# Perimetric coordinate system : Eccentricity and Half-meridian

The PTVR **perimetric** coordinate system is a **spherical** coordinate system that is used extensively by vision scientists.

In this coordinate system, the **3D position of a point P** is defined by 3 coordinates:

– The **Half-meridian **(from 0° to 360°).

– The **Eccentricity** (from 0° to 180°).

– The **Radial Distance** (in meters) between Origin and P.

The 3 coordinates of point P in Figure 1 are:

Half-meridian: 135°

Eccentricity: 20°

Radial distance: any distance in meters.

A horizontal rotation (i.e. about the Y axis) of the **perimetric** coordinate system shown in figure 1 is shown in Figure 2. This rotation aims at providing a better mental image of 3D space.

# Flat Tangent Screen