1. A drillhole centerline determining interposer, for use in robot probe locating and orienting of drillhole axes with respect to the coordinates of a master part, comprising: (a) a planarization platform having a continuous platform surface defining a platform plane, with a central physical feature with a characteristic shape having an included point derivable from the characteristic shape; and (b) mounting means to affix said planarization platform to the master part with said planarization platform surface orthogonal to a desired drillhole axis and with the included point lying on the drillhole axis.