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How reliable is Google Earth topography data?

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  • 02-03-2011 11:30pm
    #1
    Daegerty
    Closed Accounts Posts: 1,864 ✭✭✭


    Good enough to calibrate the altimeter on my GPS? I need to land a 500k UAV in a secret tunnel inside a cliff with <1m clearance know how far I went up on a peaceful sunday afternoon hill walking session


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    #2
    Alun
    Registered Users Posts: 21,467 ✭✭✭✭Alun


    Why not just use the current GPS derived altitude? That's what I do with my Oregon 550t before setting off.


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    #3
    Daegerty
    Closed Accounts Posts: 1,864 ✭✭✭Daegerty


    Alun wrote: »
    Why not just use the current GPS derived altitude? That's what I do with my Oregon 550t before setting off.

    Its notoriously inaccurate. + or - 100ft


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    #4
    Alun
    Registered Users Posts: 21,467 ✭✭✭✭Alun


    Daegerty wrote: »
    Its notoriously inaccurate. + or - 100ft
    That's the theory at least (HDOP, VDOP etc). However, my (many years of) experience of comparing GPS derived altitudes with spot heights and trig points on OS maps both on the Wicklow hills and in the UK has rarely shown much more than a few metres discrepancy, certainly nowhere near anything like 30m.

    Anyway to answer your question re: the Google maps imagery, I believe they get their elevation model from the free NASA SRTM data available to anyone. For Europe it's supposedly accurate to +/- 16m, but bear in mind that the points in the model are at 90m (3 arc second) intervals, and any points in between are interpolated which may introduce extra errors. Even the elevation model available from OSi, although derived via other methods and more accurate, is only at 100m intervals.

    BTW, the barometric altimeter in Garmin GPS's can be very sensitive to both strong winds and pressing on the case causing up/down spikes. I've seen some completely crazy data for height gain, sometimes more than double what might be considered reasonable. You'll always get a bit more than what you'd guess from looking at a map or using computer based route planning that uses an elevation model because you're registering all the little ups and downs that don't show on either.


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    #5
    Alun
    Registered Users Posts: 21,467 ✭✭✭✭Alun


    FWIW in my back garden the elevation as reported by a mapping program using OSi data, one using SRTM data, Google Earth and 3 different GPS's of various vintages and 3 different GPS chipsets was as follows ...

    51m (Mapyx Quo using OSi DEM)
    51m (Ozi Explorer using SRTM 90m data)
    48m (Google Earth)
    49m (Garmin Oregon 550t (GPS elevation))
    51m (etrex Venture HC)
    54m (etrex Venture)

    The two modern GPS's were both reporting +/-3m accuracy with WAAS/EGNOS enabled. The old one +/-7m.

    So not really a huge difference there, certainly not enough to be worrying about when measuring height gain on a walk, esp. given the sensitivity of the barometric altimeter on Garmin GPS's to the other factors I mentioned above.


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