[Shop-talk] plumping question and sandrock sewers

[pethier at comcast.net]

From: "Randall" <tr3driver at ca.rr.com>
> > USGS sea level reading - 694.10  =  Saint Paul City Datum
> 
> Phil, are you sure that's Mean Sea Level ?  Sounds to me more like an
> ellipsoidal height (meaning the theoretical model of the entire earth's
> surface that datums like WGS-84 are based on).  There are of course, some
> places that are below MSL, but even Death Valley is only 282 feet below.

Randall, I don't know whether you haven't been following the bread crumbs or if you are trolling.   Just to play along, I'll pretend you are serious.

Saint Paul is above sea level.  This makes a lot of sense because the Mississippi River flows downhill.   The magical mythical point on the Mississippi upon which early surveyors decided is now known to be 694.10 feet above USGS sea level.  Therefore, 694.10 USGS is equal to zero Saint Paul City Datum.  Therefore, a USGS elevation minus 694.10 equals a Saint Paul City Datum elevation.
 
> > Sometimes a surveyor will not call for benchmarks and just pick a random
> > place on the site and call it "100.00".
> 
> Which of course is the same basic process that produced your "Saint Paul
> City" datum.
> <G>

Yes, but the early surveyors had no choice.  The point they chose remained consistent over the entire period of the city's development.  Now that more accurate methods are available, we have a number to convert our local standard (used on 10,000 sewer maps) to a national standard.

This is not comparable to today's surveyor choosing a random point on a single building site.  Today's surveyor has an easy process to create a site plan showing elevations directly tied to either the national standard, the local standard, or both.  To create a site plan with no reference to either standard seems like sub-standard work to me.  Without a reference to existing benchmarks, a builder would have to go out to the site and shoot some elevations again to relate his site plan to existing City records of sewer elevations. 

--
Phil Ethier  West Side  Saint Paul Minnesota  USA
1962 Triumph TR4 CT2846L, 1992 Saturn SL2, 1993 Suburban,
1994 Miata C package
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I decry the textmessagization of the American-English language.