From: NavList@fer3.com [mailto:NavList@fer3.com]
On Behalf Of Frank Reed
Sent: Wednesday, August 17, 2016
3:34 PM
To: bruce@westcut.com
Subject: [NavList] Re: Historical
charts and mapping
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
