1. A method for generating an augmented image, the method comprising the steps of: receiving combined visual image data representing a combined visual image captured with a digital camera, wherein the combined visual image data comprises: site visual image data representing a visual image of a site; and optical target visual image data representing a visual image of at least a portion of an optical target disposed on a reference surface of a position and orientation sensor unit; identifying, in the combined visual image data, the optical target visual image data; calculating, based at least in part on the optical target visual image data, a position and an orientation of the digital camera relative to the position and orientation sensor unit; receiving measurements of a position and an orientation of the position and orientation sensor unit relative to a site reference frame; calculating, based at least in part on the position and the orientation of the digital camera relative to the position and orientation sensor unit and based at least in part on the position and the orientation of the position and orientation sensor unit relative to the site reference frame, a position and an orientation of the digital camera relative to the site reference frame; generating, based at least in part on the position and the orientation of the digital camera relative to the site reference frame and based at least in part on a digital model of the site, at least one graphical representation selected from the group consisting of: a virtual image associated with the site; information associated with the site; and information associated with the virtual image; and displaying on a display the at least one graphical representation superimposed on a displayed visual image selected from the group consisting of: a displayed site visual image displaying at least a portion of the visual image of the site; and a displayed combined visual image displaying at least a portion of the visual image of the site and displaying at least a portion of the visual image of at least a portion of the optical target.