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    Re: Sextant and quintant limits, and alternatives to sextants
    From: Richard M Pisko
    Date: 2009 Jan 03, 23:23 -0700

    On Sun, 28 Dec 2008 11:42:01 -0700, Richard B. Langley  wrote:
    
    > I will send a message to the UNB archivist to see if there are any  
    > examples of the
    > "Douglas reflecting protractor" or any other devices he may have left  
    > the university.
    > -- Richard Langley
    
    Thank you.  I just now (Jan 03, UT -7 = 22:24) found this in my trash.   
    There is no record in my machine of having sent it, nor do I see it in my  
    navigation folder.
    
    Now if the "Douglas reflecting protractor" could be taken out for a field  
    trial this spring with the surveyors; and a report written on the ease of  
    use, drawbacks, advantages . . . would there be enough for a student  
    thesis?
    
    "Give me a inch, and I'll try for a mile" sounds strange in metric.
    
    I think a construct of George' s double mirror device would make a useful  
    comparison, or put more originality into the thesis, should one be  
    written.  I have been playing around with a couple of old porro prisms (45  
    - 45 - 90) from a binocular.  Although I think front surface mirrors would  
    work better for clarity, keeping the relative alignment of the prisms is  
    very much easier (just set short flat to short flat and keep them in  
    contact while rotating).  So far, I don't see a solution to the consequent  
    "rotating" of the images seen through the mirrors.  Perhaps it would not  
    matter for stars, as point source of light could be matched with the  
    illuminated edge of the moon, or with the horizon.   I think that pointing  
    the telescope at the star with the index set to zero, and then maintaining  
    the mirror image while opening the arc until the more easy to find object  
    is in view directly, would be fairly easy.
    
    Unfortunately, determining the angular distance between two vertical  
    objects such as power poles or church steeples seems to be more difficult;  
    as the direct view of the one pole would still be vertical, but the  
    reflected view of the other pole would be horizontal as seen from a 90  
    degree included angle at the observer's position.  Using the base of the  
    reflected object as a matching point (if both objects were on a level  
    surface) might be accurate, but I haven't run tests on that.  Each of my  
    prisms has a groove across the long flat, which serves somewhat as an  
    aiming cross when the prism relative rotation is at any angle other than 0  
    or 180 degrees.  This may turn out to be useful, or may be unnecessary.
    
    Whether this new instrument would be useful for anyone in Geodesy or  
    Geomatics, I don't know.  I do know that the old pocket sextants are  
    pretty expensive if they can be found, but I would like to have one for  
    historical interest, fun, and rough sketching -- perhaps a little  
    triangulation survey in conjunction with a magnetic compass traverse.  As  
    George said, the Douglas reflecting sector skips having to read an angle,  
    transfer instruments, and set an angle on a protractor to be used on a  
    plane table or reconnaissance sheet; if there is not an actual plane table  
    board, alidade, and tripod set up on a stable surface.  I think George's  
    invention could be made inexpensively, and be used more easily than the  
    Douglas instrument for marking the angles.
    
    Another useful comparison might be with the Amici reflecting sector, or at  
    least half of it.  One porro prism on an index arm, and an aperture with  
    sighting vane on the frame, seems not to double the angle in the first  
    place.  The zero position is with the long face parallel to the frame's  
    aperture and sight vane.  A 90 degree angle of the index arm to the  
    initial zero has the line of sight bent 90 degrees also.  I have not  
    measured the accuracy of the intermediate angles, although I did think of  
    mounting the prism on the top of a theodolite and comparing the view  
    through the prism (as seen from a separate telescope on a stand) with the  
    view through the theodolite telescope.
    
    The temperature is about -25C, and the snow is about two feet deep; so  
    outdoor experiments will be delayed until the weather improves.
    
    By the way, is the use of the Plane Table still taught in elementary  
    surveying at UNB?  Perhaps supplemented by a small EDM to make it more of  
    a one man operation?  That's the way I had some fun down in our coulees.
    
    -- 
    Richard . . .
    
    Using Opera 9.2.4 after the "Dog" died
    
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