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Elevation benchmarks


Lothar69

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I apologize in advance if this topic has been discussed before but I didn't have time to search through the archives.

 

I have some questions of curiousity about elevation benchmarks. I asked my civil engineer father and you'd think he would know but he didn't.

 

Where exactly are elevations calculated from? Is there a single benchmark that all elevations are ultimately figured off of? If so, where is it? Is this the same point reference point used world-wide?

Is sea-level a constant point world-wide? Seems like it would be (excluding tidal factors) since the oceans of the world are interconnected. At the same time, it seems like water levels in the ocean would fluctuate based on glacier ice melt (or freeze), rainfall, evaporation, etc....so is it really a constant?

 

I am Lothar, King of the Hill people. I have many tails to tell....

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I found this info on the NGS web site from links on the benchmarking page.

 

quote:
The North American Vertical Datum of 1988 (NAVD 88) is the vertical control datum established in 1991 by the minimum-constraint adjustment of the Canadian-Mexican-U.S. leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1985 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks. ("Results of the General Adjustment of the North American Datum of 1988," Surveying and Land Information Systems Vol. 52, No. 3, 1992 pp. 133-149)


 

An interesting LINK I found on leveling, go to the last page there is a photo of a survey crew leving.

 

[This message was edited by elcamino on August 12, 2003 at 04:15 PM.]

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Most countries base levels relative to sea level derived over a fairly extensive time period (around 19.5 years depending).

 

In any case level runs, which a height datum might have been based on aren't exacly perfect either and along with many other factors there's always going to be some anomolies.

 

With GPS the situation is complicated even further as heights derived from GPS have to be "corrected" to whatever the height datum might be as GPS heights are relative to a mathematical reference surface. Things like gravity and differences in the geoid relative to a sea level model also introduce issues.

 

Actually some countries with the aid of long term GPS observations are now finding that in simple terms, No the sea isn't "level" (as such) as has been previosly adopted for height determination.

 

Your father probably gave to the best generic answer as from there on it becomes rather complicated.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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Not to slam engineers but I worked for many over my 30+ yrs as a Survey Tech for the MDOT which is 99% engineers. I found very few knew anything about surveying or cared to know. The college here (MTU) only has a 2 week summer course mandatory of the BS in Civil Engineering degree.

 

We always had Co-ops (interns) from MTU or Ferris etc working on our crews, most of the Engineer majors hated surveying and wanted off the crew. Most hated the bugs and dirt, I got those complaints a lot. And also working away from home all the time.

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Yes I'd have to agree with that about engineers but mind you there are a few (just a few) that are the exception. Unfortuneately one has to be "nice" to engineers for several reasons but

 

Engineer jokes?

 

Hardest 10 years of an engineers life? First grade.

 

Surveyor Jokes?

 

Hardest 10 years of a surveyors life? Teaching engineers the basics of surveying.

 

icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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