02 October 2014

Coordinate conversions

Knowing where you want to go is a good thing, but what happens when you're not given a street address but a set of coordinates like this: 39.4511 -077.4981? Most GPS units should have a way that allows you to drive to that set of coordinates, but what if you're given a set of coordinates like this: 39°27'03.96"N 077°29'53.16"W? I believe that most GPS units would be able to handle that as well though I don't use mine this way.

This type of coordinate, 39.4511, is called a decimal degree (DD) of latitude. Because it's positive it's known to be in the northern hemisphere, a negative number would be in the south. A negative longitude means west of the prime meridian. For the most part, unless you're in some parts of Alaska, all coordinates will have a positive latitude and a negative longitude.

This way of writing coordinates, 39°27'03.96"N, is called degree, minutes and seconds (DMS) of latitude. Some people use decimal degrees and others use degree, minutes and seconds. It's easier in the long run to keep to one system.

So, how does one convert between the two? It's actually quite a simple calculation. First we'll go between DMS and DD, then DD and DMS. I'll use 39°27'03.96"N for this example.

Step 1:
Take your seconds, in this case it's 3.96 and divide by 60.

Step 2:
Take the minutes and add the number from the last calcualtion.

Step 3:
Take the last number and divide by 60 again.

Step 4:
Take the degree and add the last number to it and you're done.

To go the other way is the same basic steps just backwards, I'll use -077.4981 in this example.

Step 1:
Take the decimal part of the number, here .4981, and multiply by 60.
Here you've already found your minutes and it's 29, the remaining decimal points are your seconds.

Step 2:
Take the decimal part of the previous number, .886, and multiply by 60 again.

Put those together and you have 77°29'53.16"W, remember the negative means west. This can be done in a spreadsheet, but it's also possible to do with a hand calculator, or you're head if you really want.

Now you can go back and forth between the two systems on the go if need be. This can be useful if you're looking for a cemetery that doesn't have an address, such as smaller family cemeteries. You can also plug either type of coordinates in to Google Maps and it will drop a marker to see where that point is. The coordinates I used in my example is for the High Knob/Main family cemetery just west of Frederick.


  1. The FCC, for some reason, has a handy coordinate converter page here: http://transition.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html . Your tax dollars at work. Also, if you just paste the coordinates, like 9°27'03.96"N 077°29'53.16"W into the Google Maps search box, it will display the appropriate map, AND show both styles of coordinates below the search box.

    1. That's rather interesting about Google maps, I knew it could deal with both, I've just never seen it display them both like that.

      I'm not too surprised that the FCC has such a conversion, they probably use it for locations of transmitters and the like. I know that National Oceanic and Atmospheric (NOAA) and National Geodetic Survey have converters as well. I thought it might be useful for someone in the field if all they had were notes and no good way to access the internet, much like my phone at times.