An apparatus for modelling a shaft in 3D, comprising: - a sensor head adapted to be axially moved within said shaft by means of a suspension system , said sensor head comprising: ∘ 3D sensors placed along the circumference of said sensor head , adapted to take images along an inner circumference of said shaft , each of said images comprising depth information about the part of the inner surface of said shaft within the field of view of the respective 3D sensor ; ∘ a measuring apparatus adapted to determine a measured height position of said sensor head within said shaft ; - a processing unit comprising: ∘ a placement module configured to place said images in a virtual space, based on said measured height position and the positioning of said image sensors on said sensor head , resulting in a rough placement of said images , thereby obtaining a first reconstruction of the inner surface of said shaft ; ∘ a correction module configured to correct said rough placement , based on comparing overlapping images and/or based on a measured deviation of said sensor head with respect to a central axis of said shaft , resulting in a 3D model of said shaft . - a sensor head adapted to be axially moved within said shaft by means of a suspension system , said sensor head comprising: ∘ 3D sensors placed along the circumference of said sensor head , adapted to take images along an inner circumference of said shaft , each of said images comprising depth information about the part of the inner surface of said shaft within the field of view of the respective 3D sensor ; ∘ a measuring apparatus adapted to determine a measured height position of said sensor head within said shaft ; ∘ 3D sensors placed along the circumference of said sensor head , adapted to take images along an inner circumference of said shaft , each of said images comprising depth information about the part of the inner surface of said shaft within the field of view of the respective 3D sensor ; ∘ a measuring apparatus adapted to determine a measured height position of said sensor head within said shaft ; - a processing unit comprising: ∘ a placement module configured to place said images in a virtual space, based on said measured height position and the positioning of said image sensors on said sensor head , resulting in a rough placement of said images , thereby obtaining a first reconstruction of the inner surface of said shaft ; ∘ a correction module configured to correct said rough placement , based on comparing overlapping images and/or based on a measured deviation of said sensor head with respect to a central axis of said shaft , resulting in a 3D model of said shaft . ∘ a placement module configured to place said images in a virtual space, based on said measured height position and the positioning of said image sensors on said sensor head , resulting in a rough placement of said images , thereby obtaining a first reconstruction of the inner surface of said shaft ; ∘ a correction module configured to correct said rough placement , based on comparing overlapping images and/or based on a measured deviation of said sensor head with respect to a central axis of said shaft , resulting in a 3D model of said shaft . <EOS>