Positions are always specified using coordinates. You may often need to use coordinates when you enter positions, and you will always be given coordinates when you query a position.
Each two dimensional coordinate is made up of two numbers, called X and Y, and each three dimensional coordinate is made up of three numbers, called X, Y and Z. You can always use either 2D or 3D coordinates in SIS. If you use 2D coordinates SIS will assume the Z value is zero.
The interpretation of the X, Y and Z numbers depends on the coordinate system.
Cartesian coordinates have the important property that the X, Y and Z-axes are straight and at 90° to each other at all places (i.e. orthogonal). This means that items can be easily and accurately measured for length, area and volume. An example of a Cartesian coordinate system is Geocentric. This is centered at the centre of the globe, with its X and Y-axes pointing towards the equator and its Z-axis pointing towards the North pole.
Latitude/Longitude coordinates are not Cartesian, as they are based on lines of longitude and latitude, which are curved.
Some mapping coordinate reference systems give rise to pseudo-orthogonal coordinate systems. Their X and Y-axes are approximately 90° to each other (and to the Z axis at all points near the coordinate reference system’s origin). They are not truly orthogonal since their surfaces are arranged to match the surface of the Earth. Examples are Universal Transverse Mercator, the Transverse Mercator for GB and Ireland, and the Transverse Mercator (19 pseudo-orthogonal) coordinate reference systems products.
In mapping, spherical (X, Y, Z) coordinates are often used for Longitude, Latitude, and height above sea level.
In SIS, longitude is normally measured Eastwards from the Greenwich Meridian, and latitude is measured Northwards from the equator. Negative values mean angles West and South respectively.
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