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11 de Febrero 2006

Why Google Earth and Prime Meridian Don't Line Up

Prime Meridian in Google EarthOne thing several people have noticed since the Google Earth application was released is that the Prime Meridian (0 degrees in Longitude) does not line up with Greenwich, England like they expect. So, if you go to Greenwich in GE, and turn on lat-lon lines by going to the menu "View->Lat/Lon Grid", you will see the Meridian line is about 100 meters east of the Greenwich observatory (viewable quite clearly in the satellite photo). This is not a mistake on Google's part. The developers of Google Earth (originally known as Keyhole) chose to support the same coordinate system as that used by GPS technology known as WGS-84 World Geodetic System. Here's an excellent post by one of the GE developers at the Google Earth Community which explains this in detail.

So, if you go to Greenwich, England with your GPS, it will in fact require you to walk about 100 meters to the east of the observatory before you will reach the GPS prime meridian. And don't believe anyone who tries to tell you Google is trying to realign the planet.

Enviado por FrankTaylor at 11 de Febrero 2006 a las 08:26 AM

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  • Comentarios

    Your comments about wgs84 and google - and the explanation by Seer, raise questions about using GE for navigation worldwide.
    Using GE as the map and a GPS, my house in Queensland, Australia appears about 50 metres from where it actually is.
    My remedy was to screenshot GE for the area, then calibrate the image using Oziexplorer and a number of known points.
    Using this method, I can now be viewed using GPS(and a recalibrated GE image)to an accuracy of about 2 metres.(on the end of a fishing jetty)

    I wonder how the accuracy of google earth (WGS84)
    appears at different points on the globe - apart from Greenwich.

    Anyone know?

    I had considered that perhaps the hi-res "tiles" for some areas, might not have been calibrated accurately by GE mapmakers.
    "globemakers"

    Enviado por: bill mcleod at 11 de Febrero 2006 a las 05:57 PM

    Reminds me of the time I took my GPS to a monument in Ecuador where the equator was surveyed by a European scientific team back in the day.

    So while everyone is shuffling by, checking out the monument and enjoying the day, there was a geeky gringo flipping through all his different datums struggling in vain to get exactly 0 latitude to come up on his Garmin.

    Man, those were good times.

    Enviado por: Brian Timoney at 11 de Febrero 2006 a las 10:25 PM

    Your comments about wgs84 and google - and the explanation by Seer, raise questions about using GE for navigation worldwide.
    Using GE as the map and a GPS, my house in Queensland, Australia appears about 50 metres from where it actually is.
    My remedy was to screenshot GE for the area, then calibrate the image using Oziexplorer and a number of known points.
    Using this method, I can now be viewed using GPS(and a recalibrated GE image)to an accuracy of about 2 metres.(on the end of a fishing jetty)

    Enviado por: faisl livy at 5 de Abril 2006 a las 05:45 PM

    I'm using a very handy tool named GoogleOzi http://flygps.host.sk/GoogleOzi to make a 'screenshot' of Google Earth. Excelent is that it makes an OziExplorer map file, so it is calibrated already with 1 m accuracy!

    Enviado por: Marian at 14 de Junio 2006 a las 01:58 AM

    Comparing sailor-maps (paper-version) with Gearth, I found for the Dutch coastal waters Gearth grid position inaccuracies of 85 m lon to the east and 90 m lat to the north. Combined a 125 m @45 degrees vector. This inaccuracy seems to be an intentional offset, as it is found at many points in lat/lon.
    Map reference is "Hydrografische kaart 1812, Waddenzee oostblad, submap 1812.3, insert Lauwersoog, position is centre of crossing coastal road and lock, lat 53.408198 and lon 6.196590. Gearth gives lat 53.4091667 and lon 6.1980

    Kind regards,

    Uulke Visser
    Enschede, Netherlands.

    Enviado por: Uulke Visser at 19 de Noviembre 2006 a las 03:29 AM

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