Thecameras need not be separated by thesame spacing as theeyes, but can be separated by any amount. Thegreater thecamera spacing, thegreater thethree- dimensional effect. Aerial photographs used in mapping are taken from points several kilometers or more apart. Air-photo interpreters use a device much like thestereopticon. A pair of aerial photographs is viewed through a stereoscope, a viewer constructed so that each photograph is seen by one eye.
Theapparent landscape seen through thestereoscope is called themodel.
If a small dot is drawn at thesame location on each photograph, a viewer will see thedot appearing to rest on theground on themodel. If thedot is drawn on a treetop, thedot will appear to be above theground on themodel. If a dot is drawn at some point on one photograph, and at a random point close to that location on theother photograph, thedot will appear to float above or below themodel surface when viewed through thestereoscope. If theparallax shift corresponding to thetwo dots is greater than that of themodel, thedot will appear closer to theviewer, that is, above themodel. If theparallax shift of thedots is less than themodel, thedot will appear to be below themodel surface.