A drillhole centerline determining interposer having a central cavity enables a robot to probe the interposer to learn the location and orientation of the desired drillhole. The interposer is a mushroom shaped device with a planar head and a stem dimensioned to fit snugly in the hole to be drilled. The interposer, in addition to the planar head from which the perpendicular can be calculated through multiple probes, has a central pocket concentric to the interposer stem.The method of using the drillhole centerline determining interposer is to place a number of such interposers manually in position in holes in a master part located in the work envelope of the robot. The robot operator prepositions the probe at an initial position facing the cavity of the interposer for a drillhole locating sequence for the related drillhole. In sequence, the robot moves the probe from the initial position sufficient to clear the edge of the cavity and by multiple probes determines the plane of the interposer surface platform. The robot then adjusts the yaw and pitch of the probe to orient the probe orthogonal to the platform plane (a vector parallel to the desired drillhole centerline axis). The robot, with orientation vector stored, now locates the XYZ coordinates of the drillhole by cavity probing actions determining the epicenter of the pocket.With orientation vector (pitch and yaw) known, and with coordinates (XYZ) known, the drillhole is determined.