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    Re: Dip Angles from Blue Hill Observatory
    From: Marcel Tschudin
    Date: 2013 Apr 6, 15:52 +0300

    Bruce,
    
    High and low altitude locations have each certain advantages and
    disadvantages. Here what comes at the moment to my mind (some of it
    has already been mentioned):
    
    High altitude location:
    Pros: Altitude error (in meter) has less influence. Horizon less prone
    to abnormal refraction because of higher incident angle.
    Cons: An error in dip measurement corresponds to a greater angle.
    Horizon is far away from observer and more prone to be hidden in
    haze/fog/smog
    
    Low altitude location:
    Pros: An error in dip measurement corresponds to a smaller angle.
    Horizon is nearer to observer and less of the time hidden in
    haze/fog/smog.
    Cons: Altitude error (in meter) has a greater influence. Horizon more
    be prone to abnormal refraction because of smaller incident angle.
    
    Practicality of location:
    This aspect was for my observations quite important. The location
    should be close to home so that one can decide at short notice to go
    and do some observations. This helps to increase the number of
    observations. It also should be easy to get installed with
    instruments, pencil and paper etc; this helps to avoid making
    mistakes.
    
    Note also that a relative error in height results - because of the
    square root - for the dip only in half this relative error. Or, in
    other words, a change in hight of eye by e.g. 10 percent changes the
    dip only by 5 percent.
    
    Marcel
    
    On Fri, Apr 5, 2013 at 3:56 PM, Bruce J. Pennino
     wrote:
    > ________________________________
    >
    > 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 Hirose
    > To: bpennino.ce---net
    > Sent: Wednesday, April 03, 2013 11:23 PM
    > Subject: [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.
    >
    > --
    > I filter out messages with attachments or HTML.
    >
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123308
    >
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123349
    

       
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