Geodetic elevation question

[Macossay]

I'm looking at a drawing of a tank foundation. The drawing says "0'-0" elevation equals 32' geodetic elevation". I go out to the field. Not far from the tank is a surveyers monument. It has an elevation of 23.94'. The base of the tank sure isn't 8 feet above the monument. It's only a few inches higher. Where did the 32' come from?

 

[fel3]

Several things come to mind.

[1] If the tank predates the monument, the tank designer may have selected an arbitrary elevation for a temporary datum for just that project. The elevation could have been estimated from a USGS quad sheet, older plans and maps for the site, or just picked out of thin air. (I have run across this myself and have even had to do it a time or two when there were no benchmarks anywhere nearby, exact elevations were not required, and relative elevations were OK.) Later, the benchmark at the site would have been established at the "correct" elevation based on other, more distant benchmarks. Has this benchmark been checked against other benchmarks?

[2] If the monument predates the tank, the surveyor and/or engineer may have had a major brain freeze, may have missed the monument completely, etc. This is unlikely, though.

[3] If the monument predates the tank, the surveyor for the tank project may have determined from other benchmark(s) in the area that the monument at the site was in error and should not be used. This could be due to an error in the survey for setting the benchmark or could be due to a subsequent datum change (see #4). I don't know if the surveyor would have had the authority to remove or re-establish the monument, but perhaps he alerted the agency owning the monument and they didn't do anything about the error.

[4] There could have been a datum change between designing the tank and setting the benchmark -or- between setting the benchmark and designing the tank. I have run across this situation several times. For years the central part of the City of Fresno, CA, was on a different datum (by 4.26') from the rest of Fresno County (it's a long story). This datum was finally phased out a few years ago, but it was the cause of must consternation for surveyors and civil engineers. Also, in areas with land subsidence the benchmark elevations are good for very long and resurveys are needed every few years..

There may be other possibilities as well.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[dik]

Just ask the person that prepared the document for clarification advising them of the adjacent benchmark.

Dik

 

[IRstuff]

Since it already exists, is the question anything beyond just academic interest? After all, the implementor obviously ignored the erroneous drawing information.

TTFN
faq731-376

 

[Macossay]

Thanks for the input. The monument predates the tank and is consistent with other, newer momuments whose accuracy have been verified. There's been a little shift from the 1929 Datum (1978 adjustment) -- but not seven feet! The surveyor is deceased, the engineering company that designed and built the unit has vanished into history. We're expanding the unit and the team is divided between choosing a baseline for the new equipment that is consistent with the existing equipment and a baseline that is consistent with the monuments. It's not a critical question for the project -- just baffling to us ChemEs.

Ciao,

Macossay

 

[fel3]

As you can see, there are two options: [1] continue to use the site-specific datum rather than the benchmark or [2] convert everything to the benchmark's datum. Each choice has its advantages and disadvantages and these are often site specific. Here are my thoughts on the matter:

A) If all (or at least most) of the existing improvements are on the same site-specific datum as the tank ***AND*** if you do not need to worry about how your elevations interact with the world outside the project site, it would probably be simplest to continue using the site-specific datum for everything, including the proposed improvements. This eliminates the need to convert the old elevations and dimensions to the new datum. Such conversions introduce yet more chances for error. I would recommend adding a note to your plans (either as a call-out for the benchmark itself or in a note describing the benchmark) similar to the following: "BENCHMARK ELEVATION: xx.xx' (USGS) = yy.yy' (LOCAL & PROJECT DATUM)". This would be the only time you would need to refer to the benchmark's "correct" elevation since all of your work would be based on your local datum. Not only will this note help the surveyors for the proposed construction project, it will help future engineers when they review your drawings for a new project. It would also be helpful to get a complete description of the benchmark from your surveyor and add that to the drawings along with a reference to the plans that originally established the local datum. I have done a lot of "engineering archaeology" in my life (i.e. digging through old plans, specs, reports, maps, etc) and I really appreciate it when I find that extra bit of information on the drawings and don't have to dig through the project files to find it.

B) If most of the existing improvements are on the benchmark's datum (obviously not including the tank) ***OR*** if you need to conform to elevations outside the project site, then it would be best to convert everything old and new to the benchmark's datum. Just be sure to double and triple check the converted elevations and dimensions. In this case, you may want to add some notes reagrding the conversions and reference the plans that used the other datum (again to help future engineers). Conforming elevations to the outside datum may be required for grading and drainage purposes, flood plain issues, local agency permitting, etc.

I hope this helps.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill