Dear Capt. Paul:
I am interested in joining a geocaching club. Part of the activities are finding benchmarks shown on topographical maps, taking a photo of the benchmark and logging it and the photo in a club book. I have several topographical maps that show specific numbers next to a small X. Is that the benchmark for that area?
Captain Paul’s response:
Geocaching is a really great way get out and use your GPS unit, and doing so will definitely sharpen your GPS skills. Plus, it allows you to get some physical exercise in the outdoors. Most geocaching benchmark sites can be found without actually using a GPS receiver, but using a GPS will certainly allow you find the benchmark faster with a greater degree of accuracy. In addition, it gives you a way to improve your use of your GPS.
The number and small “X” you see on your topo map is not a benchmark (BM), but the location could be next to one. The small “X” is the vertical designation for that point based on feet above sea level. Benchmarks when shown are usually small red triangles.
The pictured topographical maps indicate a hill top with a small “X,” which indicates that position is 185.8 feet above sea level. The small triangle just to the north of it indicates a benchmark very near the hilltop. You will note that they are NOT exactly on top of another. The BM is just to the northeast of the indicated hilltop.
These and other topo map features and definitions can be accessed and printed at https://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf
It is worth any outdoors person who uses maps to download that file to understand what the symbols mean.
The term benchmark originates from the chiseled horizontal marks that surveyors made in stone structures. They generally consist of an angle-iron placed to form a “bench” for a leveling rod, thus ensuring that the rod could be accurately repositioned in the same place in the future. These marks were usually made with a chiseled arrow below the horizontal line in a stable stone or structure, a point of reference from which measurements could be made and one that serves as a standard by which others may be measured or judged.
Nowadays these benchmarks can range from a brass description plate set in the top of a small concrete post, to a pipe sunk vertically in the ground, to a state highway marker, bridge end or even the top of water tanks. Now they are set using a grid system (Latitude/Long, UTM or State Plane) and are placed on location with a great degree of accuracy.
I have a friend who started using his smart phone’s GPS capability. He, however, is lost when a cellular tower in not within the phone’s range, and as you know the accuracy of the smart phone is about 400 feet — where the accuracy of a WAAS equipped GPS unit is about nine feet (9). The difference is obvious.
If you haven’t yet done so check the internet for geocaching sites, or use the one that I use: https://www.geocaching.com/mark/
It gives the user a broad understanding of ways and definitions used in the sport. By all means check them out.
As noted on these sites, the use of a computer can allow you to find the locations of the benchmarks. These are sites that allow you to search and verify a location by accessing the benchmark search engines. Naturally the data can be printed and should be carried with you during your quest.
National Geodetic Survey Data Explorer at https://www.ngs.noaa.gov/GPSonBM/index.shtml offers data on their benchmarks, including a data search site for finding benchmarks and their associated data. This includes a detailed description how to find the site by following physical descriptions.
A typical NGS shows the grid position as:
“UTM 15 - 3,574,964.332 493,286.219 MT 0.99960056 -0 02 17.2. And describes the location as “A U.S. Coast and Geodetic survey disk set in the top of a 12 inch cylindrical concrete monument that is flush with the ground. It is stamped hard R.M. 2 1971. It is 221 feet northwest of Power Pole Number 249, and 192 feet south-southeast of the center of a gravel road. The azimuth mark is a standard Louisiana Department of Highways U.S. Coast and Geodetic survey disk set in the top of a 12-inch cylindrical concrete monument that is flush with the ground. It is stamped hard AZ. MK. 1971. It is 103 feet north-northeast of Power Pole Number 252, 30 feet south-southwest of a 14-inch Black Jack Oak Tree, 19 feet west of a 16-inch Black Jack Oak Tree, and 3 feet southwest of southwest of a road marker, height of light above station mark 1 meter.”
Most positions are also shown in a latitude/longitude grid format — if not you will have to rely on converting the UTM format to lat/long. They also state the horizontal DATUM used for determining the exact location.
Remember to bring along the datasheet for each BM, either on paper or in a portable computer. The coordinates will get you within 15 feet or so.
When looking for pipes, you may need a metal detector. Always ask for permission if the mark you wish to hunt is on private property. As with any outdoor activity, courtesy to the landowner is top priority. Try to determine from the location description whether or not the mark is on private property. If it is, ask for permission from the landowner. Most landowners will give permission and many are interested in the history of the mark and the hunt and may want to help you find it. These sites are actively used in surveying, and are protected by law.
Some useful web sites may be found at https://geodesy.noaa.gov/web/tools/updates/windesc5/dformat.documentation.htm#A.11 or https://www.ngs.noaa.gov/GPSonBM/Recover.shtml.
If you need any additional information contact me and I will be happy to try and answer your questions.