GIS (geographic information system)

            Photogrammetry is the technique of measuring objects (2D or 3D) from photo-grammes (= photographs).  Photogrammetric camera systems have automated film advance and exposure controls, as well as use long continuous rolls of film. Aerial photographs are taken in a continuous sequence with approximately 60% overlap. This overlap (conjugate) area of adjacent images enables 3 dimensional analysis for extraction of point elevations and contours.  Parallax is the relative displacement of features on two overlapping photographs.  Overlap can be as much as 60% in the vertical direction and 40% in the horizontal direction. Taking into account parallax, shadows on an image can be used to determine the heights of vertical features (buildings, trees, towers, cliffs, etc.), as well as the time of day.

            Harry T. Kelsh introduced the Kelsh optical projection stereoplotter in 1945.   This German photogrammetric instrument was used create a 3 dimensional image from overlapping analog images (conjugate images) for extracting precise elevation information (i.e. point elevations and contours).  Accurate elevation measurements were derived by adjustment of a floating point.  By placing the floating point to the perceived surface of the 3 dimensional image accurate point elevations or contours were derived.  This is still a common method for creating topographic maps today.

                                               

            Analytical Stereoplotters combine capabilities of computer technology with stereoplotter functionality in order to limit operator intervention in the adjustment of parallax (the relative displacement of features on two overlapping photos).  With most analytical stereoplotters, operator assistance is required for fine adjustment of parallax so that the floating dot in the visual field is placed directly on the 3 dimensional surface.  There are 4 basic types of analytical plotters:  anaglyphic, polarizing, stereo image alternators, stereo optical training, digital photometric (softcopy) workstations. 

Anaglyphic stereoplotters pass each image through separate colored filters (red and blue-green) so that both images can be displayed on a single monitor.  The operator is required to wear special colored glasses to view the 3 dimensional image.

Polarized stereoplotters pass light through polarized lenses so that the images are polarized in different directions.  Both images are then displayed on a single monitor.  The operator is required to wear special polarized glasses to view the 3 dimensional image.

Stereo image alternators change the view on the monitor between the 2 stereo images at a high frequency to create a false 3 dimensional image that can be viewed without the aid of glasses.

Stereo optical training passes light from each image through a series of mirrors and prisms to binocular type lenses.  The operator views the stereo image through the binoculars.

Digital Photometric (softcopy) workstations view conjugate digital stereo models on a single computer monitor as a polarized image.  The operator is required to wear special polarized glasses to view the 3 dimensional image.

            An orthoscope is an optical distortion device used to remove distortion from a photograph. When aerial photo is taken it has a central perspective.  This means that it is being viewed downward at the central point causing distortion near the edge. To reduce the amount of distortion a planemetric perspective will be achieved. It is when a photo appears as though it is being looked at from straight above the earth at every point. A conversion will be made from a central to a planemetric perspective. This will be achieved by stretching the distortions.

            Today there is a different way to remove distortion from photos by digital alterations. This is accomplished by first scanning in the photographs to create a raster image. Pixels can then be removed or moved to eliminate distortion. This method provides a more efficient way to correct images.