The latitude/longitude system

Longitude lines (example in blue at right) run north-south and meet at the North and South Poles; they are also called meridians. Latitude lines (example in red at right) run east-west and don't meet. For this reason they are also called parallels.
Longitude divides the Earth's surface into 360 thin slivers of one degree each. Latitude divides the Earth's surface into 180 thin rings of one degree each. These divisions and their numbering are easier to see if you map the Earth as a flat sheet (below) instead of a sphere. In the sphere to the right and in the flat map below, the lat/lon gridlines are drawn five degrees apart.



The 360 one-degree longitude lines are numbered in two 180-degree lots running east and west from the 0° line of longitude (also called the prime meridian). The 0° line runs through Greenwich, England. Halfway around the globe (in the mid-Pacific) is the longitude line which is both 180° east and 180° west of Greenwich. The main island of Tasmania lies between 143°E and 149°E longitude.

The 180 one-degree latitude lines are numbered in two 90-degree lots running north and south from the 0° line of latitude, which is the Equator. The North Pole is 90°N and the South Pole is 90°S. The main island of Tasmania lies between 40°S and 44°S latitude.

Each degree of latitude and longitude is divided into 60 minutes, and each minute is divided into 60 seconds. Both latitude and longitude are usually written as degrees (symbolised ° ), minutes ( ' ) and seconds ( " ). In the lat/lon system, a location is usually specified by giving its latitude first, then its longitude. For example, the Queen Victoria Museum's art gallery at Royal Park in Launceston is at 41°26'16"S 147°08'01"E.

It's important to remember that even though the lat/lon grid on the flat map looks like a grid of squares, it isn't that way on the real Earth (see the sphere above). The distance between one-degree lines of longitude depends on where you are. It's largest at the Equator and smallest at the Poles.

Lat/lon formats

41° 26' 16"S is in degree-minute-second format, usually abbreviated DMS. Three other formats are in common use for lat/lon:

Decimal degrees, or DD. This format gives degrees latitude (or longitude) as a simple decimal. For example, 42.5000°S is 42 and a half degrees, which is the same as 42° 30' 00"S.

Decimal minutes, or DDDMM.mm. This format gives whole degrees and decimal minutes. For example, 42° 24.33'S is 42 degrees, 24 and one-third minutes, which is the same as 42° 24' 20"S.

Decimal seconds, or DDDMMSS.s. This format gives whole degrees, whole minutes and decimal seconds. For example, 42° 15' 31.7"S is 42 degrees, 15 minutes and 31 and seven-tenths seconds. In DMS format this could be rounded up to 42° 15' 32"S.

Converting from one lat/lon format to another is straightforward arithmetic:

  • DDD° MM' SS" (DMS)
    = (DDD + MM/60 + SS/3600)° (to get decimal degrees)
    = DDD° (MM + SS/60)' (to get decimal minutes)
  • DDD.dddd° (decimal degrees)
    = DDD° (60 x 0.dddd)' (to get decimal minutes)
  • DDD° MM.mm' (decimal minutes)
    = DDD° MM' (60 x 0.mm)" (to get decimal seconds)