Elements, Aids, Techniques,
Methods & Procedures of Airphoto Interpretation
“You see but you do not observe.”
- Sir Arthur Conan Doyle: The Memoirs of Sherlock Holmes
I. Definitions
Photo Interpretation: The act of examining
aerial photographs/images for the purpose of identifying objects and judging
their significance.
Photography: The art or process of producing
images on a sensitized surface by the action of light or other radiant
energy.
Image: A reproduction or imitation of the
form of a person or thing. The optical counterpart of an object produced
by a lens or mirror or other optical system.
Photogrammetry: The science or art of obtaining
reliable measurements by means of photography.
II. Activities of Airphoto/Image Interpretation
Detection/Identification - This
is primarily a stimulus and response activity. The stimuli are the elements
of image interpretation. The interpreter conveys his or her response to
these stimuli with descriptions and labels that are expressed in qualitative
terms e.g. likely, possible, or probable. Very rarely do interpreter use
definite statements with regard to describing features identified on aerial
photography.
Measurement - As opposed to detection
and identification, the making of measurements is primarily quantitative.
Techniques used by air photo interpreters typically are not as precise
as those employed by photogrammetrists who use sophisticated instruments
in making their measurements. Measurements made by photo interpreters
will get you close; given high quality, high resolution, large-scale aerial
photographs and appropriate interpretation tools and equipment, you can
expect to be within feet; whereas with photogrammetry if you employ the
same type of photography and the appropriate equipment you could expect
to be within inches.
Problem Solving - Interpreters are
often required to identify objects from a study of associated objects
that they can identify; or to identify object complexes from an analysis
of their component objects. Analysts may also be asked to examine an image,
which depicts the effects of some process, and suggest a possible or probable
cause. A solution may not always consist of a positive identification. The
answer may be expressed as a number of likely scenarios with statements
of probability of correctness attached by the interpreter.
Air photo interpretation is to photogrammetry
as statistics is to mathematics; one deals with precision the other with
probability.
III. Elements of Airphoto/Image Interpretation

A. Basic - 1st Order
Tone/Color |
Tone can be defined as
each distinguishable variation from white to black. Color may be defined
as each distinguishable variation on an image produced by a multitude
of combinations of hue, value and chroma. Many factors influence the
tone or color of objects or features recorded on photographic emulsions.
But, if there is not sufficient contrast between an object and it's
background to permit, at least, detection there can be no identification.
While a human interpreter may only be able to distinguish between
ten and twenty shades of gray; interpreters can distinguish many more
colors. Some authors state that interpreters can distinguish at least
100 times more variations of color on color photography than shades
of gray on black and white photography. |
Resolution |
Resolution can be defined
as the ability of the entire photographic system, including lens,
exposure, processing, and other factors, to render a sharply defined
image. An object or feature must be resolved in order to be detected
and/or identified. Resolution is one of the most difficult concepts
to address in image analysis because it can be described for systems
in terms of modulation transfer (or point spread) functions, or it
can be discussed for camera lenses in terms of being able to resolve
so many line pairs per millimeter. There are resolution targets that
help to determine this when testing camera lenses for metric quality.
Photo interpreters often talk about resolution in terms of ground
resolved distance which is the smallest normal contrast object that
can be identified and measured. |

Tone, Centimeter vs. Meter Resolution
B. 2nd Order - Geometric Arrangements
of Tone/Color
Size |
Size can be important
in discriminating objects and features (cars vs. trucks or buses,
single family vs. multifamily residences, brush vs. trees, etc. ).
In the use of size as a diagnostic characteristic both the relative
and absolute sizes of objects can be important. Size can also be used
in judging the significance of objects and features. The size of the
crowns of trees can be related to board feet that may be cut for specific
species in managed forests. The size of agricultural fields can be
related to water use in arid areas, or the amount of fertilizers used.
