Triangulation stations

[bugo]

Since I was a kid, I've studied road maps of all sorts. One of my favorite series of maps was the Arkansas county maps from the 80s and back. They showed much more detail than the newer ones do. One of the icons in the legend was for a "triangulation station" and the icon was a triangle with a dot in the middle. Oklahoma county maps also show some of these. I was fascinated with these mysterious triangulation stations and asked pretty much everybody I came in contact with what they were. I now know they are little metal discs much like benchmarks and bridgemarks that were placed in certain places that have something to do with surveying. That's about all I know about them. What was the exact purpose of these discs? They are located in many different areas: on mountain peaks, next to fire towers, at certain landmarks like hospitals, the center of small communities, and next to roads. There are large areas with none of these stations, and there are areas with several in the same general area.

Another oddity about these stations is the names didn't always match up with the locations. For example, in Polk County, Arkansas, the Mena triangulation station is not located in Mena but rather on top of Rich Mountain just west of Queen Wilhelmina State Park about 15 miles from Mena. The Acorn station is at the side of US 71-270 several miles northeast of the community of Acorn. The Dallas station is off AR 8 near the airport, not at the former county seat of Dallas. The Hatfield station is east of Hatfield near the old Jefferson Highway. The Hatton station is not located in the community of Hatton, but rather in the town of Vandervoort. Some of them have names that are not placenames, and some are cryptic like "No. 52". There is one called "Eagle" and one called "Eagle Mountain" less than a mile from each other. The Eagle station is located on a ridge on Eagle Mountain and the Eagle Mountain station is located at the former site of the Eagle Mountain firetower.

Why are some of these stations not located where you would think they would be? It looks like the Mena station would be in Mena, not in a rock outcropping off of the Talimena Drive (AR 88).

Why did they put these on county maps? Who would use the maps for this purpose?

Here's a link to the 1983 Polk County map showing triangulation stations all over the county:

https://www.dropbox.com/s/33llata443rxcoh/mpolk_1983.pdf?dl=0

[briantroutman]

My understanding is that the names are unimportant and that the stations are there just to establish reference points–with known latitude, longitude, and elevation. A triangulation point might be called "Smithville"  just because it's somewhere near Smithville and it's an easy name to remember, even if it isn't within the town's boundaries.

As to why they put them on the county maps–I'd assume for the benefit of surveyors, hikers, etc., even though I'd expect those people would more likely be using USGS quadrangle maps.

[wxfree]

They're horizontal control points, points at which the exact location has been measured very accurately.  Because of this, they can be used to survey other locations, just like elevations can be measured based on benchmarks (vertical control points).  Old Texas county maps also show them, but I don't know why.  They're not related to roads or other transportation.

They tend to be located on higher points, so that they can be sighted from far away.  This makes surveying based on the marks much easier.  A rod is extended vertically above the center of the mark, and can be sighted with surveying equipment.  After getting a few of these, and measuring the angles, calculations can determine the location of the survey equipment.  Having these scattered around on high areas makes it much easier to conduct future surveys (and reset marks that get removed).  These days, high-end GPS can do the job pretty accurately.

The reason they don't always line up with the place name is, at least partly, in order to locate them in high areas.  I have no idea why they're on county maps, which are not generally at a scale that makes them useful.  The USGS ones, and some of the NGS ones, are on the topographic maps, which are at a scale that makes them useful.

There are other control marks that aren't monumented.  Sometimes an antenna, a steeple, a stack, flagpole, or other high point is surveyed and recorded in the books.  I don't think I've ever seen those marks on maps, but if you look up the NGS sheets, which are online, you can find them there.
http://www.ngs.noaa.gov/cgi-bin/datasheet.prl

[empirestate]

If you want to know more about methods of topographic surveying than you ever thought you would, read this:
http://pubs.usgs.gov/circ/1341/