I recently tackled a mystery letterbox that gave longitude and latitude coordinates. Not being a geocacher, I first had to find out just a little bit about how they work.

So here’s a real quick lesson.

Latitude are imaginary lines that run horizontally along the sphere of the earth (parallel to the equator).

Longitude are lines that run vertically from north to south poles.

The coordinates are given as a latitude number first, then a longitude number.

Format

Each number is simply this:
Hemisphere Degrees Minutes Seconds

Hemisphere
You always must begin with what hemisphere you’re in since the same coordinate exists in two places.

So for example, if you leave the minutes and seconds blank and just look at hemisphere and degrees:

42 (or 42N) degrees lattitude, -71 (or 71 W) degrees longitude is in Massachusetts, U.S.A.
42 71 is in Uzbekistan
-42 71 is in the ocean between South America and Australia
-42 -71 is in Argentina

Degrees
Latitude: can go from 0 (at the equator) to 90 (at either pole)
Longitude: can go from 0 (at the Prime Meridian) to 180 (at the International Dateline)

Minutes
These are a further breakdown of degrees and usually are represented with ‘. There are 60 minutes in a degree.

Seconds
These are a further breakdown of minutes and usually are represented with “. There are 60 seconds in a minute.

Example:
42 14? 16? , -71 28? 25” is a point in Ashland State Park in Ashland, Massachusetts

More Formats
So even though the format is H DDD MM SS, you can have more variations on this.

For example, this is a valid coordinate:
42.23, -71.47

Look closely; you’ll see there is just a single number with two decimal places. What that is saying is
Lat: Northern hemisphere, 42.23 degrees
Long: Western hemisphere, 71.47 degrees

That format is just leaving the minutes and second blank, and specifying the degrees with two decimal places. This is known as decimal format (but usually with more decimal places than just two).

You could also have:
42 18.23 -71 38.22

which is
Lat: Northern hemisphere, 42 degrees, 18.23 minutes, and leaving the seconds blank
Long: Western hemisphere, 71 degrees, 38.22 minutes, and leaving the seconds blank

Google Maps and GPS Devices

Most of the web pages including Google use a decimal format. They don’t specify minutes at all, but just degrees such as 42.237855 -71.473617. Most GPS devices let you choose the format you want to use to input coordinates.

Converting Between Formats

It’s not hard to convert from one format to another, knowing that there are 60 minutes in a degree and 60 seconds in a minute.

So for example,
42 18 means 42 degrees, 18 minutes. Suppose you wanted to convert this to just degrees.

18 minutes is how many degrees?
18/60 = .3

So 42 18 is the same as 42.3 degrees.

Useful Tools
This webpage will quickly convert between the decimal version of the coordinate and the degree, min, and seconds format:
http://andrew.hedges.name/experiments/convert_lat_long/

This webpage will let you put in coordinates and then will show you where it is on a map, or click on the map and it will give you the coordinates. You can also use it to convert since you can put in DDD MM SS and it will show you the equivalent decimal format:
Latitude and Longitude of a Point.

I found this page to be extremely helpful for one box. This webpage has multiple tools.
http://www.movable-type.co.uk/scripts/latlong.html
You can put in two coordinates and get the distance between them, put in one coordinate, a bearing, and a distance and it will show you the resulting coordinate, and the intersection of two paths given the starting points and bearings. Which box did I use this tool for? Well, I could tell you, but then I might have to have you…disappeared.

Datums

A quick word about this. You know the imaginary latitude and longitude lines? Well different maps show these in different ways, which result in different coordinates for the same point, depending on which map you are using. These maps are called Datums.

Most web pages including Google Maps and GPS devices use the datum WGS84. It stands for World Geodetic System 1984 because it came out in 1984. These datums are updated every 30 years or so, due to changing tectonic plates in the Earth’s crust. So land masses actually move and they have to update their maps.

Don’t worry much about this, other than to make sure your GPS device is set to match the datum you use, if you have that setting.