The size of runways gives an indication of the types of aircraft that
can be accommodated. |
Shape |
The shape of objects/features
can provide diagnostic clues that aid identification. The Pentagon
building in Washington is a diagnostic shape. Man-made features
have straight edges, natural features tend not to. Roads can have
right angle (90°) turns, railroads can't. Other examples include
freeway interchanges, old fortifications (European cities), military
installations (surface to air missile sites). |
 
Pentagon, Freeway, Salt Spiral
C. 2nd. Order - Spatial Arrangement
of Tone/Color
Texture |
Texture is the frequency
of change and arrangement of tones. This is a micro image characteristic.
The visual impression of smoothness or roughness of an area can often
be a valuable clue in image interpretation. Still water bodies are
typically fine textured, grass medium, brush rough. There are always
exceptions though and scale can and does play a role; grass could
be smooth, brush medium and forest rough on higher altitude aerial
photograph of the same area. |
Pattern |
Pattern is
the spatial arrangement of objects. Patterns can be either man-made
or natural. Pattern is a macro image characteristic. It is the
regular arrangement of objects that can be diagnostic of features
on the landscape. An orchard has a particular pattern. Pattern can
also be important in geologic or geomorphologic analysis; drainage
pattern can reveal a great deal about the lithology and geologic structural
patterns of the underlying strata. Dendridic drainage patterns develop
on flat bedded sediments, radial on domes, linear or trellis in areas
with faults etc. It must be noted here that pattern is highly scale
dependent. |
D. 3rd. Order - Locational or Positional
Elements
Site |
Site refers to how objects
are arranged with respect to one another, or with respect to terrain
features. Aspect, topography, geology, soil, vegetation and cultural
features on the landscape are distinctive factors that the interpreter
should be aware of when examining a site. The relative importance
of each of these factors will vary with local conditions, but all
are important. Just as some vegetation grows in swamps others grow
on sandy ridges or on the sunny side vs. the shaded sides of hills.
Crop types may prefer certain conditions (e.g. orchards on hillsides). Man
made features may also be found on rivers (e.g. power plant) or on
hilltops (e.g. observatory or radar facility). |
Association |
Some objects are so commonly
associated with one another that identification of one tends to indicate
or confirm the existence of another. Smoke stacks, cooling ponds,
transformer yards, coal piles, railroad tracks = coal fired power
plant. Arid terrain, basin bottom location, highly reflective surface,
sparse vegetation = playa, which typically have halophytic vegetation
e.g. saltbush. Association is one of the most helpful interpretation
clues in identifying man made installations. Aluminum manufacture
requires large amounts of electrical energy. Schools of different
grade levels typically have characteristic playing fields, parking
lots and clusters of buildings. Nuclear power plants are associated
with a source of cooling water, weather patterns can be associated
with pollution sources etc. |
E. 3rd. Order - Interpreted
from lower order elements
Height |
For some types of analysis
e.g. land forms, forestry and some intelligence applications, some
interpreters believe that after tone/color height is the most important
element for identification. This is a point of debate, but height
can add significant information in many types of interpretation tasks,
particularly those that deal with the analysis of man-made features
and vegetation. How tall a tree is can tell something about the expected
amount of board feet. How deep an excavation is can tell something
about the amount of material that was removed (in some mining operations
excavators are paid on the basis of material removed as determined
by photogrammetric measurement of volume). |
Shadow |
Geologists
like low sun angle photography because of the features that shadow
patterns can help identify (e.g. fault lines and fracture patterns).
Church steeples and smokestacks can cast shadows that can facilitate
their identification. Tree identification can be aided by an examination
of the shadows thrown. Shadows can also inhibit interpretation. On
infrared aerial photography shadows are typically very black and can
render targets in shadows uninterpretable. |
Washington Monument (height-shadow), Objects
(shadow), Bella Lagosi (shadow)
IV. Techniques of Photographic/Image
Interpretation
Collateral Material
A review of all existing source
material that pertains to a given area, process, type of facility or object,
can aid in the interpretation process. The use of collateral material
may also result in a better definition of the scope, objectives and problems
associated with a given project. Also called "ancillary data", collateral
material may come in the form of text, tables, maps, graphs, or image
metadata. Census data, a map or description of the flora of a given area,
a land use map, meteorological statistics, or agricultural crop reports
can all be used in support of a given interpretation. Basically, collateral
material represents data/information that an interpreter may use to aid
in the interpretation process. Material contained within a Geographic
Information System (GIS) that is used to assist an interpreter in an analysis
task can be considered collateral data. Two classes of collateral materials
deserve special mention: interpretation keys and field verification.
