Stereoplotter





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.

Stereoplotter process
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 digitize the desired elevations on the terrain.

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 USGS (United States Geological Survey). The USGS produces topo-sheets for the entire US. The sheets are most often produced as 7.5’ maps. This means they cover 7.5 minutes (.125 degrees) of latitude by 7.5 minutes (.125 degrees) of longitude.

Stereoplotter evolution
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.

Future outlook
LIDAR (light detection and ranging) has largely replaced the stereoplotter for gathering elevation data. LIDAR uses a laser pulse directed at features and detects the amount of time between when the pulse is emitted and when it is detected to determine the difference in elevations.