These are derived from an ellipsoid and the origin of the coordinates is the centre of the ellipsoid.
They are usually referenced to the ‘Greenwich’ meridian that runs through the Greenwich observatory just east of London in the UK. Meridians increase from 0° at Greenwich to 180° east and west of Greenwich. The ‘International Date Line’ runs through the Pacific and is nominally at 180° east or west of Greenwich. However, different island groups in the Pacific decide to be one side or the other of the Date Line, and the line is drawn at various longitudes to defer to national boundaries. On older maps the ‘0’° meridian is not always Greenwich. There are several other reference meridians, mainly in Europe. A list of these can be found in the EPSG parameter database.
In order to facilitate loading of data in some software applications, the convention is that North and East are ‘positive’ and South and West are negative. However the reader should beware that local applications that do not apply outside their ‘quadrant’, may not obey this convention.
Projection coordinates are usually called eastings and northings. Sometimes they are referred to as ‘x’ and ‘y’. However, this can be confusing as in about 50% of the world, easting is represented by ‘y’ and northing by ‘x’. Caution is advised!
More information about how these two types of coordinate system relate will be discussed in other chapters.
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