Rectification in the photogrammetric sense means the removal of the effects of tilt on the images in a tilted photograph. It is the process of making equivalent vertical photographs from tilted photo negatives. The resulting equivalent vertical photos are called rectified photos. The process of rectification may entail: i) Plotting from the tilted photo onto another object sheet such that the plotted points are free from tilt displacement. This is called graphical rectification. An example of this is the paper strip method. ii) Optical projection of the photo image onto the map plane and alignment of corresponding features before plotting the missing features in the map by direct tracing. The aero sketch master is an example of this. iii) Photographic rectification (Optical- mechanical rectification) whereby the tilted photograph is projected optically such that the projected image is tilt-free and the image is then photographed to obtain the equivalent vertical photograph. Anharmonic Ratios