Google Earth: Long/Lat accuracy?

Discussion in 'Global Navigation Satellite Systems' started by Terry Pinnell, Aug 25, 2005.

  1. Anyone know how accurate the longitude & latitude co-ordinates are in
    Google Earth please? A couple of test locations I tried (in the
    Dolomite area of N Italy) seemed some way off (30" or so) when
    compared with an alternative source.

    Are there a few easily identifiable locations, ideally in Europe,
    which we could use to test?

    --
    Terry, West Sussex, UK
     
    Terry Pinnell, Aug 25, 2005
    #1
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  2. Terry Pinnell wrote:
    > Anyone know how accurate the longitude & latitude co-ordinates are in
    > Google Earth please? A couple of test locations I tried (in the
    > Dolomite area of N Italy) seemed some way off (30" or so) when
    > compared with an alternative source.
    >
    > Are there a few easily identifiable locations, ideally in Europe,
    > which we could use to test?


    The Greenwich Meridian passes through West
    Sussex.................................
     
    Dennis Pogson, Aug 25, 2005
    #2
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  3. "Dennis Pogson" <> wrote:

    >Terry Pinnell wrote:
    >> Anyone know how accurate the longitude & latitude co-ordinates are in
    >> Google Earth please? A couple of test locations I tried (in the
    >> Dolomite area of N Italy) seemed some way off (30" or so) when
    >> compared with an alternative source.
    >>
    >> Are there a few easily identifiable locations, ideally in Europe,
    >> which we could use to test?

    >
    >The Greenwich Meridian passes through West
    >Sussex.................................
    >


    Yes, I know, but I'm really suggesting pin-pointing locations and
    specifying their true longitude *and* latitude. It could be that my
    test was flawed, but it would be reassuring to test GE to within say
    5" for both.

    --
    Terry, West Sussex, UK
     
    Terry Pinnell, Aug 25, 2005
    #3
  4. Terry Pinnell

    Shayne Guest

    My question is what datm does google use so we can set our gps to that one
    before entering the coordinates.
    If you are set to the wrong datum on gps you could be off.
    After you enter the coordinates in the right datum then you can change the
    datum back to the one you use which is probobly wgs84 and you will have the
    same location with different coordinates.
    "Terry Pinnell" <> wrote in message
    news:...
    > Anyone know how accurate the longitude & latitude co-ordinates are in
    > Google Earth please? A couple of test locations I tried (in the
    > Dolomite area of N Italy) seemed some way off (30" or so) when
    > compared with an alternative source.
    >
    > Are there a few easily identifiable locations, ideally in Europe,
    > which we could use to test?
    >
    > --
    > Terry, West Sussex, UK
     
    Shayne, Aug 25, 2005
    #4
  5. Terry Pinnell

    Shayne Guest

    Just did a search on google datum help file, here's what I got.
    As Google Earth currently uses WGS84 datum projection.

    "Shayne" <> wrote in message
    news:LHhPe.195737$9A2.111469@edtnps89...
    > My question is what datm does google use so we can set our gps to that one
    > before entering the coordinates.
    > If you are set to the wrong datum on gps you could be off.
    > After you enter the coordinates in the right datum then you can change the
    > datum back to the one you use which is probobly wgs84 and you will have
    > the same location with different coordinates.
    > "Terry Pinnell" <> wrote in message
    > news:...
    >> Anyone know how accurate the longitude & latitude co-ordinates are in
    >> Google Earth please? A couple of test locations I tried (in the
    >> Dolomite area of N Italy) seemed some way off (30" or so) when
    >> compared with an alternative source.
    >>
    >> Are there a few easily identifiable locations, ideally in Europe,
    >> which we could use to test?
    >>
    >> --
    >> Terry, West Sussex, UK

    >
    >
     
    Shayne, Aug 25, 2005
    #5
  6. Terry Pinnell <> wrote:

    > Anyone know how accurate the longitude & latitude co-ordinates are in
    > Google Earth please? A couple of test locations I tried (in the
    > Dolomite area of N Italy) seemed some way off (30" or so) when
    > compared with an alternative source.


    it's not so bad (30ArcSec), but there is a noticable shift at the hi res
    images. I had to move all trackpoints 0.0002 degrees to the East in my
    testfile <http://mac-im-netz.de/dateien/tricias.html> to get my foot on
    the path ;-)
    The roads are more accurate than the satellite image.

    hth
    Anton
     
    Anton Betzler, Aug 25, 2005
    #6
  7. Terry Pinnell

    sierra Guest

    Just checked the corner of my terrace in SE Spain

    GPS **.**.32.0" **.**.38.3" Altitude 143m
    GE **.**.32.1" **.**.38.8" Altitude 142m

    (** degrees and minutes correct), The Google pointer is a bit hit and
    miss - doesn't come to a point.
    GPS set to WGS84
     
    sierra, Aug 25, 2005
    #7
  8. Anton Betzler wrote:

    > Terry Pinnell <> wrote:
    >
    >
    >>Anyone know how accurate the longitude & latitude co-ordinates are in
    >>Google Earth please? A couple of test locations I tried (in the
    >>Dolomite area of N Italy) seemed some way off (30" or so) when


    30 inches is just 75 cm, which is way better than the pixel resolution
    in most areas, so I assume you meant something else?

    >>compared with an alternative source.

