A method comprising: providing three-dimensional surface data for a subject; acquiring an image set for a next three-dimensional image of the subject; when the next three-dimentional image derived from the image set cannot be fitted to the three-dimensional surface data, entering a landing mode comprising the steps of: superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the current two-dimensional image representing a view of the subject from a position from which the image set was acquired, and the previous two-dimensional image representing a view of the subject from which at least a portion of the three-dimensional surface data was acquired; acquiring at least one subsequent three-dimensional image; fitting the at least one subsequent three-dimensional image to a second three-dimensional surface reconstruction; test fitting the at least one subsequent three-dimensional image to the three-dimensional surface data; and when the next three-dimensional image derived from the image set can be fitted to the three-dimensional surface data, superimposing the three-dimensional surface data and the next three-dimensional image onto a two-dimensional image of the subject in a display and adding a second three-dimensional surface reconstruction to the three-dimensional surface data. providing three-dimensional surface data for a subject; acquiring an image set for a next three-dimensional image of the subject; when the next three-dimentional image derived from the image set cannot be fitted to the three-dimensional surface data, entering a landing mode comprising the steps of: superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the current two-dimensional image representing a view of the subject from a position from which the image set was acquired, and the previous two-dimensional image representing a view of the subject from which at least a portion of the three-dimensional surface data was acquired; acquiring at least one subsequent three-dimensional image; fitting the at least one subsequent three-dimensional image to a second three-dimensional surface reconstruction; test fitting the at least one subsequent three-dimensional image to the three-dimensional surface data; and superimposing a current two-dimensional image of the subject onto a previous two-dimensional image of the subject in the display, the current two-dimensional image representing a view of the subject from a position from which the image set was acquired, and the previous two-dimensional image representing a view of the subject from which at least a portion of the three-dimensional surface data was acquired; acquiring at least one subsequent three-dimensional image; fitting the at least one subsequent three-dimensional image to a second three-dimensional surface reconstruction; test fitting the at least one subsequent three-dimensional image to the three-dimensional surface data; and when the next three-dimensional image derived from the image set can be fitted to the three-dimensional surface data, superimposing the three-dimensional surface data and the next three-dimensional image onto a two-dimensional image of the subject in a display and adding a second three-dimensional surface reconstruction to the three-dimensional surface data.