This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
垙 combines 土 (earth) with 光 (light), but the exact historical development and original meaning are uncertain.
The left side is earth 土 and the right side is light 光. Picture a patch of earth glowing with light, but its meaning remains unclear.
For コウ, imagine a glowing 光 (こう) patch of earth: 光 -> コウ, and the earth 土 lights up.