The angle in the plane of projection between a straight line and the central meridian of a rectangular coordinate system is called _______ azimuth.

The term used to describe the angle in the plane of projection between a straight line and the central meridian of a rectangular coordinate system is "grid azimuth." This designation is specific to coordinate systems that are projected onto a flat surface, such as in cartography and geographic information systems (GIS).

Grid azimuth accounts for the adjustments made due to the projection, allowing for accurate representation and navigation within the geometric framework of the map. When working with grid-based systems, the grid azimuth provides a straightforward means to measure angles concerning this projected coordinate framework.

In contrast to grid azimuth, geodetic azimuth measures the angle based on the true North direction, which corresponds to angular positions on the actual surface of the Earth without projection effects. Magnetic azimuth takes into account the magnetic North, which can vary significantly due to magnetic declination. Astronomic azimuth involves measurements taken in relation to astronomical observations, referencing celestial bodies and the true meridian.

Focusing on the context of rectangular coordinate systems and their use in navigation and mapping, grid azimuth is the appropriate term for angles defined in such projected planes.