Sam,
I know at least two people who have found that their Garmin GPS
chartplotters disagree with reality in the BVIs. There is a good
discussion about potential reasons why at:
http://stuffinbox.blogspot.com/search?q=wgs84
The US DoD disagrees with the rest of the world about where 0 degrees
Longitude is. And because they invented GPS, if you take a standard
GPS unit to the Prime Meridian in Greenwich, London, you'll find that
it's out of whack by 300 feet.
Many charts predate GPS. You must only use your GPS to directly plot
your position on a chart if that chart's datum specifies that it is
"WGS-84", meaning that its lat/long will corresponds with what GPS
reports. Over 100 or so different datums (data?) are in use across the
world and can differ by several hundred feet in latitude and longitude
from WGS-84. Most newly printed charts are WGS-84 (solely because of
the proliferation of GPS). However, older charts use a different
lat/long datum. This came about, I suspect, because the Earth is a
little wobbly on its surface and not a perfect sphere. All good GPSs
let you program in a different datum which will offset your position
by some fixed amount. You'd hope that modern GPSs with charts included
would also set the datum to match the chart currently being viewed.
>From what I've heard, either they don't, or the manufacturers didn't
realize that the charts they bough don't conform to WGS-84.
As with switching around the buoyage, clearly this is just another
means to confuse those damn tea swigging redcoats and dash their tubs
on the rocks. (Hmmm.... I wonder if people suffer this same issues in
the US Virgin Islands?)
- Stuart
North Star, N41
Also see:
http://www.sailtrain.co.uk/gps/positions.htm
http://en.wikipedia.org/wiki/WGS84
http://en.wikipedia.org/wiki/Prime_Meridian
http://en.wikipedia.org/wiki/International_Meridian_Conference
On Tue, Aug 5, 2008 at 6:28 PM, Sam Densler <[EMAIL PROTECTED]> wrote:
>
> While Gina and I were sailing the BVI over the last 2 weeks, we noticed
> a weird anomaly with the charts. I have both rasters and vectors of the
> BVI and noticed an offset when going through the pass between Virgin
> Gorda and Mosquito Island to the east. When overlaying the charts over
> Google Earth:
>
> http://demo.geogarage.com/noaa/
>
> the same offset exists. Only place I have seen this happen. Anyone
> have any ideas?
>
> Sam
>
>
> --
> ****************************
>
> Sam Densler
> mailto:[EMAIL PROTECTED]
> http://www.islandfx.com/lady
>
> S/V Lady of the Lake
> PDQ 36, Hull #15
> Melbourne, FL
>
> S/V Stories She Could Tell
> Endeavour 37, Hull #454
> Destroyed by Hurricane Frances 2004
> Reborn and sailing again in 2007
>
> ****************************
>
> "If you ever wonder why you ride the carousel,
> you do it for the stories you can tell."
>
> Jimmy Buffett
> "Stories We Could Tell"
> A1A
> _______________________________________________
> Liveaboard mailing list
> [email protected]
> To adjust your membership settings over the web
> http://www.liveaboardnow.org/mailman/listinfo/liveaboard
> To subscribe send an email to [EMAIL PROTECTED]
>
> To unsubscribe send an email to [EMAIL PROTECTED]
> The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/
>
> To search the archives
> http://www.mail-archive.com/[email protected]
>
> The Mailman Users Guide can be found here
> http://www.gnu.org/software/mailman/mailman-member/index.html
>
_______________________________________________
Liveaboard mailing list
[email protected]
To adjust your membership settings over the web
http://www.liveaboardnow.org/mailman/listinfo/liveaboard
To subscribe send an email to [EMAIL PROTECTED]
To unsubscribe send an email to [EMAIL PROTECTED]
The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/
To search the archives http://www.mail-archive.com/[email protected]
The Mailman Users Guide can be found here
http://www.gnu.org/software/mailman/mailman-member/index.html
