GPS coordinates are formed by two components that are its latitude and longitude.
The north-south position of a point is given by its latitude when its longitude gives information about its east-west position.
The latitude of a point is the measurement of the angle formed by the equatorial plane with the line connecting this point to the center of the Earth.
By construction, it is comprised between -90 ° and 90 °. Negative values are for the southern hemisphere locations, and latitude is worth 0 ° at the equator.
The principle is the same for the longitude, with the difference that there is no natural reference like the equator for the latitude. Longitude reference has been arbitrarily set at the Greenwich Meridian (it passes through the Royal Greenwich Observatory in Greenwich in the suburbs of London), and the longitude of a point is the measurement of the angle formed by the half plane formed by the axis of the earth and passes through the Greenwich meridian and the half-plane formed by the axis of the earth and passing through the point.
Careful readers would have alreay noticed that a third element is required to locate a point, its altitude. In the most typical use cases, GPS coordinates are needed for locations on the surface of the Earth, making this third parameter less important. However, it is as necessary as the latitude and longitude to define a complete and accurate GPS location.
As we saw, the above definitions take into account several parameters that must be fixed or identified for future reference:
- the equator plane and the model choser for the shape of the earth,
- a set of reference points,
- the position of the center of the Earth,
- the earth axis,
- the reference meridian.
These five criteria are the basis of the different geodetic systems used through history.
Currently, the most commonly used geodetic system is WGS 84 (used notably for GPS coordinates).
The two main units of measurement are the decimal and sexagesimal coordinates.
The latitude and longitude are a decimal number, with the following characteristics:
- latitude between 0° and 90°: Northern hemisphere,
- latitude between 0° and -90°: Southern hemisphere,
- longitude between 0° and 180°: East of the Greenwich meridian,
- longitude between 0° and -180°: West of the Greenwich meridian,
The sexagesimal coordinates have three components: degrees, minutes and seconds.
Each of these components is usually an integer, but the seconds can be a decimal number in case of need of a greater precision.
One angle degree includes 60 angle minutes and one angule minute consists of 60 angle seconds of arc.
Unlike decimal coordinates the sexagesimal coordinates can not be negative. In their case, the letter W or E is added to the longitude to specify the position east-west from the Greenwich meridian, and the letter N or S to the latitude to designate the hemisphere (North or South).
|Decimal coordinates||Sexagesimal coordinates|
|0° to 90°||0° to 180°||N||E|
|0° to 90°||0° to -180°||N||W|
|0° to -90°||0° to 180°||S||E|
|0° to -90°||0° to -180°||S||W|
On the website homepage, when you enter coordinates in one of the formats (on the left column), they are automatically converted to the other format. Also, when you visualize an address on the map, or after clicking on a point on the map, its coordinates in the two units are displayed in the left column.