Features such as holes in a component which may be an aerofoil component of a gas turbine engine are re-drilled after initial formation, for example to remove blockages created in a coating process. In order to ensure that re-drilling occurs at the precise location of the initial holes, a selected number of the holes are selected, and their exact positions identified, for example by camera imaging and image processing. The actual positions of the selected features are compared with the nominal positions, and an algorithm is generated and then used to calculate the actual positions of the non-selected holes of the array.