GPS coordinate formats: Explanations and conversions

Discussion in 'GPS Use, Tracks & Maps Discussion' started by JB2112, Dec 21, 2011.

  1. JB2112

    JB2112 Ol'Timer

    Maybe this will help readers to understand what they are seeing on their units a little better. I hope I do not confuse anyone!

    Basic explanation of geographical (spherical) coordinates
    It is easier to show this on a sphere, but the equator splits the world into two halves: North and South. A similar circle between the North Pole and South Pole would split the world in half. It was decided years ago that the main circle of reference would go through Greenwich, England, so that arc of a circle, or meridian, is called the Greenwich Meridian. This is the basis of units for East and West coordinates.

    Latitude: North or South from the Equator with a maximum of 90 degrees (The North Pole!).
    Longitude: East or West from the Greenwich Meridian with a maximum of 180 degrees. The opposite side of the earth from the Greenwich Meridian is the International Date Line and I won't even begin to discuss that here!

    When you are reading a Lat and Long position you can actually visualize a line running north/south meeting a line running east/west. It helps to see this on a map, maybe this link can help:

    Converting GPS coordinates manually:
    The 3 formats I see on my Garmin are
    ddd.ddddd°, ddd° mm.mmm' and ddd° mm' ss.s".

    Since we are using geographical coordinates these are based on a circular measurement system: Degrees (°), minutes(') and seconds(").

    A circle consists of 360 degrees. On the earth you can see if you follow a meridian you would enter four "areas" each contain 90 degrees of latitude. Longitude splits the world east/west into two 180 degree "areas."

    One degree contains 60 minutes. One minute contains 60 seconds. This may be confusing as they are also time measurements, but you have to start thinking distance measurements now! (I think they are actually angular measurements, but for our discussion, please think LENGTH!)
    Note: When you are reading formats ddd° mm.mmm' and ddd° mm' ss.s" always remember that, by definition, the minutes and seconds cannot be 60 or higher. Why? They have now become a higher unit of measurement raised by one.

    Conversion 1: ddd° mm' ss.s" to ddd° mm.mmm'. Here you are converting the seconds, or ss.s, into decimal format and appending it to the minutes.
    Example: N 20° 19' 15.5" becomes N 20° 19.258'.
    How? 15.5" / 60 = .258

    Conversion 2: ddd° mm.mmm' to ddd.ddddd° Now you do the same conversion to the minutes area which will result in a full decimal appendum to the degrees of latitude or longitude.
    Example with same units from above: N 20° 19.258' becomes N 20.32097°.
    How? 19.258' / 60 = .32097

    That should do it. I hope I got this all right. It has been a long time since university.
    If you get a handle on how to do the conversions you should get a better feel for your GPS, navigating (or navi-guessing!) , and the world in general!

    Good luck! JB
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