Apparatus for determining a spatial property in a 3D-space, wherein the apparatus comprises a physical pointer, a camera and a position and orientation measurement system, wherein a first object in the 3D-space comprises a pattern on it, wherein the physical pointer is configured to contact a surface area of a second object in the 3D-space, wherein the camera is configured to capture, at a position of the camera with respect to the first object, image data comprising an image of the pattern; wherein the position and orientation measurement system is configured to determine a heading, position, verticality, attitude, and/or inclination of the camera based on the image of the pattern, and determine, based on the determined heading, position, verticality, attitude, and/or inclination of the camera and a relative position of the physical pointer with respect to the camera, the spatial property of the surface area by the physical pointer.