Latitude and Longitude Formats

What do Lat Longs Really Represent ?

Latitude and Longitude can be represented in many different ways, and although all they amount to are two numbers that each describe an angle there are many different ways to write those numbers down. For example

50°30'30"	[50 Degrees 30 Minutes and 30 Seconds]	{DDDMMSS}
50°30.50'	[50 Degrees 30.5 Minutes]	{DDDMM.MM}
50.5083°	[50.5083 Degrees]	{DDD.DDDD}

All describe the each same angle with a different presention. In each example the degree part does not change, it is the fractional part of the angle which causes the confusion due to different ways it can be written.

Concepts

A number of key concepts can sort out what is meant by which format.

• Decimals are as always fractional parts divided by multiples of 10.
• Seconds are 60ths of Minutes (just like in time) and Minutes are 60ths of Degrees. A simple example 30 / 60 is a half (0.5 in decimal format)
• Without revisiiting the spherical geometry accept that 1 degree is 60NM along a Meridian of Longitude (in a North / South direction), which implies that 1 minute is 1NM and therefore a latitude specified to 2 decimal places of minutes is accurate to within 60 feet, if specified to 3 decimal place of minutes its accurate to 6 feet.

It may be obvious that the more places there are to the right of the Degrees in any particular format the greater the accuracy of the position described.

Where the formats are used

The confusion seems to be made worse when a user gets into the mindset of a particular GPS, all the manufacturers have differing ways of entering Latitude and Longitude and moving from one make to another without being aware of the different presentations often leads to the user trying to enter his favourite format as a sequence of numbers in the available space during data entry regardless of whether its the the right one or not. The consequences of putting in the wrong position information when navigating VMC can be embarassing at best, when navigating IMC they could prove fatal.

Format	Used in	Potential Accuracy (of the data not the unit)
DDDMMSS.S	Garmin GNS 430	10 feet
DDDMM.MM	Skyforce SkyMap IIIc, Bendix King KMD 150, Av8or, Garmin GNC 250, Garmin CNX 80	60 feet
DDDMM.MMM	Trimble 2101 I/O Plus, GNS 430, Lowrance Global Map 100	6 feet
DDD.DDDD	Most website URLs	36 feet

Manufacturers seem to pick a format and stick with it, so even if your unit is not listed above if you can find the manufacturer it may give you an idea of the format used. Some units like the Garmin GNS series can accept Lat/Longs in more than one format, in the case of the GNS 430 this is changed in the Position Format / Map Datum Screen in setup.

Summary

• If writing down Lat Longs make sure you are clear about the format you are using by writing the symbols for degrees and minutes (if specified) and any decimal points.
• Never ever use the same delimiter between different parts of a Lat Long it makes it impossible for someone else to know what you really meant.
  Some examples I've seen are 50:30:25 or 50.34.45 but what do they really mean as they cannot be reliably decoded ?
• Always write the degree portion of longitudes with three digits and latitudes with two.