Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: dip, dip short, distance off with buildings, etc.
    From: Frank Reed CT
    Date: 2006 Jan 13, 02:01 EST

    Bill, you wrote:
    "I'm not dogging Nate, it is just that times have  changed, and as pointed
    out, impact traditional navigation."
    Just to  clarify, "Nate" did not invent any tables that relate to this topic
    though the  rough estimates of the changes in dip caused by refraction in that
    era were not  far off.
    The Table 15 in 20th/21st century "Bowditch" that has caused  occasional
    puzzlement on the list appears to have been added in the 1958 edition  (which was
    a MAJOR revision of the publication). At that time it was numbered as  Table
    9. And even then the text notes that "an error may be introduced if  refraction
    differs from the standard value used..." but there's no indication at  all
    how much error a navigator should anticipate. And what I am saying is that a
    large portion of this variability is both calculable and also easy to interpret
    conceptually as a change in the effective curvature of the Earth.
    And you  wrote:
    "Back to the beach shots.  Even though the adjusted T15 constants  get us
    closer to the 23 nm target (Sears) and 23.5 nm (Hancock) with height  of eye
    15 ft and assumed base above water level of 30 ft, both calculations  leave
    us short.  Meaning the observed angles were too high by approx  0.8'.  How
    much of this difference be attributed to possible thermal  inversion?"
    Very easily, ALL OF IT could be the result of variation in  the lapse rate
    (called a "temperature inversion" if it goes positive). Such  variations are not
    merely common, they're quite normal. It is easy to find  stations, where the
    lapse rate is positive (linguistic purists would call this  negative, so just
    to clarify, by positive I mean "warming with altitude") in the  morning and
    negative in the afternoon, day after day. At such places, that 0.8  minute of
    arc variance in the measured heights of distant objects should be  expected, and
    more so, too. In fact, I've set this up in a spreadsheet where I  can vary
    the lapse rate and see the effect on the measured heights of the three  tall
    structures (Sears, Hancock, and Cooling Tower) visible from the Indiana  beach
    that day in late September. They are all consistent with a specific  moderate
    temperature inversion.
    42.0N 87.7W, or 41.4N  72.1W.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site