# CRS

**CRS** sets up the current **Coordinate Reference System**, the current **Viewing Projection** and the current **Units**.The Coordinate Reference System is used for measuring positions and storing graphics in datasets.

Select **Home > Map > CRS.**

Alternatively right-click inside the Map Window and select this option.

The right-click context menu option is only available when no overlays are present.

## Coordinate Reference System

**Name:** Select the required **Coordinate Reference System** from the drop-down box. This will change the meaning of typed coordinates and may affect measurements.

**The Viewing Projection will be:** This displays the viewing projection in accordance with the **Coordinate Reference System** chosen above

There are two different types of coordinate reference systems - Cartesian and Spherical.

- The X, Y and Z axes of a Cartesian coordinate reference system are orthogonal to each other and the units are linear (metres, miles, etc.).
- The X and Y axes of a Spherical coordinate reference system are angles of latitude and longitude. The Z axis of a spherical coordinate reference system measures height above sea-level.

**Viewing Projection**

The **Viewing Projection** is used to present graphics on screen.

**Note: **SIS Desktop displays the **Viewing Projection** in accordance with the chosen **Coordinate Reference System**.

**Name**

Select the required **Viewing Projection** from the drop-down box. The viewing projection determines how the curved surface of the Earth is mapped onto the screen.

For simplicity SIS Desktop keeps the **Coordinate Reference System** and the **Viewing Projection** compatible with each other. If you change one, SIS Desktop automatically changes the other.

**Units**

Determines how coordinates are displayed in the Status Bar and how they are displayed when you use the Measure commands, e.g. Measure Distance.