Reference plan: Perpendicular plan in O to the astronomical vertical of the place. The azimuth is the angle A = (OS, OE') measured from 0° to 360° or from -180° to 180° in retrograde sense. The elevation (or altitude) is angle (OE, OE '), positive if the star is above the horizon, or negative otherwise.
Note: Azimuth is also computed as angle (ON, OE').