A method is provided of combining first and second models of a common region of space, such as a region of the earth below the surface, to form a third model. Common points have different positions in the first and second models. A predetermined correlation exists between the positions of the common points in the first model and the positions of points of interest in the first model. The positions of the common points in the third model are derived from the common point positions in the first and second models. The positions in the third model of the points of interest are derived from the positions of the common points in the third model and from the predetermined correlation.