Apparatus and method for estimating orientation of camera relative to ground

A method of estimating the orientation of a camera (120, 320) relative to the ground. The method comprises: detecting a line segment from an image captured by a camera (120, 320); superposing a first virtual cube with three orthogonal vanishing points with random 3D directions on the image; classifying and grouping line segments of the image into 3D direction groups; and superimposing the second virtual cube on the image in the initial 3D direction. The optimal 3D direction of the second virtual cube is calculated by iteratively changing the 3D direction of the second virtual cube and measuring the vertical distances from the three orthogonal vanishing points to the three line segment groups in each iteration starting from the initial 3D direction, wherein the optimal 3D direction of the second virtual cube is the direction providing the shortest vertical distance; a covariance of an orthogonal vanishing point of the second virtual cube in the optimal direction is calculated; and a ground plane orientation is