Basics of Aerial Photography
Aerial photography has three general
- Cartographers and engineers take detailed
measurements from aerial photos
- Trained interpreters use airphotos to
map land use and all kinds of human activity
- Earth scientists use air photos to analyze
and map the environment
Although both maps and aerial photos
present a "bird's-eye" view of the earth, aerial photographs are not
generally considered maps. Maps are spatially correct representations
of the earth's surface; they are directionally and geometrically correct
with regard to a planar frame of reference. Uncorrected aerial photos
have radial distortion (due to lens curvature) as well as geometric
distortions due to inconsistencies in the attitude of the airplane (roll,
pitch and yaw). These limitations to accuracy are sometimes
quantifiable in terms of scale, resolution and angular distortion but
for the most part the "accuracy" of an air photo interpretation is a
function of the experience of the interpreter and availability of ancillary
information. Geometric distortions can be corrected but not always,
this too is a function of the experience of the person doing it.
Photogrametry is, "the art or
science of making measurements from aerial imagery". The geometry of
an air photo is fixed by the altitude, focal length and film format.
These three variables, along with measurements of the plane's three
dimensional position relative to the ground, can be used to remove spatial
distortions caused by topography, called "ortho
rectification". By removing the geometric distortions and the topographic
distortions aerial photographs can be used for very precise measurements.
Novice photo interpreters often encounter
difficulties when presented with their first aerial photograph because
objects are portrayed from an unfamiliar perspective. It's all a matter
or experience, once you become familiar with interpreting aerial photography
you will become better at identifying features and recognizing diagnostic
patterns. It's simple, the more photos you look at the better photo
interpreter you become.
Advantages of Aerial Photography over
Ground Based Observation
- offers an improved vantage point
- capability to take a "snap shot" of time
that serves as a permanent record of changes
- broader spectral sensitivity than the
human eye and much broader field of view
- better spatial resolution and geometric/radiometric
fidelity than many ground based methods
- provides repetitive looks at the same area
- can focus in on a very specific wavelength
range and distinguish subtle differences
- can also look at a number of wavelengths
Why Process Remotely Sensed Data Digitally?
Humans are adept at visually interpreting
images and learning patterns. Our eyes and brain are the most sophisticated
remote sensing instruments there are, well beyond what machines will ever
be capable of (even in the 21st century!) but computers do one thing better
than we do - the same thing over and over and over again very quickly
with flawless repetition. What our brains can to do far better than computers
can is rule out illogical inferences; when we are presented with an image
our brains immediately associate patterns with meanings, a computer has
to match the pattern to something in a digital database. Although brute
force computer horse power can effectively match patterns, with something
as complicated as an aerial photograph it is doubtful that people will
ever be replaced in this regard.
Think of "precision" as the number of
decimal places and "accuracy" as how well the results match the real
world. In the field of air photo interpretation it is more important
to be accurate with your reasoning rather than precise with your measurements.
The products of human interpretations can be highly subjective, hence,
not perfectly repeatable. Conversely, results generated by computer
programs - even when they're wrong - are perfectly repeatable. Some
foundation knowledge about camera systems and film will serve as a basis
upon which to start becoming an air photo interpreter. Although technology
has progressed significantly, the basic ideas describing cameras and
film will continue to be relevant far into the future.
Cameras are similar to your eyes;
many of the mechanisms, including the geometry, are analogous. Cameras,
like film, have undergone a steady evolution since they were invented.
lens was crucial, as was film that would react quickly (and predictably)
to light of different wavelengths. Camera lenses and filters are now a
fully evolved technology. The following diagrams depict some fundamental
ideas that pertain to the optics of cameras.
Camera vs. Eye Ball. Note the image
inversion; this is corrected by mirrors in the camera body, and
by our brains - the world your eyes see is really upside down!
Focal Length vs. Lens Shape. Focal
length is the distance between the lens and the point where the
light rays converge. Magnification is controlled by distance from
the object, film format, and/or the focal length. Lens curvature,
optical purity and film quality also play a role.
- For aerial systems, the amount of ground
coverage acquired by a camera is a function of altitude of the platform,
the focal length of the lens and the film format (size). Different
camera systems have been developed for specific purposes, e.g. military
surveillance where the balance between resolution, area covered and
altitude has resulted in a multitude of different camera system configurations.
The panoramic camera and the stereo strip camera are two examples of
camera systems developed for specific tasks; panoramic cameras covered
large areas and the stereo strip camera covered relatively small areas,
from low altitude, in detail.
- A wider field of view will capture
more area but sacrifice resolution. Digital camera systems are subject
to the same physical constraints. Longer focal length, with lens diameter
held constant, reduces image coverage but increases detail.
Focal Length vs. Size Size
Magnification, focal length, and angler
field of view.
- The figures above illustrate the the
relationship between distance (altitude) and focal length, as well as
the relationship between aerial coverage and angular field of view.
The figures below illustrate the tradeoff between lens diameter and
object size. The pair of diagrams on the left shows the relationship
between aperture, focal length, and magnification. Increasing lens diameter
compensates for shorter focal length, but increasing lens aperture,
focal length and film format, as is illustrated by the picture of the
40 inch focal length camera on the right, can greatly increase magnification
and potential image detail.
