Abstract: An ordnance piece is aimed at a terrain and the trajectory followed by an imaginary projectile from the ordnance piece is calculated. A range finder is sighted through a succession of points along the trajectory onto the terrain. Simultaneously the distance between a fixed reference point and each of the points through which the range finder is sighted is also calculated and these two distances are compared until the range finder gives a reading which corresponds to a point on the trajectory. This point corresponds to the intersection of the trajectory with the terrain and is marked so as to indicate where the imaginary projectile would have hit.