Interpretation Keys
An interpretation key is a set
of guidelines used to assist interpreters in rapidly identifying features.
Determination of the type of key and the method of presentation to be
employed will depend upon, a) The number of objects or conditions to be
identified; and, b) The variability typically encountered within each
class of features or objects within the key.
Some authors say that as a general rule,
keys are more easily constructed and used for the identification of
man-made objects and features than for natural vegetation and landforms.
For analysis of natural features, training and field experience are
often essential to achieve consistent results. Basically, an interpretation
key helps the interpreter organize the information present in image
form and guides him/her to the correct identification of unknown objects.
Keys can be used in conjunction with any type of remotely sensed data.
Such keys can differ from those employed in other disciplines in that
they can consist largely of illustrations, e.g. landforms, industrial
facilities, military installations. Many types of keys are already
available, if you can find or get your hands on them. This can often
be very difficult and a reason why people develop their own keys.
Depending upon the manner in which the
diagnostic features are organized, two types of keys are generally recognized.
1) Selective keys, and 2) Elimination keys. Selective keys are arranged
in such a way that an interpreter simply selects that example that most
closely corresponds to the object they are trying to identify, e.g.
industries, landforms etc. Elimination Keys are arranged so that the
interpreter follows a precise step-wise process that leads to the elimination
of all items except the one(s) that they are is trying to identify.
Dichotomous keys are essentially a class of elimination key. Most interpreters
prefer to use elimination keys in their analyses.
Field Verification
Field verification can be considered
a form of collateral material because it is typically conducted to assist
in the analysis process. Essentially, this is the process of familiarizing
the interpreter with the area or type of feature. This type of verification
is done prior to the interpretation to develop a visual "signature" of
how the feature(s) of interest appear on the ground. After an interpretation
is made field verification can be conducted to verify accuracy.
Fieldwork is sometimes calculated as being three times as expensive as
lab analysis. (This is why good interpreters can be so valuable).
The nature, amount, timing, method of acquisition, and data integration
procedures should be carefully thought out. Will you use windshield surveys,
point or transect sampling? Will the sampling be random or systematic?
The amount and type of field work required
for a given project may vary greatly and is generally dependent upon
the,
a. Type of analysis involved.
b. Image quality, including scale resolution
and information to be interpreted.
c. Accuracy requirements for both classification,
and boundary delineation.
d. Experience of the interpreter and the
knowledge of the sensor, area, and subject.
e. Terrain conditions, and the accessibility
of the study area.
f. Personnel availability, access to ancillary
material .
g. Cost considerations.
Handling of Imagery
Although a good deal of photo
interpretation is still done using paper prints, the use of diapositive
transparencies is increasing. Transparencies can be used either as single
frames or as a roll. Care should be taken when handling transparencies
so that they are not marred. An orderly procedure for the handling of
either prints or transparencies should be developed and adhered to in
any interpretation project. Airphotos are typically numbered with flight
name and/or frame number, and should be kept in order in so far as practical.
Different dates and flight lines should be kept separate, etc. Anytime
transparencies are used surfaces should be as clean as possible and the
interpreter should either wear cotton gloves or be sure not to touch the
emulsion surface as skin oils can cause image deterioration.
