The aerial photographs are taken on a clear day by flying over the terrain on straight (perhaps east-west) paths taking strips of photographs, snapping each several seconds apart so that adjacent pictures have at least 60% overlap.  When our USGS Forest Park maps were made, one of the more common techniques to convert the photos into contour maps was the "Multiplex" system.  A line of projectors is mounted on a bar, and each projects one of the series of pictures.  Adjacent projectors have different color filters, red or blue, over their lenses.  The map maker wears matching glasses, one red and one blue filter, so that each eye sees only one of the two overlapping images.   Where images overlap, the operator sees a stereoscopic image of the ground.  (You will likely be pretty impressed when you see this for the first time.)  The projection screen is a circular white table, about six inches in diameter, that has a tiny light spot in its center and can be raised or lowered to make the light spot below above or right at the surface of the ground as seen stereoscopically.  The table also has a pencil point that can be lowered to the sheet of drawing paper under it.  The operator first properly lines up all the equipment and then sets the light spot on the surface, lowers the pencil to the paper, and pushes the table around always keeping the light spot right on the surface.  A contour for that elevation is thusly drawn on the paper.  The operator lifts the pencil, raises or lowers the table to the level of the next contour, lower the pencil to the paper, and draws the next contour.  The process is continued until all the contours are drawn.
 

Since what he or she sees is the tops of the trees, the operator must correct, as best as can be done, for the height of the trees.  Much detail is missed in heavily forested terrain.  Several significant gulches which force Wildwood trail into significant meandering are simply not shown on the USGS map.

Nevertheless, stereoscopic photogrammetry is an ingenious way to interpret a monumental mass of data.  Older cartographers would go out into the area to be mapped and set up a carefully leveled drawing board, a "plane table."  On the plane table he would set a straight edge with a small surveyor's telescope attached, an "alidade."  An assistant then hauls a long board with gradations painted on it, a "stadia board," and finds a line of points on the ground that all have the same altitude; he follows a contour line, that is.  The alidade has several crosshairs in it and the cartographer determines the distance to the board by the number of painted gradations seen between a fixed pair of horizontal cross hairs, "stadia hairs."  He then plots the contour line on the map, point by point.  It's crude, it's tedious, and it works, but takes almost forever.  The Multiplex system takes many orders of magnitude less time.

Stereoscopic mapping extends an ordinary human perception through extraordinary human ingenuity.

So, take a pair of photographs of the same view, but move the camera several feet between shots.  If the terrain is miles away rather than fractions of a mile, move the camera as much as a fraction of a mile or more.  Then view the pair of photos so that one eye looks at one picture, the other eye at the other picture, and fuses them.  If you have a pocket stereoscope in your pocket, so much the better.  However, most people can learn to cross or spread their eyes and view the photos stereoscopically without the optical aid.

Here's a stereoscopic pair (from some special pages not linked from our regular Web site) taken from the top of Rainbow Bridge in southern Utah.  The camera was moved about 50 feet between shots.  The left picture is on the right and the right picture is on the left: so this pair is viewed by crossing your eyes.  (When images are this large eye-spreading is difficult.)
 
 

You see depth detail that cannot be seen at the scene without this trick, without this extension of perception.  It's especially good for finding routes on climbs.

A puzzle to think about.

Many feel that it's important to make the distance between the two camera positions as close to the actual inter-eye distance as possible.  They say, "If you take them farther apart, you will exaggerate the depth."  But the depth almost never looks "exaggerated."  (And in those rare cases where it does look exaggerated, the pictures might have been taken right at the interocular distance . . .  or even less!)   There's an "obvious yet unobserved" principle here, but let's leave it for now as a puzzle:  What might make sterescopic images have exaggerated depth?

The Stereoscopic Model