A stereoplotter uses stereo photographs to determine elevations. It has been the primary method to plot contour lines on topographic maps since the 1930s. Although the specific devices have advanced technologically, they are all based on the apparent change in position of a feature in the two stereo photographs. Stereoplotters have changed as technology has improved. The first stereoplotters where projection stereoplotters they used only the light rays and optics to adjust the image. The Kelsh Plotter is an example of the projection stereoplotters. The analog stereoplotters came next and were more sophisticated in that they used more sophisticated optics to view the image. The analytical stereoplotter is used today. It incorporates a computer which does the work of mathematically aligning the images so that they line up properly. The analytic stereoplotter also allows for storing the data and redrawing at any desired scale.
Analogical The stereoplotter requires two photographs that have considerable overlap (60%) and are corrected for distortion due to angle of photo. The photos are put onto transparent media and projected with a light source. Each image will be projected with overlap on the other. The operator, using a special set of optics, would then see the image as three-dimensional due to the differing perspective of each photo. The optics of the stereoplotter is what allows the operator to plot the contours and features. The light source used to project the photo is what begins the process. One photo is projected using cyan/blue filter and the other photo is projected with a red filter. The operator wears a special set of glasses that have the same color filter for lenses. Seeing the left photo in blue light while the left eye has the blue filter and the right photo projected with red light and the right eye seeing through the red filter, the overlapping image becomes three-dimensional. The images will have control points that detail how the overlap of the photos should occur. The resultant overlapping image is called an anaglyph and is a three-dimensional model of the terrain. Once the two photos are projected and the desired control points aligned the operator will then start to record the desired elevations on the terrain by "flying" a light spot along the contours. If the light spot appears to hover above the terrain or appears to dive into the terrain, the operator knows that he has moved it too far away from a slope or too far toward a slope, respectively. Originally, stereoplotters recorded the path of the flying light spot by directly scribing the path on a sheet of acetate or polyester coated with an opaque lacquer, which could be photographed to make the topographic map printing plates. If mistakes were made during contour tracing, the operator would daub some lacquer on the incorrect trace, allow it to dry and then try flying the light spot again. Current systems that use digital capture techniques allow simple erasure of part of the faulty data vector in computer memory, whereupon digitizing can resume. The digital database is then combined in a graphics program with annotations and symbols, ultimately being used to produce the map printing plates with a photoplotter. The stereoplotter serves an important need of allowing aerial photos to become the base for contours and elevation maps. In the United States the largest use of topographic maps is the United States Geological Survey (USGS). The USGS has cataloged all of the territory of the United States and has produced topographic sheets for all of it. The sheets are most often used as 7.5' maps. This means there is 7.5' (0.125 degree) latitude by 7.5' (0.125 degree) longitude.
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