A method of determining in real time a position with six degrees of freedom of an offshore structure, for example an offshore jacket, while the structure is being placed at a predetermined installation position with six degrees of freedom on a seafloor or on an existing subsea infrastructure using a crane on a vessel, the position of the structure being determined in a fixed frame of reference, for example the Earth's reference frame, by observations of the structure from a moving frame of reference, the structure comprising a plurality of markers positioned on the structure such that they are visible from a viewing direction and provide position information of the structure with six degrees of freedom by viewing said markers, the method comprising: - initially surveying the structure, for instance on land, including determining the positions of the plurality of markers with respect to a reference frame of the structure and storing survey information as to the determined positions of the markers in relation to the geometry of the structure; - after surveying, and during lowering and placing the structure on said predetermined installation position by the crane: - viewing the structure with an imaging device from the moving reference frame, to generate image data representative of the relative marker positions of the markers in the moving reference frame; - determining an instantaneous position with six degrees of freedom of the imaging device in the fixed reference frame and generating imaging device position data synchronized with said image data, and - processing the synchronized image data and the imaging device position data to compute the position of the offshore structure in the fixed reference frame with six degrees of freedom by using said survey information as to the determined positions of the markers, as well as the marker positions of the markers as derived from the image data, and the imaging device position data. - initially surveying the structure, for instance on land, including determining the positions of the plurality of markers with respect to a reference frame of the structure and storing survey information as to the determined positions of the markers in relation to the geometry of the structure; - after surveying, and during lowering and placing the structure on said predetermined installation position by the crane: - viewing the structure with an imaging device from the moving reference frame, to generate image data representative of the relative marker positions of the markers in the moving reference frame; - determining an instantaneous position with six degrees of freedom of the imaging device in the fixed reference frame and generating imaging device position data synchronized with said image data, and - processing the synchronized image data and the imaging device position data to compute the position of the offshore structure in the fixed reference frame with six degrees of freedom by using said survey information as to the determined positions of the markers, as well as the marker positions of the markers as derived from the image data, and the imaging device position data. - viewing the structure with an imaging device from the moving reference frame, to generate image data representative of the relative marker positions of the markers in the moving reference frame; - determining an instantaneous position with six degrees of freedom of the imaging device in the fixed reference frame and generating imaging device position data synchronized with said image data, and - processing the synchronized image data and the imaging device position data to compute the position of the offshore structure in the fixed reference frame with six degrees of freedom by using said survey information as to the determined positions of the markers, as well as the marker positions of the markers as derived from the image data, and the imaging device position data.