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On 2018-09-22 23:01, Geoffrey Kolbe wrote:
> There would appear to be an ad-hoc program in the US to accurately determine 
the position of existing benchmarks, but this would appear to be on a 
volunteer basis with members of the public being invited to send in the 
latitude and longitude of benchmarks determined using whatever GPS device 
they have. This program has the advantage that it costs the US Government 
essentially nothing, but it is not the same as the UK passive GNSS network, 
set up to a unified standard by a government agency.

Laymen often call any survey mark a "bench mark," but the US National
Geodetic Survey takes pains to explain:

"In a geodetic context, the terms 'survey mark' and 'bench mark' are not
the same. A bench mark is a specific type of survey mark that has a
known elevation above or below an adopted surface or datum. Survey mark
refers to any permanent marks or disks placed in the ground or attached
to a permanent structure with known latitude, longitude or height
information. Other terms used for survey mark are 'survey point' or
'control point.' Colloquially (and incorrectly) bench mark is used
interchangeably with survey mark."

https://www.ngs.noaa.gov/datasheets/SurveyMarks_FAQ.shtml#SM

That's still not very definite, since all the NGS station descriptions
seem to include elevation. However, a true bench mark is primarily about
height. Its horizontal coordinates are secondary, and often of low accuracy.

The US Coast & Geodetic Survey used different disks to indicate the type
of mark. They said BENCH MARK or TRIANGULATION STATION. Nowadays the
latter is more often called a horizontal control station, triangulation
having gone out of fashion. Professionals in the US often use "monument"
as a general term for any survey mark.

As for the NGS soliciting reports from the public on bench mark
locations, that applies to bona fide bench marks only. Back in the day,
their horizontal positions were recorded only approximately, height
determination by spirit level being the item of interest. Thus many old
BMs are hard to find today, if indeed they still exist. It saves
surveyors much labor if the NGS online database shows that a mark has
survived and includes coordinates, even from a consumer grade GPS.

https://www.ngs.noaa.gov/GPSonBM/index.shtml

By no means is the UK unique to provide accurately surveyed marks where
a base station antenna may be set up for differential GPS operations.
The US has thousands. For instance, the old historically significant
station "Meades Ranch" in Kansas has been surveyed within a few
millimeters by GPS.

https://www.ngs.noaa.gov/cgi-bin/ds2.prl?retrieval_type=by_pid&PID=KG0640

It has been said that "NAD 83 was the first civilian horizontal datum
anywhere to have the origin of the coordinate system defined as close to
Earth geocenter as the technology would allow, rather than as a specific
survey mark in the middle of a field somewhere." (Doyle, 2015)

Well, from ca. 1900 to the release of NAD83, Meades Ranch was the "mark
in a field" for the United States. Such is its celebrity that the most
recent entry in the datasheet mentions a tour by a group of 30.

Another old monument with a story is Pasadena East Base in California.
It was instrumental in the famous speed of light determination by A.A.
Michelson in the 1920s, where he measured the round trip time from Mount
Wilson to a mirror on a peak 22 miles away. Of course time is useless
unless distance is known, so the Coast & Geodetic Survey measured a
parallel baseline down in the Los Angeles basin with extraordinary care.
Theodolite observations were taken from the baseline end points to fix
the locations of Michelson's stations on the mountain peaks. The story
is told here:

http://www.otherhand.org/home-page/physics/historical-speed-of-light-measurements-in-southern-california/

Pasadena East Base survives, with modern precise coordinates and still
in use by the California Department of Transportation:

https://www.ngs.noaa.gov/cgi-bin/ds2.prl?retrieval_type=by_pid&PID=EV3083

There is concern in the surveying community that the data on many marks
will not be updated to the next edition of the North American Datum in 2022:

https://www.xyht.com/surveying/monumental-changes-part-1/

   

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