Hi Roger:
I have a web page listing the U.S. LORAN-C stations. And with the
Google imaging you can easily see the transmitter antennas which are
rather large since the frequency is 100 kHz. The transmitters are in
precisely known locations (WGS-84), but there is still a small amount
of error. see:
http://www.pacificsites.com/~brooke/Loran-c.shtml
click on the station name in the table for the Google image.
Have Fun,
Brooke Clarke
Roger W. Sinnott wrote:
List:
I'm sure Mike meant that the Greenwich Meridian Line is about 5" west,
not 5' west, of where it is shown on Google Earth. (I just looked to
confirm this!)
This discrepancy is simply due to Google Earth's adopted geodetic
datum,
WGS84. GPS receivers can often be set to show coordinates for other
datums (data?) as well (for example, GB36, the Ordnance Survey of Great
Britain 1936, on which the Greenwich meridian line is *much* closer to
0
longitude).
The datum choice probably explains the equator discrepancy in Ecuador
as
well. It's simply that each country, historically, conducted geodetic
surveys that are internally consistent to high accuracy. But they don't
join up perfectly with those done in other countries. The WGS84 datum
is
intended to apply to the entire world. We had a brief discussion of
this
issue in Sky & Telescope for August 2005, page 110.
I learned this lesson the hard way. When I went to find the gravestone
of telescope maker Russell W. Porter in Port Clyde, Maine, I had GPS
coordinates from a friend. After an hour and a half, no luck -- so I
gave up. Only later did I learn that his receiver was set to WGS84
while
mine was set to NAD27.
-- Roger
At 08:27 AM 7/23/2006 +0100, Mike Shaw wrote:
>>>>
Bill G.
said:
<<There is a large monument in Ecuador that
marks the exact
location of the equator, so they claim. Google Earth puts it about 780
feet south of the equator. Any thoughts on who is more accurate: Google
Earth or the Ecuadorian surveyors? >> I
have
also noticed that, according the Google Earth, the meridian line at the
Greenwich Observatory (surely correct) is at 5.26 minutes West. Mike Shaw 53.37N
3.02W www.wiz.to/sundials
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