Geog 280: Basic Geographic Techniques
Exercise 7: How Aerial Photography Works
Aerial Photography
Photography was first developed in the
1840s. It wasn't long afterward that people began carrying
cameras on balloons, and then on airplanes after their invention
in the early 1900s. One famous aerial photograph was taken
San Francisco of immediately after the 1906 earthquake (see
portion at right). Aerial photography developed rapidly in
the 1930s and 1940s as a useful approach to mapping Earth's
surface. It has been the source of most information on USGS
topographic maps since then. Even the contours that show
topography are derived from stereoscopic air photos (we'll see
more about this later in this exercise).
Satellites have also provided images of Earth, but they use a
different technology than aerial photography. Air photos
use conventional film, which is similar to the film you would use
in a typical camera. The film must be exposed in a camera,
developed with chemicals, and printed. Satellite imagery,
on the other hand, scans Earth electronically and stores
information as computer data. We will see more about
satellite technology later.
Some people have expected satellite imagery to replace
conventional air photos for gathering information about
Earth. For various reasons, air photos continue to provide
an important resource for Earth observation. If nothing
else, the photos that have been taken over the past 60 years
provide a tremendous historical record of landscape change.
For this reason, we will take a few pages here to examine the
characteristics and uses of air photos.
Types of Air
Photos: Vertical vs. Oblique
Nearly all photos taken of Earth that are to be used for
mapping are taken vertically. By vertical we mean
that the camera points straight down from the aircraft, which is
also toward the center of Earth. The photo at the right is
a vertical photo of Bodega Bay, California. Photos that are
not vertical are not as useful for mapping or GIS. In
actuality, few vertical air photos are absolutely vertical.
They actually are taken at a slight tilt of perhaps one or two
degrees off the vertical. This tilt can be accounted for
when measuring features for maps based on the photos.
Photographs that are not taken from cameras pointed directly
downward are called oblique. These photographs point
at an angle toward Earth. If you've taken photographs from
an airplane or even from a high vantage point, you've probably
pointed the camera across the landscape at an oblique
angle. The example below is from Antarctica.
Films for Aerial Photography
The films used for air photos are slightly different from the
film you would typically buy for your own camera. Your film
is probably natural-color film, either for prints or
slides. Aerial films don't often use the natural color
approach. You can see why when you fly in a commercial
airliner. The colors from high above Earth look rather
washed out, and everything tends to have a grayish blue
tint. This is because Earth's atmosphere scatters
some of the sunlight back to your observation point. The
scattering is especially severe for blue colors.
Instead of natural-color film, aerial photography most often
uses either panchromatic or color-infrared
film.Panchromatic film is simply a type of black-and-white film.
Actually it's very similar to the black-and-white film you can
still buy in camera shops and sometimes at department
stores. Panchromatic film is popular because not only is it
cheaper than color film, but the images are clearer than
natural-color. A yellow filter can be used to filter out
some of the scattered blue for an even sharper image. The
examples we saw above of Bodega Bay and Antarctica use
panchromatic film.
Color-infrared (CIR)
film produces a colored image but the colors do not
correspond to the colors your eye perceives. CIR film
records both visible and infrared light. Infrared is
energy that you cannot see. We will see more about infrared
and other electromagnetic energy a bit later. For now, just
remember that many objects react differently to infrared light
than to visible light. This makes detecting some objects
easier with CIR film. Vegetation is the best example of the
difference. Plants reflect infrared energy very strongly.
Since infrared isn't a color, we have to give it one on the
film. Usually the color given to infrared is red.
In other words, the more red an object on CIR film, the more
infrared it is reflecting. To make room for infrared, the
red is reassigned to green, and in turn green is assigned
to blue. Blue is not even recorded on the film,
which isn't a big deal since it gets scattered a lot
anyway. Interpreting CIR photos requires that you keep in
mind that colors don't mean what you're used to. In the
example above, trees and other vegetation show up in red, while
buildings and pavement are pale or dark.
One other important
difference between air photos and the typical hobby camera is the
size of the film. The film for your 35 mm or other
camera has negatives (or slides) that are fairly small (35 mm
wide for 35 mm cameras!). Aerial cameras, on the other
hand, typically use negatives that are 9 inches by 9 inches
(230 mm x 230 mm) -- almost the size of notebook paper!
The reason for this size is the resolving power of film.
The silver-salt grains used in the film are small but still have
some size. The bigger the negative, the more grains, and
the more detail can be recorded. If you enlarge a 35 mm
negative into a large print, say 36 inches wide, you'll notice
the "grain" in the print. A typical air photo can
be enlarged to the same 36" size and still show excellent
detail.
Cameras for Aerial Photography
Not surprisingly, the cameras for aerial photography are
large and expensive. The typical air photo camera is called
a frame camera (or metric camera, or cartographic
camera). These cameras are expensive not only because they
handle large negatives, but also because they have a set of
precisely ground lenses so that the light entering the camera is
accurately directed to the film. These cameras usually have
a large magazine that holds a large spool of film. The
whole camera is usually mounted in an airplane with an opening
cut in its belly specifically for such a camera.
Aerial cameras have a shutter like other cameras, which
opens briefly to record the scene on the ground. The size
of opening (aperture) can also be adjusted.
Interestingly, these cameras typically cannot be focused
-- in other words, the distance from lens to film cannot be
adjusted. Why? Once a camera gets beyond a few dozen
feet from objects, the objects appear to the camera as virtually
the same as objects several miles or any further distance away
(obviously they'll be different in size, but not in
geometry). The same approach is used in the simple one-use
cameras you can buy in grocery stores -- they photograph scenes
anywhere from 6 feet to infinity without changing the focus of
the lens.
Taking Air Photos
Unlike satellites, aerial cameras aren't constantly circling
Earth taking pictures. To obtain air photos, someone must
fly an airplane carrying the camera over a specific area.
The air photo mission must be planned carefully. The
mission must plan for optimum conditions for lighting, wind,
clouds (i.e., lack of them!), and ground conditions. For
example, air photos of an oak forest might be best taken in
summer when the trees have leaves.
Air photo flights
typically fly a series of parallel, straight lines. Along
each line, photos are taken every few seconds so that they
overlap with each other. The cameras are usually connected
to a device that automatically snaps photos based on speed and
direction of the aircraft. The aircraft must fly at the
proper height to get the desired scale of photo. We'll
discuss scale and other interpretation issues next.
Questions on this page
- What kind of landscape would appear very red on a
color-infrared photograph, and why?
- What does a camera used for aerial photography lack that
other less-expensive cameras normally have the ability to
do? Why?
Bryan Baker, Sonoma State
University, bryan.baker@sonoma.edu
Updated 17 February 1999