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    Re: Estimating height of eye
    From: Marcel Tschudin
    Date: 2013 Apr 10, 21:40 +0300

    {at} Rchard Langley
    
    I noticed from your contributions and publications that you seem to be
    more deeply involved in the subject of GPS. May I therefore ask you:
    Do you have an idea on the sort of accuracy one may "generally" expect
    when measuring with GPS at the same location (few tenths of meter
    distance) height *differences* between about 2 and 5 m ? Would it be
    possible to measure at the two locations the height and obtain their
    difference to about +/- 20% or even better to e.g. +/- 10% and what
    type of equipment would this require?
    
    Thanks in advance for some hints.
    
    Marcel
    
    
    On Wed, Apr 10, 2013 at 7:32 PM, Richard B. Langley  wrote:
    > ________________________________
    >
    > It would be averaging the effects of measurement (pseudorange) imprecision
    > (random over longish time intervals) and multipath (not random and not
    > repeatable except for a fixed location with the same satellites one sidereal
    > day to the next). And, to a lesser extent, tropospheric delay errors and
    > other minor model deficiencies. Two sites, some distant apart, would
    > experience different errors although the pseudorange-based DGPS technique
    > (such as that supported by the Coast Guard) uses the errors computed at a
    > reference station to improve the positions of a rover (in real time).
    > -- Richard Langley
    >
    > On 2013-04-10, at 1:20 PM, Marcel Tschudin wrote:
    >
    >>
    >> Thank you, Brad, for finding this Android app for me. I presume that
    >> the 1 m represent an absolute accuracy and that measurements of height
    >> differences may possibly attain a higher accuracy. I will give this
    >> affordable gadget a try and perform some tests with it.
    >>
    >> Marcel
    >>
    >>
    >> On Wed, Apr 10, 2013 at 6:28 PM, Brad Morris
    >>  wrote:
    >> > ________________________________
    >> >
    >> > Hi Marcel
    >> >
    >> > There is an app in the Android market called "Precision GPS Pro".  The
    >> > cost
    >> > is US$0.99
    >> >
    >> > It claims a 1 meter elevation accuracy after averaging, although they do
    >> > not
    >> > indicate how long an average you require to achieve that.
    >> >
    >> > The free version of this app does not do elevation averaging.
    >> >
    >> > I have not tried this app.
    >> >
    >> > Brad
    >> >
    >> > On Apr 9, 2013 6:21 AM, "Marcel Tschudin"
    >>  wrote:
    >> >>
    >> >> ________________________________
    >> >>
    >> >> In my sunset observations the guesstimates for the HoE of probably not
    >> >> better than +/- 20% are indeed a weak point. But how weak? The dip is
    >> >> proportional to the square root of the HoE which means that the
    >> >> estimated dip error is only half of the estimated HoE error. For the
    >> >> average HoE of 3.5m the "normal" dip is 3.3 moa and a 10% error of it
    >> >> therefore +/- 0.33 moa. This corresponds in the photo to slightly more
    >> >> than one pixel which is 18 seconds of arc.
    >> >>
    >> >> Frank, you mentioned estimating HoE within about +/-5%. I'm wondering
    >> >> whether you would also be able to attain such an accuracy by climbing
    >> >> over the rocks as shown in my photo:
    >> >>
    >> >>
    >> >> 
    https://mail-attachment.googleusercontent.com/attachment/u/0/?ui=2&ik=b33d2c81ba&view=att&th=13ddab531282f2e7&attid=0.1&disp=inline&realattid=f_hf5h7cfp0&safe=1&zw&saduie=AG9B_P-DvbGAuAxTxtruQd29-avq&sadet=1365502394438&sads=w4N-7jPt0vbdfN6eZHKSEaR8vQ0
    >> >> For my average of 3.5 m this would correspond to less than 20 cm or
    >> >> what might be more familiar to you to about half a foot.
    >> >>
    >> >> In order to verify my guesstimates I do indeed consider to perform
    >> >> some additional measurements. I'm wondering how good I could measure
    >> >> it using GPS and also the Russian version of it. I have no experience
    >> >> in using these systems. I understand that I would have to measure both
    >> >> levels, eye and sea level and then take the difference. Is there an
    >> >> Android application which I could load on my Samsung pad allowing to
    >> >> collect measurements over a longer time period which then calculates a
    >> >> mean altitude and its standard deviation? What are the sort of result
    >> >> one could obtain this way?
    >> >>
    >> >> Marcel
    >> >>
    >> >> View and reply to this message: http://fer3.com/arc/m2.aspx?i=123477
    >> >
    >> > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123498
    >>
    >>
    >> View and reply to this message: http://fer3.com/arc/m2.aspx?i=123499
    >>
    >
    > -----------------------------------------------------------------------------
    > | 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/
    > |
    > -----------------------------------------------------------------------------
    >
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123500
    

       
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