# What are Coordinates?

Physical positions on a map are always denoted using **coordinates**.

Each two dimensional coordinate is made up of two numbers X and Y. Each three-dimensional coordinate is made of of three numbers X, Y and Z.

You can use either 2D or 3D coordinates in Cadcorp SIS Desktop. If you use 2D coordinates Cadcorp SIS Desktop will assume the Z value is zero.

**Note: **The interpretation of the X, Y and Z numbers depends on the coordinate system.

## Cartesian Coordinates

In Cartesian coordinates, the X, Y and Z-axes are straight and at 90° to each other at all places (i.e. orthogonal). This means items can be easily and accurately measured for length, area and volume.

An example of a Cartesian coordinate system is Geocentric. This is centered in the middle of the globe with its X and Y axes pointing towards the equator and its Z-axis pointing towards the North pole.

## Spherical Coordinates

In mapping, spherical (X, Y, Z) coordinates are often used for Longitude, Latitude, and height above sea level.

In Cadcorp SIS Desktop, 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.