fiducial point

1. A point (or line) on a scale used for reference or comparison purposes.

In the calibration of meteorological thermometers, for example, the fiducial points are 100°C (212°F) and 0°C (32°F), which correspond to the boiling point and ice point of water at standard pressure (760 mm of mercury).

1. A reference point of known location on a map, graph, or image used to set the position for overlays of similar maps, graphs, or images.