Remote Sensing - Interpreting Imagery

Visually Interpreting Aerial Images: A Primer

The American Society for Photogrammetry and Remote Sensing defines photo interpretation as “The act of examining photographic images for the purpose of identifying objects and judging their significance,” (Paine,1981). This classical definition conveys a general idea of the theory behind air photo interpretation, but in today’s computerized age the methods of analysis have changed dramatically. To understand how this science has evolved, traditional techniques and practices should first be appreciated.

What makes a skilled photo interpreter? Primarily, the ability to make inferences about a scene by recognizing features on an aerial photograph. This talent is not developed overnight. Many hours of practice combined with visiting ground sites for interpretation accuracy assessments, and a strong background in the earth sciences all contribute to building a successful photo interpreter (Paine, 1981). Professionals in this field also rely on many theoretical principles of photo interpretation.

These principles include using image characteristics like size, shape, shadow, texture, tone or color, and pattern to aid in the identification of objects on the ground. Take color, for instance. When dealing with infrared photography one can assess the vitality of vegetation by the colors on the image. In this example, bright red indicates healthy vegetation or cropland [See Image 21]. Besides these theoretical concepts, photo interpreters employ various tools for examining photography.

Image 21: Landsat infrared image of Hayman Springs fire area. (Image: USGS)

One of the most basic and the most widely used tools is the stereoscope. In short, this tool allows the interpreter to see photographs in three dimensions. The 3-D view is simply set up by positioning two overlapping photos under the stereoscope in a prescribed fashion. This instrument permits the observer to see terrain (topography), building heights, and other objects in a much more effective manner than without stereo.

Stereoscopes come in all shapes and sizes. Some folding stereoscopes consume an entire tabletop while others can fit in a shirt pocket. The three basic types of stereoscopes are the mirror, lens, and zoom.

Figure 2: Mirror Stereoscope (Topcon, Inc.)

Mirror stereoscopes use mirrors to bring the two images to the two eyes of the photo interpreter [See Figures 2 and 3]. The main advantage of using this type, is that the stereo pair can be completely separated, thus permitting the analyst to see more of the image at once.

Figure 3: Pocket Mirror Stereoscope (Wild, Switzerland)

The lens stereoscope is the most basic variety [See Figure 4]. Most “pocket” stereoscopes are part of the lens category. These simple devices are usually composed of two lenses, one for each eye, and “legs” to support them.

Figure 4: Lens Stereoscope (Abrams Instrument Corporation)

Finally, the zoom stereoscope, as the name implies, allows the viewer to magnify, or zoom in, on areas of interest. Most zoom stereoscopes use a light table to view roll film. The film is spooled over the light table by motorized spindles, so photo interpreters can efficiently analyze a large amount of aerial photography [See Figure 5].        

Figure 5: Zoom Stereoscope (Bausch & Lomb, Inc. and the Richards Corp)

Each kind of stereoscope serves the same basic purpose, allowing flat photography to be visualized three-dimensionally. The difference between them is in the mechanical operation, the range of magnifications possible, the kinds of aerial photography they are designed for, etc.

Photographic measurement is another important tool photo interpreters use. Anything from ground distances to the diameter and height of trees is measurable. The benefit of this aspect of photo interpretation is enormous, especially in environmental science, planning, and military intelligence.

The last tool covered here relates to the evolution of new technologies. A new era of computerized equipment and sophisticated digital image analysis software has developed on a large-scale, making the interpreter’s work more diversified. These new tools enable people to store and retrieve large amounts of information and integrate imagery into other multipurpose systems like a GIS.

As the section title suggests, this is only a primer to the world of interpreting aerial images. Countless books and publications are in circulation regarding this field.


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