Help Needed!!!

Discussion in 'OTC and ACF' started by Muffy_the_minj_slayer, Oct 13, 2005.

  1. GPS/Gadget skills

  2. Map and Compass Skills


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  1. [align=center]urgent [/align]

    Today, I was asked to explain how to work out GMA can anybody assist on this please. I know what a GMA is just can't remember how to do them. God how i love GPS...GARMIN I love you.
  2. Hope this isn't a wah, but (deep breath)

    look for the magnetic deviation marked on the map you are using (unless you've trimmed it down and fabloned it into a small postage stamp :wink: )

    with that figure apply the following little ditty.

    grid to mag add, mag to grid get rid

    Then ditch your gps (its only an aid to navigation, not the key) as one with a hole through is useless, unlike a map.

    learn how to read contours.

    get lost lots, get very embarrassed, get the p*ss taken out of you, never get lost again!
  3. Any chance someone could put the manual answer up for me not to sure stil.
  4. You need to work out the Grid Magnetic Angle first of all. Here's an example - I've chosen the numbers just to make the maths easy:

    On the top of an OS map / marginal info of a military map you will find some blue writing along the lines of:

    "Grid Magnetic Angle 10 degrees (178 mils) West of Grid North June 2000 at the centre of the sheet. Annual Change 1 degree (17.8 mils) East"

    What this means is that Magnetic North is 10 degrees to the West (Left on map) of Grid North in June 2000.
    The angle between Grid North and Magnetic North is the Grid Magnetic Angle (GMA).
    Magnetic North is moving 1 degree (17.8 mils) to the East (Right on map) every year - eg. the Grid Magnetic Angle is decreasing by 1 degree every year.
    We are now in Oct 2005, which is 5 years on, so the overall decrease in the GMA will be 5 x 1 degree (5 x 17.8 mils) which equals 5 degrees (89 mils) overall. The GMA will now be 10 degrees (178 mils) minus 5 degrees (89 mils), which equals 5 degrees (89 mils).

    It is this GMA value that is the key: to convert a grid bearing to a magnetic bearing add the GMA, to convert a magnetic bearing to a grid bearing subtract the GMA.

    One thing that can get slightly confusing is that it is common to see things like "Grid Magnetic Angle 2 degrees 12' (39 mils) West of Grid North". 12' means 12 minutes - a minute is 1/60th of a degree.

    Hope this example helps!
  5. Have a look at what OS have to say about it
  6. cheers all got it sorted now great help.