A blockhead's location can be determined using several methods.
Derived from World Location Guide by Fonceai.
A player can determine their location by the direction of the movement of the sun, moon, or stars over the course of the day. For the purposes of these charts, the player is always at the "top" of the world. The more vertical the sun/moon/stars move, the closer the player is to an equator. The more horizontal they move, the closer the player is to a pole.
Now add the poles:
Near the north pole, celestial objects will move from left to right.
Near the south pole, celestial objects will move from right to left.
Near the east equator, celestial objects will move upward, from the bottom of the screen to the top.
Near the west equator, celestial objects will move downward, from the top of the screen to the bottom.
For example, between the north pole and the east equator, celestial objects move both upwards and towards the right.
Provided the area above where a blockhead is underground has been explored, it is possible to pan the camera to the surface and figure out the blockhead's location there. The altitude can be figured out by zooming out to map mode and comparing the blockhead's location to the surface level or the water level.