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TBM

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Close to where I live is EV0161 (G 708 RESET), EV0162 (G 708 TBM 2), EV0163 (G 708 TBM 1). Of course, EV0161 is a regular USC&GS bench mark, but EV0162 is a star shaped rod, and EV0163 is the metal cap to a water pipe. What does TBM mean? And what is it's purpose.

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Close to where I live is EV0161 (G 708 RESET), EV0162 (G 708 TBM 2), EV0163 (G 708 TBM 1). Of course, EV0161 is a regular USC&GS bench mark, but EV0162 is a star shaped rod, and EV0163 is the metal cap to a water pipe. What does TBM mean? And what is it's purpose.

 

temporary benchmark?

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No, I've come across these before, it simply means that when the benchmark was placed, it was guarded by a Totally Big Monkey thus the acronym TBM, but don't worry, these monkies were all captured and enslaved in 1997.

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"Bench Mark - A relatively permanent object, natural or artificial, bearing a marked point whose

elevation above or below an adopted datum is known. Usually designated as a BM, such a mark is

sometimes further qualified as a PBM (permanent bench mark), or as a TBM (temporary bench

mark)."

 

From the South Dakota Office of Road Design Survey Manual, Chapter 9, Surveying Terms and Abbreviations. http://www.sddot.com/pe/roaddesign/survey_manual.asp

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I prefer the answer from Cal W. A little fun here, but tosborn is by all means one of the guru's around here. I've run into TBM's out west here also.

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I'm always surprised when I find a temporary mark in the NGS data base. Obviously they weren't intended to be around for years, and most of them aren't. We've even had pointed out data sheets for wooden stakes set decades ago whose remains would be pretty hard to identify now.

 

But they must have been important enough in the overall measurement and computation plan that their data was entered along with the rest.

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I haven't seen any listed as 'temporary', but I've run across listings for 'a nail set in a tree' in the 1800's, and a 'nail set in the pavement of the highway' in the 1930s.

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The purpose of a TBM can be many. Some reasons to set one are;

  • Need a place to stop working at end of day
  • Working elevation point to search for lost BM
  • Need to preserve an elevation to be reset and conditions make it undesirable to set the new mark yet (construction zone, weather, time constraints etc)
  • Need an elevation spotted where it is not cost effective to place a permanent mark

Edited by Z15

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