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Gaddiel

"Newbie" Questions

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First of all, thanks to all of you who are willing to share your knowlege with the group in this forum! I've been lurking for a while, and haven't seen this addressed. I know these are probably pretty innane questions from a relative newbie, but here goes:

 

Most of the marks that we've found so far (not that many) seem to be placed pretty randomly. There seems to be no real pattern. I know that some were placed at high points (I'm guessing for line-of-sight) and there seem to be an overwhelming majority of them along railroads.

 

What I'm wondering is: What caused the marks to be originally placed where they are, and why are some marks very close to each other, while others are far apart?

 

My second question: How were/are the marks used for surveying? I originally thought that surveyors used the disks to mount a tripod in a precise location, but then we found a mark mounted vertically in a bridge abutment. How would a mark like this be used?

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Geodetic markers were, and still are, placed with respect primarily to convenience, permanence and accessibility, unlike boundary markers, which must occupy a specific location, so you are right that there is no consistent pattern to them. There are a lot of them along railways, for example, because this was the best path for the survey to follow at the time they were set, and it also made the marker locations relatively easy to describe, find and use, as the various regions were settled and developed. It is not always necessary to set a tripod over a point to use it, in fact, to use a benchmark for conventional leveling, a level rod is held on the disk and observed from a random location nearby, so any solid surface can be a suitable benchmark location. There are several old message threads also discussing these subjects.

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When looking at the mass of benchmarks, it can seem that there is a high degree of randomness to them. There are places where there are a high concentration of marks and other places that have large voids. But looks can be deceiving. There are generally two types of benchmarks - vertical and horizontal - and when these two types are looked at independently of each other, there is more of a pattern. Particularly with the horizontal triangulation stations.

 

The ultimate goal of the triangulation stations was/is to measure the shape of the earth. In order to do this accurately, a web of triangles was designed to cover the nation, and then the angles between them were measured. A clear line of sight was necessary, and the ability to have triangles that had relatively wide angles at all three corners. To accomplish this, a high degree of thought and planning went into their placement and there is a pattern in their placement. These are commonly found on high spots so they can be seen from farther away. In areas where the lines of sight are shorter, the triangles may be smaller and consequently, there may be more triangulation stations in that area. Also, these stations can have nearby reference marks that can have their own PID making it seem like there are two (or more) nearby marks when they are actually part of the same station.

 

The vertical benchmarks do have a more random placement. They are usually placed in locations that were going to provide a stable platform that would most likely be free from any vertical movement or subsidence. Railroad bridge abutments, waterway bridge abutments, foundations of large, old buildings, exposed rock formations, etc, don't move up & down much and are stable places to mount a station for elevation purposes. Many times these were placed when such a stable place was found. There still is a pattern to them, it is just not as easily seen and was more affected by the availability of stable locations. Also, these are generally the stations that local surveyors use, and their work, as they set new benchmarks based on the older marks, do sometimes make it into the NGS database, adding to the seeming randomness of the whole thing.

 

With regards to your second question, usually the tripods & surveying instruments are not set up over the elevation stations, but they are measured to using a process called leveling. A survey rod is held on the elevation station and the elevation is measured from another point. (Believe me, the last place you want to set you survey instrument in on the side of a railroad bridge abutment. You, or the instrument, or both, will have problems if a train comes by.) On an elevation station that is mounted vertically in the side of a bridge abutment or building, there is a line scored across the middle of the mark at the proper elevation.

 

Keep on Caching!

- Kewaneh

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