An image processing apparatus comprises processing circuitry configured to: obtain a respective identity and location for each of a first plurality of landmarks in a reference volume; obtain a respective weighting for each landmark of the first plurality of landmarks; obtain a respective identity and location for each of a second plurality of landmarks in a further volume; and determine a transform based on a fit between the first plurality of landmarks and the second plurality of landmarks, wherein the fit is determined in dependence on the weightings.