Method for detecting a line above ground from a helicopter during landing of the helicopter in a terrain unfamiliar to the pilot, wherein the method is comprising the steps of: - providing a first image from a first viewpoint and a second image from a second viewpoint by means of a monocular camera, wherein the camera is mounted to the helicopter and directed towards the ground below the helicopter; and wherein the helicopter is in motion between the first viewpoint and the second viewpoint and wherein the first image and the second image has an overlapping region; - performing an image registration process resulting in a mapping of image points (qA) in the first image to their corresponding image points (qB) in the second image; - determining a common reference system; - determining a ground plane in the common reference system, wherein the ground plane is a planar approximation of the ground; - determining a position and an orientation of the viewpoints (VPA, VPB) in the common reference system; - using the results of the image registration to calculate a difference image comprising all image points of the overlapping region and the difference in pixel values between the corresponding image points (qA, qB) at these image points of the overlapping region; - identifying any positive lines (Lpos) in the difference image having positive difference values; - identifying any negative lines (Lneg) in the difference image having negative difference values; - identifying the originating image (A, B) for each positive line (Lpos) and each negative line (Lneg); - identifying any compliant line pair as two lines where one originate from the positive lines (Lpos) and the other originate from the negative lines (Lneg) and where one originates from the first image and the other originate from the second image; - reconstructing a 3D line (L3D) in the common reference system from each compliant line pair based on triangulation from the position and the orientation of the viewpoints (VPA, VPB); - determining that a line above ground is present if the reconstructed line (L3D) is compliant with geometric constraints formed by the viewpoints (VPA, VPB) and the ground plane. - providing a first image from a first viewpoint and a second image from a second viewpoint by means of a monocular camera, wherein the camera is mounted to the helicopter and directed towards the ground below the helicopter; and wherein the helicopter is in motion between the first viewpoint and the second viewpoint and wherein the first image and the second image has an overlapping region; - performing an image registration process resulting in a mapping of image points (qA) in the first image to their corresponding image points (qB) in the second image; - determining a common reference system; - determining a ground plane in the common reference system, wherein the ground plane is a planar approximation of the ground; - determining a position and an orientation of the viewpoints (VPA, VPB) in the common reference system; - using the results of the image registration to calculate a difference image comprising all image points of the overlapping region and the difference in pixel values between the corresponding image points (qA, qB) at these image points of the overlapping region; - identifying any positive lines (Lpos) in the difference image having positive difference values; - identifying any negative lines (Lneg) in the difference image having negative difference values; - identifying the originating image (A, B) for each positive line (Lpos) and each negative line (Lneg); - identifying any compliant line pair as two lines where one originate from the positive lines (Lpos) and the other originate from the negative lines (Lneg) and where one originates from the first image and the other originate from the second image; - reconstructing a 3D line (L3D) in the common reference system from each compliant line pair based on triangulation from the position and the orientation of the viewpoints (VPA, VPB); - determining that a line above ground is present if the reconstructed line (L3D) is compliant with geometric constraints formed by the viewpoints (VPA, VPB) and the ground plane.