Final answer:
An angle is measured relative to a reference axis, which in two dimensions is usually one of the perpendicular Cartesian axes, most commonly the x-axis. In three-dimensional space, angles can be defined relative to a reference plane or via the rotation angle from the axis of an object's rotation. The azimuth angle is an example of an angle measured from a fixed direction, usually north.
Step-by-step explanation:
An angle is typically measured relative to a reference axis. When we discuss measuring angles in two dimensions, we measure the angle relative to one of the perpendicular axes, which are commonly the x-axis and y-axis in a Cartesian coordinate system. The horizontal line or the x-axis is the usual reference line from which angles are measured. The measure of an angle in a plane can vary depending on the context and the system being used. For instance, in polar coordinates, the angle is measured from the positive x-axis. Furthermore, in navigation or astronomy, the azimuth angle is often measured from the north direction in a horizontal plane.
While dealing with three-dimensional space, angles can also be measured relative to a reference plane. For instance, if one wants to define the angle between a line and a plane, one must refer to the angle which completes it to 90 degrees, namely, the angle between the given line and a line perpendicular to the plane. Thus, in surveying, the elevation angle measures how much a telescope is lifted above the horizontal, with positive values indicating upward angles and negative for downward angles.
When talking about the rotation of objects, such as a CD, the angle of rotation is measured relative to a radial line from the rotation axis to the object's edge. This rotation angle can be characterized by the arc length swept over time, compared to the radius of the circular path followed by the object.