The main difference between aerial
camera systems and traditional cameras is the need for aerial systems
to be spatially accurate. Metric precision is necessary because aerial
photography is often used to measure very small distances, and to create
high resolution elevation models from stereo imagery. For these purposes
it is necessary to have photography that is extremely accurate. It is
possible to do your own photo reconnaissance by pointing a regular camera
out the window of a plane and taking a picture of the ground, but in order
to make reliable measurement you need a more stable setup. Certified "metric
quality" cameras are expensive sensitive devices but necessary if precise/accurate
measurements are required.
Aerial cameras consist of essentially
1. Lens Assembly; 3.5, 6, 8.25 and 12
inches are typical focal lengths. The lenses of aerial systems have
the focus fixed at infinity.
2. Focal Plane; this is a perfectly perpendicular
plate aligned with the axis of the lens, a vacuum system is used to
fix the film to the plate so the focal plane is perfectly flat during
3. Lens Cone; this holds the lens and
filter, and covers the front part of the camera preventing light from
leaking into the camera body.
4. Body; encloses the camera, the mounting
bolts and stabilization mechanism.
5. Drive Assembly; the guts of the camera,
the winding mechanism, shutter trigger, the vacuum pressure system and
6. Magazine; holds the roll of unexposed
film, advances the film between exposures, holds the film in place and
winds-up the exposed film.
Aerial camera systems also have a mounting
bracket, power supply, vacuum lines, heating jackets, filters, forward
motion compensation (FMC) and an Interial Motion Unit (IMU). Also part
of the system is a viewfinder for targeting the camera, an intervalometer
that determines the rate at which exposures are taken (the amount of
overlap) as the plane flies along the flight path, a navigation control
system and an exposure control system. Computers have assumed many of
these tasks which were once manual/mechanical. Global Positioning Systems
(GPS) are now integrated into the camera system to provide very precise
in-flight positional control.
Multispectral cameras were the
precursor to the development of digital mulispectral satellite remote
sensing systems. Different configurations of cameras, lenses, filters
and film types were experimented with to determine optimal wavelength
regions for the remote sensing of different landscape features. These
experiments led to the selection of the multispectral wavelength ranges,
or "bands", that were later used for satellite based remote sensing systems.
Some Military Camera Systems
Left: Four Hasselblad 60 mm cameras
setup to acquire synchronized multispectral images using different
Right: BW CIR film using four filters; A blue
filter, B green filter, C red filter, D near IR filter.
WWII Mosquito being loaded with
700,000 candlepower photoflashes in the bomb bay to take high
definition flash pictures of the ground surface at night.
K-25 WWII Recon Camera
- KA-18A Stereo Strip Camera
|The Stereo Strip Camera demonstrated
its military surveillance potential in a time when near real time
high resolution images were of the utmost importance.
These cameras were pulled out of
storage specifically for the task of low level high speed reconnaissance.
Prior to the Cuban Missile Crisis, they were used in WWII and
for determining the winner of horse races that were too close
to judge by the human eye.
It does not use a shutter like
a regular camera, but instead uses a slit. The film rolls past
the slit with the same proportional speed of target, which in
the case of aerial photography is the speed of the plane. Two
contiguous strips of stereo photography are produced which would
not be possible using a conventional shuttered camera and image
motion compensation because low flying jet aircraft move way too
- Ektachrome 64 mm & Hasselbladd
500 EL/M 70 mm Cameras
- NASA has modified standard off-the-shelf
Hasselblad 70 mm cameras (right in the image below) to operate in
zero gravity aboard the Space Shuttle. A data recording module (DRM)
has also been installed on each camera to record the date, time, mission
number, roll number, and frame number on each exposure. The camera
utilizes a 70 mm film format and uses one of three lens (50, 100,
or 250 mm) to acquire high quality photographs through the four viewing
ports on the Shuttle. The Ektachrome 64 mm camera was also used aboard
the Space Shuttle.
- Gateway to Astronaut Photography
of Earth: http://eol.jsc.nasa.gov/sseop/
Ektachrome 64 mm & Hasselblad
70 mm cameras.
- Lihof Aero Technika
NASA has modified the Linhof cameras
to operate in zero gravity on board the Space Shuttle as well. The camera
utilizes a five-inch film format and is equipped with interchangeable
lens (90 mm and 250 mm). A data recording module (DRM) is also mounted
on the camera to record the date, time, mission number, roll number, and
frame number for each photograph. http://www.linhof.de/english/
- Large Format Camera
The Large Format Camera (LFC)
was a high altitude aerial mapping camera scaled up to operate from the
Space Shuttle. It was used on one mission in October 1984. The camera
weighs about half a ton and a single frame covers 23,400 square miles
at about 10-20 m resolution.
- LFC specs: Film Format: 9 x 18 inches
(23 x 46 cm), Aperture: F/6.0, Focal Length: 12 inches (30.5 cm),
Exposure Interval: 7.5 sec., Ground Resolution: 20 meters at 160 nautical
- High Quality Photographic Systems
and the Digital Modular Camera
Aerial cameras manufactured by
Carl Zeiss and LH Systems are of the highest quality and are the most
widely used. Z/I Imaging (Intergraph - Carl Zeiss) introduced the Digital
Modular Camera (DMC) pictured on the left below. The advantages of an
all-digital camera system are extremely precise planar coordinate registration,
reliability and the automation of manual tasks, such as triangulation,
which are typically expensive and time consuming. The images on the right
are of two other metric quality camera systems, the most popular are the
RMK cameras. RMK TOP cameras have been in production for many years and
are the most recognizable aerial camera system. LH Systems cameras incorporate
the higher fidelity capabilities of film based cameras with the exacting
digital precision of computers and in-flight GPS.