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    Re: Wobbly Prime Meridian
    From: John Huth
    Date: 2010 Nov 24, 12:14 -0500
    Frank -

    Well, it very well could've been a couple of factors.   In my case, I was comparing my coordinates with landmarks - mainly points of land.   I entered the landmarks from topo's, so the datum might very well be the culprit.   I don't know.   This was also at a time when I suspect that my receiver was degrading due to the effects of salt water, because it completely failed a few months after making a number of sightings where 100 m was my typical error.   It might be a combination of the two.   

    I frequently end up in locations where the alignment of satellites may not be terribly good.   That is to say, canyons or deep valleys - this may also be a factor.   

    John H. 

    On Wed, Nov 24, 2010 at 1:01 AM, Frank Reed <FrankReed{at}historicalatlas.com> wrote:

    John, you wrote:
    " 100 m sounds about right for a hand-held device under decent conditions."

    I get about 3-5 meter accuracy typically using the GPS in my cell phone. Most of the time, probably about 75% of the time, the position fix is that good. The huge exception is in those dreaded "urban canyons". Then you can get reflections and the GPS fix can jump around by hundreds of meters. Many modern smartphones now include the capability to get a much more accurate fix in cities by triangulating off WiFi signals (which have been mapped by various companies including Google). With a GPS receiver that's linked to the satellite view in Google Maps (as in all smartphones I've seen), it's very easy to verify the accuracy at least in areas that have a lot of features on the ground. I'm assuming here that most of the apparent position discrepancy is in the GPS position fix with no significant errors in the alignment of the Google Maps satellite imagery. This does seem to be the case.

    The discrepancy between the WGS84 zero of longitude and the traditional line at the Greenwich Observatory is quite real, and consumer-grade GPS units, either stand-alone or in smartphones, can detect it easily and with considerable accuracy. Also, just go to Google Maps. If you right-click (Windows users) and select "What's here?", it should display the decimal latitude and longitude in the search box. You can find the zero of longitude very nearly 100 meters east of the original zero line from the old transit instrument's building. That building and the stripe out front where tourists get photographed with a foot in each hemisphere is at lat/lon=51.477895,-0.001506. If you work your way east from there in Google Maps, you will find a spot where two paths in the park intersect that is very close to the WGS84 Prime Meridian. I noticed just now that the paths in the "map" view in the park do not match the paths in the satellite view very well --another case where the satellite view is a good sanity check on the map view (and less frequently, vice versa).

    -FER

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