Cross section

Or set A and an azimuth and length, which is how a section is usually planned:

Ground heights come from the public-domain terrarium tiles, and they cache like map tiles — build the section once with signal and it redraws for ever without.

A bed cut obliquely looks shallower than it is. What you draw on a section is the apparent dip: tan(apparent) = tan(true) × cos(angle between the section and the dip direction).

A contact crops out somewhere along the line and dips away. This carries it down at its apparent dip and tells you how deep it is at any other point.

The last line is the transposition back to the map: if the horizon has not been eroded away, that is where to walk to find it. A drawn prediction is only as good as the assumption that the bed stays planar between the two points.

The oldest trick in structural geology. Three places where the same contact reaches the ground, and the plane through them is fixed — no compass needed at all.

Points that are nearly in a line give a badly conditioned answer — spread them out, and get as much height difference between them as the ground allows.