This document discusses different types of meridians used in surveying: astronomical, magnetic, assumed, and true meridians. It describes how each is defined and how direction is measured in relation to the different meridians. Specifically, it explains that azimuth is the angle measured from the meridian plane to the plane containing the surveyed line, and can be a true, magnetic, assumed, or grid azimuth depending on the reference meridian used. The document also mentions that accurate azimuth determination requires observing stars or the sun to establish the true meridian orientation.