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Bruce J. Pennino wrote:
> On April 1, 2013 I went to Blue Hill Observatory near Boston. It is easy to 
reach and has(had) a National Geodetic Survey Station (Bench Mark BM) on the 
top of the mountain. I was told there is a clear view to the horizon over 
Massachusetts Bay to the NE. The BM PID MY 3472 is covered by an 18 " 
concrete base for a flag pole.

Too bad the mark itself could not be found, as the station has a long
history. Possibly the copper bolt that marks the station is original
from 1845.

http://www.ngs.noaa.gov/cgi-bin/ds2.prl?retrieval_type=by_pid&PID=MY3472

Now, about the height. Apparently it wasn't measured very accurately.
If you search for PID MY3472 (Blue Hill) at the NGS site, the list of
search hits shows a 1 in the H column (meaning first order horizontal
station) and blank in the V column. So they don't consider the mark a
vertical control station. The height determination was incidental to the
horizontal survey. In this case, the height was computed by adjusting an
old NGVD 29 vertical datum height, which in turn came from vertical
angle observations.

C&GS vertical angle procedure was to record six pointings, reversing
face each time. "It is desirable that these observations be made between
12:00 noon and 4:00 P.M. since refraction is smaller and more constant
during that part of the day." Each L/R pair was meaned to yield three
determinations of zenith distance free of index error. If all three
agreed within 10 seconds, the observation was complete. Such angles
would have been observed both ways between Blue Hill and the other
adjacent stations in the triangulation chain. A tie to a bench mark was
desirable about every third quadrilateral in the chain.

In other words, vertical angle height determinations at triangulation
stations were a far cry from geodetic leveling.

Nevertheless, is the height good enough for your purposes? If I had to
guess, I'd say yes. Just don't think of it as a gold plated value.

If you're really serious, maybe you could rent a GIS data collection GPS
receiver. (The purchase price would be in the range of a quality
sextant.) My old Magellan Promark X is repeatable to 1.0 meter RMS
vertical with a 3 minute observation.

By the way, I know only one way to create an URL like the one above:
save one in your browser to serve as a prototype. Viewing a datasheet at
the NGS site doesn't give a usable URL. However, the PID, which is shown
at the beginning of each line, can be copied and pasted into a prototype
URL in the obvious place.

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