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: Historical charts and mapping
    From: Frank Reed
    Date: 2016 Aug 17, 15:25 -0700

    Don Seltzer, you wrote:
    "But which reference meridian would have been in use in 1847?  Would surveys be referenced to the Washington meridian of that time, providing a different offset to WGS84 coordinates?"

    Well, I don't think anyone was really talking about a Washington meridian in 1835, but by the time of the re-printing in 1847 someone may have worried about it. There's only one longitude referenced in this particular chart: the Stonington Lighthouse. Let's suppose for now that they performed actual astronomical observations there. They would have used terrestrial instruments, but we can imagine them using a high-quality nautical sextant and a Mercury-filled artificial horizon. An altitude when the Sun bears more or less east or west gives local time, and the chronometer provides Greenwich time. Subtract and you get your longitude. But how, exactly, do we know that the chronometer reads correct Greenwich time? It has to have its error and rate checked at some location with a known longitude. So really our measured longitudes are referenced to that known longitude. I suspect, but don't know, that the Coast Survey in this era would have checked their chronometer errors at the nearest major port, probably New York in this case. Tricky, huh?

    Frank Reed

    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