    >
    >
    > it's not so bad (30ArcSec), but there is a noticable shift at the hi res
    > images. I had to move all trackpoints 0.0002 degrees to the East in my


    So about 6 m offset? Still not too bad. :)

    > testfile <http://mac-im-netz.de/dateien/tricias.html> to get my foot on
    > the path ;-)
    > The roads are more accurate than the satellite image.


    Here in Oslo, Norway the roads have very good coordinate quality, and
    GE's images seem to match them very well.

    When I display roads on top of the sat images, they all seem to match
    within a meter or so, when I just now compared the location of the
    closest road junction outside our house, the fit was comparable.

    Terje

    --
    - <>
    "almost all programming can be viewed as an exercise in caching"
     
    Terje Mathisen, Aug 25, 2005
    #8
  9. "sierra" <> wrote:

    >Just checked the corner of my terrace in SE Spain
    >
    >GPS **.**.32.0" **.**.38.3" Altitude 143m
    >GE **.**.32.1" **.**.38.8" Altitude 142m
    >
    >(** degrees and minutes correct), The Google pointer is a bit hit and
    >miss - doesn't come to a point.
    >GPS set to WGS84


    I'd call that spot on!

    --
    Terry, West Sussex, UK
     
    Terry Pinnell, Aug 25, 2005
    #9
  10. Terje Mathisen <> wrote:

    >Anton Betzler wrote:
    >
    >> Terry Pinnell <> wrote:
    >>
    >>
    >>>Anyone know how accurate the longitude & latitude co-ordinates are in
    >>>Google Earth please? A couple of test locations I tried (in the
    >>>Dolomite area of N Italy) seemed some way off (30" or so) when

    >
    >30 inches is just 75 cm, which is way better than the pixel resolution
    >in most areas, so I assume you meant something else?
    >
    >>>compared with an alternative source.

    >>
    >>
    >> it's not so bad (30ArcSec), but there is a noticable shift at the hi res
    >> images. I had to move all trackpoints 0.0002 degrees to the East in my

    >
    >So about 6 m offset? Still not too bad. :)
    >
    >> testfile <http://mac-im-netz.de/dateien/tricias.html> to get my foot on
    >> the path ;-)
    >> The roads are more accurate than the satellite image.

    >
    >Here in Oslo, Norway the roads have very good coordinate quality, and
    >GE's images seem to match them very well.


    Both Anton and myself used the " symbol to mean seconds of arc. 30" is
    1/120 of a degree, so that's around 3000 feet, or nearly 1km.

    --
    Terry, West Sussex, UK


    >When I display roads on top of the sat images, they all seem to match
    >within a meter or so, when I just now compared the location of the
    >closest road junction outside our house, the fit was comparable.
    >
    >Terje
     
    Terry Pinnell, Aug 25, 2005
    #10
  11. Terje Mathisen <> wrote:

    > > it's not so bad (30ArcSec), but there is a noticable shift at the hi res
    > > images. I had to move all trackpoints 0.0002 degrees to the East in my

    >
    > So about 6 m offset? Still not too bad. :)


    it's not too bad - but it's more than 20 m
    You can notice it if you use Google Earth or Google Maps to display
    hiking tracks on high resolution images. I hope GE will correct this
    shift in the future - because it's strange to have such a perfect tool
    and you cannot use it without dirty tricks ;-)

    Anton
     
    Anton Betzler, Aug 25, 2005
    #11
  12. Anton Betzler wrote:

    > Terje Mathisen <> wrote:
    >
    >
    >>>it's not so bad (30ArcSec), but there is a noticable shift at the hi res
    >>>images. I had to move all trackpoints 0.0002 degrees to the East in my

    >>
    >>So about 6 m offset? Still not too bad. :)

    >
    >
    > it's not too bad - but it's more than 20 m


    Hmmm, where's my mistake...

    Each degree is 111.111 km when going north-south, so 0.001 degrees would
    be 111 m and 0.0002 is 22 m, which is your value I guess.

    However, I assumed from your email that you're in Germany, so offsets
    east-west must be multiplied by cos(latitude), which is 0.5 in Oslo, and
    a little larger (but not by much) in Germany, right?

    Assuming cos(lat) = 0.6 results in an offset of 13 m, so I was still
    wrong by a factor of two, sorry!

    Terje

    --
    - <>
    "almost all programming can be viewed as an exercise in caching"
     
    Terje Mathisen, Aug 26, 2005
    #12
  13. Terje Mathisen <> wrote:

    > so I was still wrong by a factor of two, sorry!


    Your maths are correct - but not your assumption ;-)
    The track from my link is on La Palma at 28 N. cos(lat) = 0.9
    My home in Germany at 48 N has no hi res image yet.

    ;-) Anton
     
    Anton Betzler, Aug 26, 2005
    #13
  14. Terry Pinnell

    SC_guy Guest

    There is a bit of discrepency in tahoe:

    http://www.geocities.com/webspace314/google_earth.jpg

    Interestling, the elevation is also a bit off, but the actual elevation
    USGS gives me for a spot is about the same as the one at the place
    where google earth points, which is a little bit off. I think that
    there may be a little bit off random error added for security purposes.


    Anton Betzler wrote:
    > Terje Mathisen <> wrote:
    >
    > > so I was still wrong by a factor of two, sorry!

    >
    > Your maths are correct - but not your assumption ;-)
    > The track from my link is on La Palma at 28 N. cos(lat) = 0.9
    > My home in Germany at 48 N has no hi res image yet.
    >
    > ;-) Anton
     
    SC_guy, Sep 25, 2005
    #14
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