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    Re: Estimating height of eye
    From: Brad Morris
    Date: 2013 Apr 10, 19:12 -0400

    USGS GPS GRADES

    The four accuracy grades of GPS receivers identified by the United States Geological Survey are recreational grade, commercial mapping grade, differential mapping grade and survey grade. Manufacturers must meet specific horizontal accuracy requirements to classify their products in each grade. Generally speaking, a recreational grade must be accurate to within about 50 feet; commercial mapping, 10 feet; differential mapping, 3 feet and survey grade within one centimeter.

    The USGS sets vertical measuring specifications only for survey grade GPS receivers, requiring an accuracy of within two centimeters.

    On Apr 10, 2013 2:46 PM, "Marcel Tschudin" <marcel.e.tschudin---.com> wrote:

    {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
    

    View and reply to this message: http://fer3.com/arc/m2.aspx?i=123505

       
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