As the sensor scans across each line, the distance from the sensor to the ground increases further away from the centre of the swath. Another distortion (B) occurs due to the rotation of the scanning optics. Again, the displacement increases, moving towards the edges of the swath. As the sensor scans across the swath, the top and side of objects are imaged and appear to lean away from the nadir point in each scan line. There is no displacement directly below the sensor, at nadir. They too exhibit relief displacement (A), similar to aerial photographs, but in only one direction parallel to the direction of scan. Images from across-track scanning systems exhibit two main types of geometric distortion.
Geometric variations between lines are caused by random variations in platform altitude and attitude along the direction of flight. The geometry of along-track scanner imagery is similar to that of an aerial photograph for each scan line as each detector essentially takes a "snapshot" of each ground resolution cell. If the objects are tall or are far away from the centre of the photo, the distortion and positional error will be larger. at the nadir) will have only their tops visible, while all other objects will appear to lean away from the centre of the photo such that their tops and sides are visible. Objects directly below the centre of the camera lens (i.e. The primary geometric distortion in vertical aerial photographs is due to relief displacement.