Grid azimuth is the same as geodetic azimuth only when the point of observation falls on the _______ meridian.

Grid azimuth and geodetic azimuth represent angles measured in different frames of reference. The grid azimuth is measured relative to the grid north on a map projection, while geodetic azimuth is measured relative to true north, which is aligned with the Earth's meridian.

The point of observation must be on the central meridian of the projection for the two types of azimuth to be the same. This is because the central meridian is the line where distortion is minimized in a map projection, allowing the grid north to align closely with true north. Consequently, at this meridian, the angular differences between grid and true north are effectively zero, which is the condition for the equality of grid azimuth and geodetic azimuth.

Other options such as the observer's meridian, Greenwich meridian, or prime meridian do not provide the same alignment of grid north and true north, especially since the observer's meridian can vary by location and the Greenwich and prime meridians have specific geographic significance that does not inherently relate to the characteristics of a specific grid system. Therefore, the central meridian is indeed the key factor that allows grid azimuth to equal geodetic azimuth.