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  1. #1

    Exclamation Different datums conversion.

    I was given a GPS mark for a spot x taken from an iphone photo that the " internet " can ID the longitude and latitude from, some weird thing.

    But, the co-ords come up in the Google Earth datum, which of course is different to Garmin datum.

    So, how to calculate it back to Garmin datum. ( WGS 84 )

    Difference is Google use ( GIS... Geographic Information System ) Decimal Degrees and Garmin ( WGS ... World Geodetic System ) use Degrees ( degrees, minutes and seconds )

    There is of course a formula to do all conversions and one looks like this :-




    I did a manual thingie and came up with a WGS84 mark is 365mtrs at 286 degrees from the Google Earth mark. No idea how correct this will be until I hit the water and look for said Spot X

    But the manual thingie I did at home was to simply turn on my 2 Garmins and ID the co-ords............ then go to Google Earth and id those co-ords and do the math.........

    Whether this works or not due to me doing the sums here rather than at spot X some 500k's north of my home, time will tell.

    Maybe someone has a better system or can tell me if I got it wrong ?

    Cheers LP
    Last edited by Lucky_Phill; 11-08-2019 at 11:16 AM.
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  2. #2

    Re: Different datums conversion.

    Use your mapping software (mapsource or homeport)..

    change the preferences to the correct datum for the mark.
    enter your mark.
    Change preferences to WGS84 or the datum you want to use in your unit..

    Hey presto..

    Regards
    Honda.

  3. #3

    Re: Different datums conversion.

    1. There are Datum's and then there are Formats, two totally different entities

    2. Google Earth uses WGS84 datum, no such thing as "Garmin Datum" which is user configurable/selectable, including WGS84 among many others

    3. WGS84 Datum can use a variety of different formats

    4. A difference of 365m @ 286 degrees is not logical apart from being determined incorrectly or this "internet" thing being totally obscure, meaningless and useless

    5. If the datum is known, if the format is known, then it doesn't matter what correctly configured device the position is input, the result will basically be the same

  4. #4

    Re: Different datums conversion.

    Yep. Just a different position format. If you set your unit or homeport to that setting as Honda suggested, enter the waypoint and then switch it back you will be all good

  5. #5

    Re: Different datums conversion.

    mmmm thanks guys.

    So When I determined my position on earth via Google Earth and then by my Garmin GPS units and the " information " = datum was different, that didn't mean squat ? Even though the info from both put me 365mtrs and 286 degrees from GPS location via WGS84. minutes

    I am unsure if I should fiddle with my Homeport and Mapsource and even my Garmin and change formats ? Knowing my luck the whole 1500 waypoints will take " a jump to the left " Visions of Rocky Horror !!!!

    Well , I took the plunge.. entered the spot X to Home port, but before I did I changed the format to Degrees Minutes and seconds, ( hddd mm'ss.s ) as the existing format was Degrees Decimal Minutes ( hddd mm.mmm ). Then changed the format back and my spot is now 1.3k's and 318 degrees from where I calculated it to be.... and some 971 mtrs and 320 degrees from the mark I entered as per given in Degrees Minutes Seconds, but entered with format at Degrees Decimal Minutes.

    Now I have 3 marks for the same spot

    Now before we go any further, can someone tell me what format this would be ?

    25* 35' 7.6" S 151* 17' 3.5" E


    cheers
    Kingfisher Painting Solutions:- Domestic and Commercial.

    For further information, contact details, quotes or advice - Click Here





  6. #6

    Re: Different datums conversion.

    deg Min sec.sec that format...has noting to do with the datum as all datums use all of three different ways to record the mark. Your problem you are having is your waypoint has been created on one datum and plotted on another datum giving the error.
    With nearly all gps units on the market you can enter d mm ss.ss or d m.mm and switch back and forth in the settings and you will never lose position of that mark.
    Jack.

  7. #7

    Re: Different datums conversion.

    deg Min sec.sec that format...has noting to do with the datum as all datums use all of three different ways to record the mark.
    Your problem you are having is your waypoint has been created on one datum and plotted on another datum giving the error.
    With nearly all gps units on the market you can enter d mm ss.ss or d m.mm and switch back and forth in the settings and you will never lose position of that mark.
    Jack.

  8. #8

    Re: Different datums conversion.

    So if you change your Homeport to LAT/LON hddd.ddddd° and then enter the number you got directly from Google Earth, then change it back to LAT/LON hddd° mm.mmm' , does it line up?

  9. #9

    Re: Different datums conversion.

    On another note, I just checked - you can change the readout of your google earth to degrees, decimal minutes too.

  10. #10

    Re: Different datums conversion.

    download GPS Babel to change co-ords easily between formats.
    nil carborundum illegitimi

  11. #11

    Re: Different datums conversion.

    Quote Originally Posted by GBC View Post
    download GPS Babel to change co-ords easily between formats.
    or just use this site, nice and easy and no need to download software.

    https://www.directionsmag.com/site/latlong-converter/

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