A signal frame has a reference point that is nationally defined: <ul> <li>a lamp, typically the red lamp, or</li> <li>the centre of a set of lamps, or</li> <li>the edge of a frame nearest to train gauge.</li> </ul> This reference point has: <ul> <li>a spot location.</li> <li>an horizontal offset, e.g. 1,8 metres from the nearest rail.</li> <li>a vertical offset, e.g. 3,6 metres above the top of the highest rail.</li> </ul>