1. A surgical guidance system comprising: a storage medium for storing pre-operative image data associated with a patient; an integrated surface topology imaging and optical position measurement device comprising: an optical projection device for projecting optical radiation onto an exposed surface of the patient, such that backscattered optical radiation is suitable for optical surface topology detection; an optical source having a wavelength selected to illuminate a set of fiducial markers provided on a movable instrument; two or more cameras, wherein at least one of said two or more cameras is configured for imaging the backscattered optical radiation, and wherein at least two of said two or more cameras are configured for imaging the set of fiducial markers when illuminated; and a surgical guidance controller operatively connected to said integrated surface topology imaging and optical position measurement device and said storage medium, wherein said surgical guidance controller includes a processor configured to: control said optical projection device to illuminate the exposed surface and obtain, from said at least one camera, topological image data associated with the exposed surface; and control said optical source to illuminate the set of fiducial markers and obtain, from said two or more cameras, positional image data associated with said set of fiducial markers; determine a position and an orientation of said movable instrument relative to said exposed surface; and register said topological image data, and said position and orientation of said movable instrument to said pre-operative image data; wherein said optical projection device, said optical source, and said two or more cameras are rigidly mounted on a frame, thereby maintaining a fixed calibration of said system without requiring inter-operative recalibration.