3. Stereoscopes

In photogrammetry, camera positions define the perspective positions, Figure D-8.

Figure D-8 Aerial Photographs

To view the overlap as a 3D model requires the left eye looking at the left overlap area and the right eye the right. Photographs from mapping cameras are usually 9" x 9".

When placed side-by-side, the overlap is wider than the viewer's eye base. Laying one photo atop the other, Figure D-9, partially obscures the 3D view.

Figure D-9 Obscured View

Not everyone is able to comfortably view two separate images simultaneously without an aid. A stereoscope is an instrument which makes viewing the 3D model easier. A basic  stereoscope has two eyepieces to aid each eye view a specific image, Figure D-10.

Figure D-10 Basic Stereoscope

The eye base is unaltered so one photo must still be on top of the other.

A mirror stereoscope, Figure D-11, uses first-surface mirrors and right-angle prisms to allow wider photograph spacing.The entire 3D model can be viewed and measurements more easily made.

Figure D-11 Mirror Stereoscope