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GPS Accuracy

Discussion in 'Boat Electronics' started by wg992000, Jan 27, 2004.

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  1. wg992000

    wg992000 Guest

    Does anyone know if there is a set relationship between the accuracy / DOP/
    Differenctial indicated on the GPS page to the accuracy of the compass
    direction? ( of the GPS)

    ie. Trying to determine how far off the compass rose would be off if WAAS
    sat's were being received and there was high accuracy.

    Kind of new at this. Any thoughts appreciated.
  2. The compass course given by the gps depends more on how fast you are
    going than whether WAAS is on. The magnetic course depends then on
    whatever the software has for variation. Variation is a slowly
    changing function of location, so won't depend on WAAS at all.

    But, since you don't really know what the software does, you would be
    better reading true if you are trying to swing your compass with the
    GPS. Then use the variation from your chart.

    Rodney Myrvaagnes J36 Gjo/a

    Entering your freshman dorm for the first time, and seeing
    an axe head come through the door on your right.
  3. Harry Krause

    Harry Krause Guest

    You forgot time.

    And how long is the "so long" you refer to in indicating a change in
    heading or speed? It doesn't seem "so long" on my GPS instruments.
    Perhaps you wired them up wrong, eh?

  4. The "compass" in a GPS is surely historical, it tells you where you've been,
    not where you are going? A very rough comparison with the magnetic compass
    might indicate if the latter had major errors, but compass swinging using
    GPS, someone must be kidding! Much better to use transits, or a hand bearing
  5. Since the position of magnetic north is changing perpetually, it is standard
    practice to give all bearings etc, in true notation, and apply the
    correction factor appropriate to the time and location of the navigator.

    The compass rose on charts, where magnetic north has moved almost as soon as
    the chart is published and sold, is for guidance only.

    GPS bearings and calculated courses are corrected for magnetic north as at
    the time and place of the fix shown on the screen, but the user has the
    option to display in true notation if he/she so pleases. I personally set my
    GPS to true readout, but others may prefer to set theirs to magnetic.

    Remove "nospam" from return address.
  6. Sorry, I tried to make that clear. The 'true' measurement is derived
    first from successive Lat-Lon measurements, so it is historical as
    others have said. The 'magnetic' depends on a lookup table in memory
    if it is done by the gps.

    As you will see if you read the compass rose legend on a chart, the
    variation changes slowly over the years. Unless you know if your gps
    updates its table, and/or when its data were entered, you would be
    taking a chance.

    Even though the 'true' is historic, it is possible to use it for
    swinging. You must motor along an ad hoc range line at constant speed,
    the faster the better. A motorboat will be easier., but if the
    direction reading doesn't change at all for some time. you should be

    If you can hold the range and the speed over ground, your true
    heading will also hold steady and be correct. If conditions prevent
    you from holding, it's the wrong time to swing your compass.


    Rodney Myrvaagnes J36 Gjo/a

    Entering your freshman dorm for the first time, and seeing
    an axe head come through the door on your right.
  7. wg992000

    wg992000 Guest

    Thanks for all the input -
    It was the swinging the compass input I was after.

    Thought process was that maybe with decent sat reception - swinging the
    compass (adjustments and compass card) may be easier and more accurate with
    the GPS then using traditional methods i.e.. lining up bearing points,
    shadows etc with my 13 year old at the wheel and me trying to do this. Have
    a 21 coastal boat with compass in 5 degree increments. Sounds like the best
    idea is stick with traditional methods or farm it out. In any case thanks
    for all the help and thoughts.
  8. BrianR

    BrianR Guest

    Tried a Google search for "gps speed doppler" and got the following message:

    Your search - "gps speed doppler" - did not match any documents.

    - Make sure all words are spelled correctly.
    - Try different keywords.
    - Try more general keywords.
    Also, you can try Google Answers for expert help with your search.
  9. BrianR

    BrianR Guest

    Thanks, I left the quotes in originally as I was looking for the exact
    Now get the 13,300 hits, much appreciated.
  10. If you go to and enter GPS Speed Doppler
    without quotes, and use the "all the words" function, you will get
    34,344 hits.

    I don't know if the extra 21,000+ hits matter to you, but there is
    a wealth of information on some of these pages.

    I hope this helps!
  11. Alot of Coastal Towns have such Ranges setup out here in the west.
    In Seattle, all the compass rebuilders use the same range that is
    setup in Lake Union. In the spring the whole Seattle Based fishing
    fleet can be seen out there doing compass adjusting.

    Bruce in alaska
  12. Doug,

    Yea, there is a fellow that I used a lot in the past.
    Eknes Instruments, in Ballard 826 NW 49th St 98107 (206) 783-3303
    this fellow is a third generation Compass Adjuster/rebuilder.
    Well respected in the North Pacific Fishing Fleet.

    Bruce in alaska
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