Remote Sensing - Interpreting Imagery
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
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.