ABOUT SURVEY MONUMENTS AND BENCHMARKS
by Richard L. Carey, revised January 25, 2008.
All across the continental US as well as in Alaska and Hawaii there is a network of survey monuments which are bronze disks about 8 to 10 cm. in diameter set in rock or permanent structures. The exact number can only be guessed at, but I read that the number is one million or more in place in all manner of locations from city sidewalks and bridges to remote ridges and mountain tops. These have been set by surveyors since 1879 and are the basis for horizontal and vertical control for all the mapping done in the US. The survey monuments used for horizontal control are called triangulation stations, triangulation marks, control stations, or simply stations. A benchmark is a monument that is part of a leveling network and is a point of precisely measured elevation. The term is derived from "bank" as in the elevated land along a river. I tend to use the term benchmark for simplicity, but may use the term station in the following discussions.
Looking at most any 7.5 minute or 15 minute topographic map you can probably find several benchmarks on it. Most of those at lower elevations are labelled with BM and an elevation next to an "X" mark. The ones of interest to the hiker, which are on high ridges or mountain tops, will be shown with a small triangle and most of the time an elevation is shown. There are some, maybe less than 5%, which will not have an elevation value. The name of the benchmark will be shown in a smaller type than the name of the summit. If the summit is unnamed then only the benchmark name will show. Many times the name of the benchmark is the same as the name of the mountain and is not shown separately.
Benchmark Placement on Mountain Tops
Most of you reading
this have probably been to a lot of summits which have had a benchmark
in a prominent location that was easy to see. There may be a large
rock at the summit and the benchmark will be cemented in place
at the very highest point. Surveyors are concerned with visibility
to other survey monuments and thus may place the benchmark near,
but not actually on the highest point. The elevation of the benchmark
will in these cases be lower than that of the true summit and
the position as measured in latitude and longitude will not be
that of the highest point.
In addition to the primary benchmark, surveyors usually will place two others nearby called reference marks or RM1 and RM2. These will usually be within about 50 feet of the primary mark and be at right angles to each other. They will be marked with an arrow pointing to the primary site to aid in locating it should it be obscured by rocks or plant growth or to help relocate it should it be destroyed.
The primary benchmark will most always have the station name and date it was placed stamped onto it. Only rarely is the elevation put on. The name is the one the station goes by in the National Geodetic Survey data sheets. Benchmark names are not unique and may be used again even in the same area so a number is assigned to them called a Permanent Identifier (abbreviated as PID) which consists of two capital letters followed by four numbers. The PID is unique and is never repeated anywhere in the U.S.
Geodetic Data Sheets
A data sheet accessable
to the public exists for many, but not all benchmarks shown on
the maps. These contain the name and location of the benchmark
indicating the state, county, and USGS quad or topo map that it
is on as well as precise survey data such as latitude, longitude,
elevation and numerous other parameters mainly of interest to
Shown below is the major part of a data sheet for FRISCO which is for Humphreys Peak, the highest peak in Arizona. The benchmark in this case is not named directly for the peak, but for San Francisco Mountain of which it is a part. Note that the PID is entered down the left side in front of every line so it definitely cant be missed! Under the CURRENT SURVEY CONTROL heading is the latest latitude and longitude for the site referenced to NAD 83 the North American Datum of 1983. The Datum is the mathematical model for the shape of the earth. (See the article on GPS for a more complete explanation of this.) Also shown is the elevation above sea level in meters and feet. If the note VERTCON is shown then it is the latest adjusted elevation. If the word SCALED is shown then it has been scaled from a map and not accurately measured.
Below other explanatory notes is the position in other coordinate systems such as the State Plane Coordinate system and Universal Transverse Mercator which are abbreviated SPC and UTM. Then under Reference Object other nearby benchmarks sighted during the survey are listed showing their PID, name, distance, and bearing. Note that in this case there is only one close reference mark called FRISCO RM which is about 10 meters away toward the southwest (on a bearing of 220 degrees).
Looking further down the data sheet there is the HISTORY which shows when the benchmark was first placed and the date of subsequent visits called recoverys. Then there is a STATION DESCRIPTION which gives the location relative to towns and nearby landmarks. STATION RECOVERY, the last item, can be many pages if the station has been visited many times. This gives a description of how to reach the station including driving and hiking directions. The older ones shown first can be interesting for they show a record of changes in the area. Many times highway numbers have changed, small towns are no longer there, and ranches have changed names. Details of the summit area often record lookout towers being torn down or put up in which case the benchmark or some of the reference marks may have been destroyed. To save space I have truncated the last part of the station descriptions for FRISCO. The complete data sheet is shown under AZ in the benchmark files.
Starting Datasheet Retrieval...
National Geodetic Survey, Retrieval Date = APRIL 28, 1998
FQ0624 DESIGNATION - FRISCO
FQ0624 PID - FQ0624
FQ0624 STATE/COUNTY- AZ/COCONINO
FQ0624 USGS QUAD - HUMPHREYS PEAK (1983)
FQ0624 *CURRENT SURVEY CONTROL
FQ0624* NAD 83(1992)- 35 20 46.83090(N) 111 40 40.60394(W) ADJUSTED
FQ0624* NAVD 88 - 3851.8 (meters) 12637. (feet)VERTCON
FQ0624 LAPLACE CORR- -0.55 (seconds) DEFLEC96
FQ0624 GEOID HEIGHT- -22.51 (meters) GEOID96
FQ0624 HORZ ORDER - FIRST
FQ0624.The horizontal coordinates were established by classical geodetic methods
FQ0624.and adjusted by the National Geodetic Survey in August 1993.
