A method for performing a stereotactic brain surgery. The method includes first interposing a left and a right anthropological baselines on a patient's face, each trago-orbital anthropological baseline connecting an orbitale point and an auricular point on a corresponding side of the patient's face. A headgear unit having a pair of temple elements and a front housing, where the front housing includes at least two cameras and a 3D compass, is then positioned on a patient's head. Each of the two cameras is adjusted such that its view field picks up one of the two trago-orbital anthropological baselines on the patient's face. These view fields of the two cameras are then used to construct a Human Skull Base Plane as a plane formed by the left and right anthropological baselines when these anthropological baselines are parallel and aligned. The same view fields of the two cameras are also used to identify a precise center of the patient's face, and a center of the front housing is then positioned in alignment with the center of the patient's face. The 3D compass is then positioned into a measuring plane coinciding with the Human Skull Base Plane, and is used to quantitatively measure a three-dimensional orientation of the patient's head at the time when the 3D compass is positioned within the measuring plane coinciding with the constructed Human Skull Base Plane. An image of a patient's brain is then obtained and stored in correlation with the measured three-dimensional orientation of the patient's head. Prior to the surgery, the patient's head is positioned into the same position in which the image was taken. A stereotactic brain surgery is then performed using the stored image.