A method of constructing a three-dimensional model of an internal surface of a tubular structure comprises obtaining image data from an area of the internal surface of the structure, obtaining measured height profile data from the internal surface in a plurality of sub-regions of the area, for example using a multi-finger caliper tool, determining image properties from the image data, correlating the measured height profile data with the image properties in the sub-regions, and constructing expected height profile data for at least part of the area outside the sub-regions using the correlated measured height profile data and image properties.