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Why were these benchmarks placed?


mjcongleton
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I just got back from a benchmark hunt and I am wondering, most of the benchmarks in my county follow along a single roadway or railroad.

 

I don't understand what purpose these benchmarks really had, or have. Why was highway 38 in Monroe county Ohio and one, of many, railways littered with benchmarks every couple miles or so, and all the other railways and highways ignored?

 

Can somebody explain to my what the purpose of placing all of these benchmarks was to begin with? Most were placed in 1934/1935.

 

Thanks.

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The Coast & Geodetic Survey began long ago finding elevations of bench marks and latitude/longitude of other points (loosely called benchmarks here but not by professionals). In the 1930's a major push was made to connect the whole country into one elevation network and one lat/lon network so everybody was working off the same system.

 

Elevation measurements require setting up an instrument about every 200 yards, reading the height on a rod behind, and the height of a rod ahead (oversimplified). Then the level instrument is moved up and the process repeated. This means it was easiest to follow a reasonably level path without excessive vegetation, and railroads and roads were the obvious paths to use.

 

For horizontal lat/lon, most of the measurement was done by theodolites taking angles to various positions in all combinations, with typical sight lines of a few to 20 miles. The obvious places were hilltops, with additional sights to "intersection stations" like towers and water tanks.

 

The primary network maps look like a fish net stretched across the country. Your highway happened to fall close to where they wanted a path in the mesh.

 

After the field measurements were done, it took years of manual computation to tie all the data together into the best estimate of all the elevations and positions.

 

Somewhere on the ngs web site under About NGS/history you can find stories and pictures of the history of these operations and maps of the progress.

 

Today the elevation bench marks remain very important reference points for all types of construction, drainage, and flood control work. GPS cannot yet adequately replace them because GPS knows almost nothing about variations in gravity from place to place and elevation is based on gravity (it is preferred if water runs downhill). Horizontal marks are less important than they used to be, but are sometimes still useful and certain selected ones are very important references for the whole network.

Edited by Bill93
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I just got back from a benchmark hunt and I am wondering, most of the benchmarks in my county follow along a single roadway or railroad.

 

I don't understand what purpose these benchmarks really had, or have. Why was highway 38 in Monroe county Ohio and one, of many, railways littered with benchmarks every couple miles or so, and all the other railways and highways ignored?

 

Can somebody explain to my what the purpose of placing all of these benchmarks was to begin with? Most were placed in 1934/1935.

 

Thanks.

 

I'm sure the other highways/RRs weren't ignored, they just aren't listed on GC.com. As a former roadbuilder, I can attest that grade and elevation is VERY important when building things like roads. These benchmarks give surveyors and engineers a reference point to work with. They shoot off the known values of the benchmarks to find the values of whatever they are surveying. I was told by some of the surveyors that there is a benchmark/survey mark within eyesight no matter where you are in the country.

 

Sometimes they use existing structures instead of those disks. Iron nails and RR spikes driven into the walls of old building, and even trees are not that uncommon. (Trees grow from the top up, so a spike driven into the bottom of a tree would hold its elevation).

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Another thing to point out is that there is a hierarchy or chronology to how leveling was done and how it was/is used.

 

The primary lines of leveling were mostly done by C&GS now NGS to establish the higher orders of elevations across the country. In that exercise utility was not the primary concern, and so they follow easier and accessible routes to get from point A to point B generally following roads and railroads. Levelling cross country through the woods and over the hills and dales would be much more costly and prone to difficulty with increased error budget.

 

Later for mapping control USGS would run loops between those to wherever they needed elevations to control their mapping and generally to a lower quality since the use was less critical.

 

Other individuals or agencies such as transporation departments, water departments, reclamation departments, state, muncipalites, counties would do the same thing. Starting and ending their level lines or loops on NGS points and to various orders. Some of that may be high order and blue booked and monumented and some of it not.

 

Anyone needing elevations for a project, such as civil engineering for drainage of a subdivision, water project, etc. would essentially use whatever existing control was in the area and extend it to their project by work of their own. NGS's primary networks were not intended to provide elevations for specific projects, but to get elevations out into the general area for others to use or 'densify'.

 

Then there were various eras or campaigns of improvements or releveling by NGS to suit particular needs.

 

This is analagous to the horizontal network, where C&GS provided the first control, often only on mountain tops, USGS densified for mapping purposes. Other agencies bring control into thier areas of interest and projects and do the same everyday. Much of it not monumented but some of it is. This followed by some improvement and intermediate densification by NGS.

 

And the cycle continues again.

 

- jlw

Edited by jwahl
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