Latitude and Longitude
       comes in three flavors 

This is the most common format used to mark maps. It's also the most cumbersome to work with. It's a lot like telling time…

There are sixty seconds in a minute (60" = 1') and

There are sixty minutes in a degree (60' = 1°).

Keeping in mind a few easy conversions between seconds and decimal minutes will help when working with maps that use degrees, minutes and seconds.

15 seconds is one quarter of a minute or 0.25 minutes

30 seconds is one half of a minute or 0.5 minutes

45 seconds is three quarters of a minute or 0.75 minutes

This is the format most commonly used when working with electronic navigation equipment.

This is the format you'll find most computer based mapping systems displaying. The coordinates are stored internally in a floating point data
 type, and no additional work is required to print them as a floating point number.

Often the N-S and E-W designators are omitted. Positive values of latitude are north of the equator, negative values to the south. Watch the sign on the longitude, most programs use negative values for west longitude, but a few are opposite. This saves a lazy western hemisphere programmer from having to type in a minus sign before most of their longitude values.

Which format should you use?

                              First off, if you are working with other people who have agreed upon a format to use,
    then you should probably use that format.

    Next, you will want to look at the maps, lists of coordinates, and any software
    you may be using. If you can find a consistent format among them, your work will be easier.

 You can set your GPS to display any one of these three formats. Locations can be entered into the GPS with the selected format, and then by switching the display format setting, viewed in a different format.

I frequently choose to use the Degrees and Decimal Minutes format, even though the USGS maps I'm using are marked in Degrees, Minutes and Seconds. The markings on the map are all at either 0, 15, 30, or 45 seconds. By remembering the "quarter minute conversions" of 0.00, 0.25, 0.50, and 0.75, I can quickly do the conversions in my head.

Thanks to John Carnes of MapTools for use of his copyrighted material
http://www.maptools.com/index.html