As a lover of all things esoteric [as well as being of a mathematical bent] I have always been interested in the intersection between the spiritual and the geographical. Particularly as this relates to actual artifacts placed in the world and the abstract representation of the physical world [the map].
Examples might be omphalos / trig markers / boundary stones / the locations where four [or more] states/countries meet / ley lines / etc
Though Earth Rod covers in pavements mark nothing more mysterious than the location of an earthing / grounding pole for electrical discharge, I admit the first time I saw one I was forced to imagine it as the site of a missing pole for keeping the firmament up, akin to Piero Manzoni’s 1961 sculpture SOCLE DU MONDE, socle magic n. 3 de Piero Manzoni, homage a Galileo, which is supporting the Earth.
I then happened across other instances of Earth Rod markers from time to time and so came to see them as a more mundane, but still secret, network of tent pole base plates for the erection of huge marquees to completely enclose whole neighbourhoods given undisclosed emergency criteria.
Whatever my fanciful imaginings of supposed purposes, I remain convinced that a mapping of Earth Rod locations must surely reveal some wonderfully elaborate net of points mirroring the carved knotted nets on ancient omphaloi.