FQ0624.The NAVD 88 height was computed by applying the VERTCON shift value to
FQ0624.the NGVD 29 height (displayed under SUPERSEDED SURVEY CONTROL.)
FQ0624.The Laplace correction was computed from DEFLEC96 derived deflections.
FQ0624.The geoid height was determined by GEOID96.
FQ0624; North East Units Scale Converg.
FQ0624;SPC AZ C - 482,020.995 235,058.044 MT 0.99990580 +0 08 17.2
FQ0624;UTM 12 - 3,911,663.013 438,397.666 MT 0.99964677 -0 23 32.0
FQ0624: Primary Azimuth Mark Grid Az
FQ0624:SPC AZ C - MESA BUTTE 347 32 59.2
FQ0624:UTM 12 - MESA BUTTE 348 04 48.4
FQ0624| PID Reference Object Distance Geod. Az |
FQ0624| dddmmss.s |
FQ0624| FQ0611 DEADMAN LOOKOUT TOWER APPROX.11.2 KM 0272657.4 |
FQ0624| FQ0625 AGASSIZ PEAK APPROX. 2.3 KM 1792529.6 |
FQ0624| FQ0662 WOODY MOUNTAIN LOOKOUT TOWER APPROX.23.6 KM 1963245.8 |
FQ0624| FRISCO RM 10.351 METERS 22038 |
FQ0624| FR0861 MOUNT SITGRAVES APPROX.29.8 KM 2692426.4 |
FQ0624| FQ0674 KENDRICK PEAK APPROX.17.1 KM 2933506.0 |
FQ0624| FQ0675 KENDRICK PEAK LOOKOUT TOWER APPROX.17.1 KM 2933513.1 |
FQ0624| FQ0706 MESA BUTTE APPROX.34.4 KM 3474116.4 |
FQ0624 SUPERSEDED SURVEY CONTROL
FQ0624 NAD 83(1986)- 35 20 46.82905(N) 111 40 40.60708(W) ADJUSTED
FQ0624 NAD 27 - 35 20 46.80200(N) 111 40 38.02900(W) ADJUSTED
FQ0624 NGVD 29 - 3850.5 (meters) 12633. (feet) VERT ANG
FQ0624.Superseded values are not recommended for survey control.
FQ0624.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
FQ0624.See file format.dat to determine how the superseded data were derived.
FQ0624_MARKER: DS = TRIANGULATION STATION DISK
FQ0624_SETTING: 80 = SET IN A BOULDER
FQ0624 HISTORY - Date Condition Recov. By
FQ0624 HISTORY - 1920 MONUMENTED CGS
FQ0624 HISTORY - 1936 GOOD CGS
FQ0624 HISTORY - 1958 GOOD CGS
FQ0624 HISTORY - 1965 GOOD USGS
FQ0624 STATION DESCRIPTION
FQ0624'DESCRIBED BY COAST AND GEODETIC SURVEY 1920 (CLG)
FQ0624'STATION IS ON THE HIGHEST AND NORTHERN PEAK OF THE SAN
FQ0624'FRANCISCO MOUNTAINS AND ON WHAT IS LOCALLY KNOWN AS HUMPHREYS
FQ0624 STATION RECOVERY (1936)
FQ0624'RECOVERY NOTE BY COAST AND GEODETIC SURVEY 1936 (JDT)
FQ0624'STATION MARK, REFERENCE MARK, WERE FOUND AS
FQ0624'DESCRIBED AND WERE IN GOOD CONDITION. A NEW AZIMUTH MARK,
FQ0624'WAS SET AND...
Searching for Data Sheets on the Web
Getting data sheets used to require a call or letter to the National Geodetic Survey where your request was handled by the staff. This was a slow and expensive process and a group of ten stations from one 7 1/2 minute block would cost $20.00 in paper form. Due to the cost and lack of information the general public did not see these data sheets. With the advent of the Internet and the world wide web the NGS has put this information on their home page readily accessable to the public for free. You can search the database of approximately 750,000 benchmarks by three methods: First you can select the state to be searched and enter the name. Next you can search an area by entering a latitude and longitude. Finally you can search by entering the PID. The web addressess for these are:
Search by state and name.
Search by area.
Search by PID.
Search by Quad (map name).
Another method is to purchase the data on CDROM. To cover the entire U.S. the data has been divided into five regions which sell for $50.00 each. These can be purchased by credit card if you wish by calling (301) 713-3242.
Please note that the National Geodetic Survey, formerly the Coast and Geodetic Survey is part of NOAA the National Oceanic and Atmospheric Administration under the Department of Commerce. It is concerned with the maintenance of the National Spatial Data Infrastructure a part of which has been described. The US Geological Survey under the Department of the Interior is the agency responsible for the production of topographic maps. Sometimes these agencies are confused or thought to be the same organization. For more information see the home page of the National Geodetic Survey at: http://www.ngs.noaa.gov.
Benchmark Data Sheets Included on this Web Site
I have searched the web pages mentioned above for each of the fifty state highpoints and gathered benchmark data sheets together in a file for each state. Of the fifty states, I found thirty-six with some type of data sheet that pertains to the summit. For some states there are numerous entries which include not only a benchmark at the highpoint, but nearby ones and also sightings of structures such as observation towers. So to be complete all data sheets for anything within a few hundred feet of the summit are included. These are simple text files and may be viewed in Wordpad or most any word processing program.
For many of the peak lists I have identified where there is a benchmark on the summit and have noted the PID number if it is in the USGS database. If the PID is blue then I have saved this file and it is available from this site. Not all of these are completed at this time.
In the peak lists, if the benchmark name is in blue then there is a photo of it.