Literally, triangulation signal.
Triangulation is a method of measuring based on trigonometry theory in mathematics.
It is used for calculating distance that cannot be measured directly, for instance, the distance across a wide river.
In the KENJI's days, many sankaku-hyo towers were built in order to measure geographical distance of land in Japan.
They were tall wooden towers with signals (lights) on the tops, which can be easily marked from a distance.

You see many sankaku-hyos in KAGAYA's paintings.
Lights of sankaku-hyos are painted for the real stars which are larger or brighter in the Milky Way.
The constellations of the lights are drawn just as they really are.
And brightness of the lights are painted just as they really are in terms of magnitude scale of 1~6 in astronomy, while towers are variously designed according to respective magnitude of light.
Materials other than sankaku-hyo such as signals, flowers, grasses, stones, or even sands in the picture are drawn, in accordance with their respective brightness, to signify innumerable smaller or farther stars in the Milky Way.

You may well be aware that triangulation lights (stars) are really used in measuring distance of the stars in astronomy.

SANKAKU-HYO restored by The Geographical Survey Institute