NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Dip Angles from Blue Hill Observatory
From: Bruce J. Pennino
Date: 2013 Apr 5, 08:52 -0400
From: Bruce J. Pennino
Date: 2013 Apr 5, 08:52 -0400
Bruce
Hello Paul and All:
I was hoping you would review my Blue Hill
posting. I really liked the idea of going to Blue Hill because any small
(feet) error in vertical measurements would not be significant (relatively) in
the dip angle when I'm 670 ft more or less above sea level. I carefully asked
the folks at Blue Hill if the datum was MSL, and they immediately said
yes. I recollect that the 1929 datum was MSL, and I also think there is
about 0.75 ft , more or less here in the east with the water going up and shore
moving whichever way. I think it is good enough also, but happy you think so.
I 'm glad to learn about the Magellan Promark
X, and I'm hoping other List members will provide postings on
hand held equipment that they personally have verified against a known vertical
datum. I'd like to buy something that I know works and gives repeatible
results. I remember once seeing a hand held device that had output which
provided the number of the satelites "in view".
This would be a confidence builder.I really don't
want to buy an expensive gadget .
For small heights of eye, less than maybe 50 ft or
so.....maybe 75 ....., I'm trying to pick sites where I can easily walk
down to the water line, or stand on an elevated dock and drop a tape to the
surface. I now have a short prism pole which fits inside an upside
down basket/crate type frame, which I can leave at the water line
(one person "dip" surveying most of the time). I then "shoot" the prism
with the EDM and know precisely the height of eye/instrument after adding in the
height of the prism pole.
My most recent idea is to find sites with a
light house. I'll post some data later on results from Avery Point on Long
Island Sound.
The lamp is a known height above HW. If I can put
my prism pole on the light centerline and get the horizontal distance, I should
know my height of eye above HW. Your opinion? The old Avery Point
"light" is some sort of perforarted device, but I could estimate the center of
it probably within 6 inches. I'll post more of this after I review the
data.
Bruce
----- Original Message -----From: Paul HiroseSent: Wednesday, April 03, 2013 11:23 PMSubject: [NavList] Re: Dip Angles from Blue Hill Observatory
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. --: http://fer3.com/arc/m2.aspx?i=123308
