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    Re: Estimating height of eye
    From: Richard B. Langley
    Date: 2013 Apr 11, 10:32 -0300

    On 2013-04-10, at 3:51 PM, Brad Morris wrote:
    
    > Hi Marcel
    > 
    > I just tried "GPS Averaging" by Destil.   When I tried to take the waterline 
    vs my eye height, I found the report was inverted.  That is, it reported the 
    water over my head.  nonsense.  I think that this is a function of the poor  
    GPS receiver in my phone, because my GARMIN GPSMap 76 reported a fairly 
    accurate 17 feet HoE.
    
    Some receivers used to mix up geodetic height (obtained directly from GPS) and 
    orthometric height (or height above sea level). At sea level, it is possible 
    for the geodetic height to be a negative or a positive value. The difference 
    between the two types of height is the geoidal undulation. Perhaps some 
    software mixes up these heights, too.
    
    > Richard Langley
    > 
    > Forgive me for not understanding the technical terms in your description of 
    this HoE measurement by GPS.  Could you please explain that in layman's 
    terms?  Is the HoE measurement by reported GPS altitude feasible with/without 
    averaging?
    
    Depends on the accuracy you are looking for. If you want 1 metre accuracy 
    (1-sigma level; i.e. a value with an error of 1 metre or less 68% of the 
    time), then using a handheld receiver (making pseudorange measurements) will 
    require averaging. A single spot value will not be accurate enough. How much 
    averaging? Will depend on local conditions. One way to do this would be to 
    store the positions as they accumulate in the receiver, download them, and 
    use Excel or something else to do the averaging, noting how the position 
    uncertainty becomes better as more values are averaged.
    
    After sending my previous message, I recalled we had an article on the GPS 
    Standard Positioning Service accuracy in GPS World a few years ago. Here's a 
    link to it:
    http://gge.unb.ca/Resources/gpsworld.february03.pdf
    The topic is probably worth re-visiting in my column -- 10-years on.
    
    -- Richard Langley
    
    > Brad
    > 
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123506
    > 
    
    -----------------------------------------------------------------------------
    | Richard B. Langley                            E-mail: lang---.ca         |
    | Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/ |
    | Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142   |
    | University of New Brunswick                   Fax:      +1 506 453-4943   |
    | Fredericton, N.B., Canada  E3B 5A3                                        |
    |        Fredericton?  Where's that?  See: http://www.fredericton.ca/       |
    -----------------------------------------------------------------------------
    
    

       
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