Without a doubt, the most commonly asked question I get from divers has to do with reading and using GPS coordinates.  This is not surprising since a GPS coordinate can be represented in so many different ways.  Books, magazines, and online sources often display coordinates differently, some using a notation that includes values represented as pure decimal numbers, others using a notation that includes minutes and seconds of degrees.  And some use NORTH, SOUTH, EAST, and WEST indicators, while others use plus and minus symbols instead.  I've even seen books that list coordinates one way on some pages, and other ways on other pages -- within the same chapter! 

But once you understand the different notations and you learn to recognize the patterns, using the coordinates and converting between the different formats becomes pretty easy.  Now before I get into the details, let me just say that the coordinates on DiveSpots.com can be displayed in three different formats, making it easy for you to see the coordinates any way you want them to appear.  Just click a button and we handle the complex math needed to convert between the different formats (ok, it's not that complex).  Here's how...

For any dive spot listed on the site, you'll see the GPS coordinates presented in the simulated GPS unit on the right side of the screen (shown here to the right).  The coordinates are initially displayed in what is probably the most common format, which includes the hemisphere, the hours of degrees, and a decimal value representing the minutes of degrees.  You can flip to the next display format by clicking the "mode" button at the bottom of the simulated GPS unit.  In this second format, the minutes of degrees are represented as a whole number and the seconds are also represented as a whole number.  Click the mode button again and you can see a third format which shows the GPS coordinates as pure decimal numbers (very useful for doing coordinate math, like calculating distances).

Let's start with a short primer.  As you may recall, the earth is divided into hemispheres, divided north-from-south by the Equator, and west-from-east by the Prime Meridian.  What's significant about these imaginary lines on the earth is that GPS coordinates are given relative to these positions.  For example, the latitude N 25° 07.450' is a little more than 25 degrees north of the Equator.  The latitude S 25° 07.450' is a little more than 25 degrees south of the Equator, which can also be written as the negative number -25° 07.450'.  The same applies to the Prime Meridian.  Values to the West of the Prime Meridian (such as points in the United States) are represented with a "W" or a negative number, and values to the East (such as points in Japan) are represented with an "E" or a positive number. 

The hemisphere is sometimes a source of confusion since publishers frequently omit the hemisphere from printed coordinates. This can create problems when using these coordinates with some computer programs since computers always assume a positive number when you don't enter a sign (+ and -, or E and W).  So, suppose you're diving in the United States and you see the coordinates for the Christ of the Abyss as:

     25 07.450, 080 17.800

You enter these coordinates directly into your favorite computer program and suddenly you're somewhere in India!  That's because the longitude was entered as a positive number, which would put it East of the Prime Meridian.  Not good.  Be sure to include the hemisphere!

OK, let's talk about the actual numbers.  Using the latitude N 25 07.450 as an example, we can represent this same location as N 25° 07' 27", or simply 25.124167.  They all refer to the same location.  This is the most common source of confusion.  I get e-mails saying things like, "your site lists the Christ of the Abyss as 25 07 450, but this book I have has it as 25 07 27."  See the problem?  Don't ignore the decimal point!  If it's there, then the seconds are represented as a fraction of the minutes.  If it's not there, then the third value is the number of seconds.  Let's break it down:

N 25 07.450 is read as 25 degrees and 7.450 minutes North of the Equator. 

To convert this to the second form, simply take the fraction, .450, and multiply it by the number of seconds in a minute.  .450 x 60 = 27.  Thus, 07.450 minutes is the same as 07 minutes and 27 seconds, or simply 07' 27". 

The third format, 25.124167 is just the degrees as a pure decimal number.  Take the fraction, .124167, and multiply it by the number of minutes in a degree.  Thus, .124167 x 60 = 7.450 minutes. 

Piece of cake, right?  Right!   Again, we publish all GPS coordinates in these three common formats.  Just use the "mode" button on the simulated GPS unit to find the format that suits you best.  If you download the coordinates directly from our site to your GPS unit, we handle the conversions for you.  If you're entering them manually into your GPS unit, be sure you're using the correct entry format or you may find yourself way off the target!

Comments:

	Posted by seagoat, Mar-16-2006 @ 07:11 PM CST Thanks! Very good article.
	Posted by FaaGaaWee, Mar-17-2006 @ 07:29 AM CST That’s right. Oh ya, now I remember. What a great refresher.
	Posted by Airburnr, Mar-23-2006 @ 01:09 AM CST Thanks Very informative and easy to understand
	Posted by Reborn1ce, May-27-2006 @ 09:09 AM CDT You know what? I knew it, just did not remember. Great article, for all of us. Thanks, and keep up the good work.
	Posted by sschocker, Aug-20-2006 @ 11:32 PM CDT This was a great first lesson. Thanks
	Posted by Junior8, Oct-4-2007 @ 02:57 AM CDT Brilliant glad to see how easy and clear it can be made THANK YOU!!!
	Posted by robl756, Jan-13-2009 @ 07:22 PM CST Great Primer for my students, I WILL recommend it, and I'll be talking to you about linking.
	Post a Comment