me so I decided to cross over and talk to him. Was he aware there was a national geodetic benchmark right across from his house, I asked? Yes, he said, and in fact he not only remembered when it was placed there in the 1950’s, he proudly said there was another one located on his side of the road right by the corner of his barn, and would I like to see it? I said, you must have lived here for a long time. The gentleman said, "I have lived here for 89 years. The bed I sleep in is the bed I was born in!". How many people in the world can say that?
Looking benchmarks is a lot of fun and you don’t even need a GPS to find them. In fact, the benchmark database is geared toward finding benchmarks from the descriptions, not the coordinates. It has taken me awhile to realize that using the descriptions is actually best. The only problem is that sometimes you read "41 feet southwest of the side porch of the house on the southeast corner of X Road and Y Road" and the house was actually torn down forty years ago and replaced with a shopping mall. Other benchmarks may have been at an intersection of two roads that is now built over by an Interstate Highway.
Some benchmarks are actually things like radio towers, church steeples and smokestacks. The top of the entity is the actual "station". It is not recommended to climb them. Just take a picture and make a log entry at geocaching.com that you found it.
While benchmarking is a fun sport, we should always have respect for them and not disturb them in any way. Many are on private property where we should not trespass, but all of them are potentially important to the economy since engineers, surveyors, and map makers use them to create new public and private facilities of all kinds. you also have to be careful as you if you downtown or near an infrastructure facility of some kind, looking around, holding something in your hand, looking in bushes and near fences or roads. If someone in a uniform approaches you, I recommend putting a smile on your face and introducing someone new to the joys of the hunt.

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