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    Re: Digital Image CN Exercise
    From: Marcel Tschudin
    Date: 2010 Apr 5, 16:16 +0300

    Thank you for the link to the NOAA chart which definitely shows more
    off-shore oil rigs than Google Earth.
    
    This exercise draw my attention since it uses the type of procedures
    which I intend to use in my program for certain parts of my work. It's
    therefore a good possibility to verify this part of the program
    (height above apparent horizon) which never has been checked before.
    And indeed, its only now that I have gone through to write up all of
    my results that I notice a bug. For some reasons my program seems to
    use a much too large dip. The location which I found and have written
    down is therefore definitely not correct.
    
    There is however an other aspect which I like to point at: the scale.
    You indicated a scale of 0.97 Minutes of arc per Pixels. For my work I
    measure this value from the diameter of the sun. For this I filter the
    sun image by removing red and blue colours, just leaving green. In
    addition I further dim the green disc. From this image (see attached
    _IMG_5689 green.tif) I obtain a slightly different scale. Several
    measurements of the width result in 321.5 Pixels. The diameter of the
    sun on this day is 0.53319 degrees, thus resulting in a scale of
    0.0995 Minutes of arc per pixel. (Note: For my work I take all photos
    with the same pixel size. The scale is the average from a lot of
    different photos. This allows then to evaluate by how much an
    individual photo has to be further corrected.)
    
    The two different scales lead to locations which are separated by
    about 10 km. Could it be that you determined the scale by calculating
    from the known location backwards?
    
    Marcel
    
    
    On Mon, Apr 5, 2010 at 12:50 AM, Greg Rudzinski  wrote:
    > Marcel,
    >
    > Go To Chart Web Sight:
    >
    > http://www.charts.noaa.gov/OnLineViewer/18720.shtml
    >
    > Zoom and print a working copy for Latitude 34�10' North ; Longitude 119�14'
    > West (A.P.)
    >
    > Sun Almanac Data for 3 April 2010 UTC 01:57:41
    >
    > GHA 208� 34'
    > DEC 5� 13.0' North
    >
    > Image Altitude for Sun Upper Limb:
    > (2468 pixels) x (.097' per pixel) = 239.4' = 3� 59.4' Hs
    >
    > Altitude Corrections:
    > Semi Diameter -16'
    > Refraction -11.7'
    > Dip -3.5'
    > Total Correction = -31.2'
    > Observed Altitude (Ho) = 3�28.2'
    >
    > This should get you rolling.
    >
    > GRudzinski
    >
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