Amateur GPSr accuracy, horizontal and vertical

From: Tom Wagner (
Date: Tue Jun 29 2004 - 18:56:12 EDT

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    I have finally figured out how to locate the precise coordinates on various
    markers in any area in the US. Today I used one to make a quick test of the
    accuracy of my Magellan SporTrak GPSr.  GPSr = GPS receiver.
    The details of my test follow:
    The location of the marker was accurately, 42 26.583 N by 92 25.240 W at an
    altitude of 872 feet. I took four readings, one reading right on to begin
    with, then three others by walking away in three different directions,
    pausing a few seconds each time, then coming back to the marker and waiting
    for my Magellan SporTrak to average 30 times (30 seconds) each before
    recording a mark. I held my unit 5 feet above the marker (above the weeds).
    My averaged reading was 42 26.586 by 92 25.239 at an altitude of 875 feet:
    minus the 5
    feet above the marker this = 870 feet. My unit was reading a WAAS satellite
    and was receiving 8 strong signals at the time. The area was wide open with
    no significant obstructions. There were fewer satellites in the N than any
    other area.
    42 26.583 - true value
    42 26.586 - my averaged value
    error = 0.003 minutes = 0.18 second too far N
    92 25.240 - true value
    92 25.239 - my averaged value
    error = 0.001 minute = 0.06 second too far E, NOT BAD!!!!
    According to the
    position error overall was 19 feet (mostly in latitude) at this latitude.
    872 feet alt - true value
    870 feet alt - my averaged value
    error = -2 feet, NOT BAD!!!!
    My actual readings ended with the following minutes:
    Lat (actual = 26.583)
    Long (actual = 25.240)
    Feet Alt (actual = 872 feet)
    As one can see, it seems like the first reading I made was the most accurate
    I have the details about how a person may obtain the information needed to
    locate suitable markers [[in the US]] to test their own GPSr. Feel free to
    contact me if you want a copy of my instructions.
    Tom  Iowa  USA
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