Secondary schools

Latitude and Longitude coordinates explained

Locating a point on the Earth is easily calculated with the aid of coordinates. The Earth can be represented as a sphere with lines running east to west (latitude) and others running perpendicular to them in a north south direction (longitude). These are more formally known as Geographic Co-ordinates. Latitude lines are parallel to the Equator while the longitude lines are perpendicular to the Equator. These are the coordinates that pilots and boat captains use (

The location of a point on Earth e.g. OSi, Co. Dublin can be expressed in terms of angles from the centre of the Earth.

The method of calculating latitude and longitude coordinates.

To locate the latitude coordinate of OSi (P in Diagram 1) an angle needs to be measured between two lines. The first line would be from the centre of the sphere to the equator (1) and the other (2) would be from that line to OSi (P).

The longitude of the coordinate can be calculated by measuring the angle between the first line created (1) and a third line which is measured from the centre of the sphere to the Greenwich Meridian (3). If the point of interest was on the equator and the Greenwich Meridian the coordinate would be 0,0 degrees. The location of OSi is 53° 21’ 52” latitude and -6° 20’ 52” longitude.


More about mapsIn this diagram the blue lines travel toward the East on the x axis

More about maps

The diagram above illustrates the Northing increasing in number as they travel toward the north on the y-axis.

The Grid

More about mapsTo take a reading off of this grid it is important to remember that the eastings are found first and then the reading for the northing. For this example above the green dot is located at 4.5 degrees east and 2.5 degrees north of the origin. The origin is the starting point i.e. 0,0.

More about maps

For a lot more on learning about maps, download So Where are You? – Mapping Ireland and Databases – Changing the World” from Science and Technology in Action. In addition, visit the Science and Technology in Action website for a great innovative and learning resource bringing industry and education together.