What point on a UTM map does the declination diagram refer to? Is it the centre? Is it the edges? Is there a standard?
The central meridian on a UTM map points to true north. Are grid north and true north arrows identical only at this central meridian? If they are identical then what point on the UTM map does the grid north arrow on the declination diagram refer to?
If all grid lines on a UTM map point to true north (causing increased distortion with distance from the central meridian) then why is there a convergence angle (grid north to true north) on the declination diagram?
This answer concerns grid declination only (not magnetic declination).
For the map series that I'm familiar with (USGS), the declination diagram refers to the center of the map.
The meridian at the center of a map often is not the central meridian aka the defining meridian of a projected coordinate reference system (CRS) like a UTM zone.
A map may have several sets of reference lines drawn on it. A grid of UTM lines on a UTM-based map will form a set of rectangular or square blocks. The up-down are in the direction of grid north, or positive Y / northing values. They are not in the direction of true or geodetic north except for the central meridian of the zone or other projected CRS. The graticule shows the latitude and longitude lines. The longitude lines are in the direction of true north.
A colleague mentioned that he has seen maps with multiple magnetic diagrams but it was for a map in high latitudes.