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TBM


Team Christiansen

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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'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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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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