A method of determining the roll angle of a camera mounted on a host vehicle includes: a) defining Cartesian axes in the real world and corresponding axes in the camera imager; b) from an image taken from the camera, determining the co-ordinates of the two points (P1, P2) of the real world, in the imager of the camera ((x I,1, y I,1), (x I,2, y I,2)); c) determining the difference (D) in the coordinates of the points in a horizontal World plane in one axis (Xw,1−Xw,2), using corresponding co-ordinate system as in step b); and d) determining the roll angle ρ of a camera located on a host vehicle from the following formula ρ=[D/h−(c2/a2−c1/a1)]/[(a2d2−b2c2)/a22−(a1d1−b1c1)/a12], where a1 and a2 are found from the following, where i=1,2 represents the two points (P1, P2): ai=sin θ*f+cos θ*yI,i; bi=−cos θ*xI,i ci=−cos η cos θ*f+sin η*xI,i+cos η sin θ*yI,I di=−cos η sin θ*xI,i+sin η*yI,i where for θ is the known value of fixed tilt angle, η is the known value of fixed pan angle and h is the known fixed camera height h; and xI,i, yI,i are the respective co-ordinates of two of the points in the imager.