Stereo Viewing
Binocular vision is natural to
all of us, but to the trained interpreter the ability to perceive stereo
is an incredibly valuable asset. Stereoviewing will be covered in detail
later, but suffice it to say that viewing high quality stereo aerial photography
though a mirror stereoscope is like seeing in another dimension. Although
the identification and interpretation of many landscapes can be accomplished
with mono, stereo is required for certain types of studies. The following
are some tips for using stereo effectively.
Basics for Stereoviewing
1. Make certain that the photos
are properly aligned, preferably with the shadows falling toward the viewer.
2. Keep the eye base and the long axis
of the stereoscope parallel to the flight line.
3. Maintain an even glare free illumination
on the prints or transparencies.
4. Arrange for comfortable sitting and
sufficient illumination.
5. Keep the lenses of the stereoscope
clean, properly focused and separated to your interpupillary distance.
6. The novice interpreter should not
work with stereo more than 30 minutes out of any hour period. You have
not had a headache until you've had one the comes from doing stereo
interpretation for too long!
Trouble Shooting Stereo
1. Your eyes may be of unequal
strength. If a you normally wears glasses for reading or close-up
work, you should also wear glasses when using the stereoscope.
2. Poorly illumination, misaligned prints
or uncomfortable viewing positions may result in eye fatigue.
3. Illness or severe emotional distress
may create sensations of dizziness in one using a stereoscope.
4. Reversal of prints may cause psuedo-stereo.
A similar problem may occur if prints are aligned with the shadows falling
away from rather than towards the interpreter.
5. Objects that change positions between
exposures cannot be viewed in stereo.
6. In areas of steep topography, scale
differences in adjacent photographs may make it difficult to obtain
a three dimensional image.
7. Dark shadows or clouds may prohibit
stereo viewing of an area by obscuring an object on one photo.
D. The Multi Concept
Multi-Station |
The successive overlapping
of images taken along a given flight line as being flown by an aircraft
or by a satellite along an orbit path. Think of it like "multi-position".
Not to be confused with multi-stage. |
Multi-Band |
Multi-band indicates
individual spectral bands within a given region of the EM spectrum
(e.g. the red green and blue bands of the visible portion of the EM
spectrum). Often seen to have an overlapping meaning with the next
term, multi-spectral. |
Multi-Spectral |
The use of images from
various regions of the EM spectrum (e.g. ultra-violet, visible, infrared,
thermal and microwave). |
Multi-Date |
The use of multiple aerial
photographs or remotely sensed images taken over time of a given area. |
Multi-Stage |
This typically
means using ground based photos, oblique low altitude photos and vertical
photographs or remotely sensed images from platforms flying at different
altitudes. Multi-stage has also been applied to sampling strategies;
A multi-stage sampling scheme as used in statistics is one where progressively
more information is obtained for progressively smaller sub-samples
of the area being studied. |
Multi-Direction |
There are times when more
information can be obtained using viewing angles other than vertical. |
Multi-Disciplinary |
Basically, no one interpreter
can know everything about a system in question. By using teams
of interpreters and experts with expertise in different disciplines
more information may be gained for a given application. In the legal
system this is most similar to the "convergence of evidence" idea;
having different viewpoints and different information sources to prove
a point adds validity. |
Multi-Thematic |
Remote sensing images
are one-time write, many times read. Many different themes (e.g. hydrology,
vegetation, transportation, urban areas, etc. ) can be extracted from
a single set of images. |
Multi-Use |
Many potential users from
environmental planners to resource managers to public policy decision-makers
can use outputs derived from image analysis and interpretation. |
E. Methods of Search
There are basically two techniques that
people tend to follow when searching for imagery. One is logical
search and the other can be termed the "fishing expedition". In the
latter, the fishing expedition, the interpreter searches the imagery
in a random fashion attempting to find recognizable features or object
that will lead to whatever the interpretation goal happens to be. At
some point even this type of interpretation begins to logically converge.
Patterns of anomalous vegetation may lead to looking for water sources,
which may lead to looking for transportation systems, illegal growing
etc. Logical search is a more systematic method of analysis most often
used by interpreters.
The logical search involve these things:
1. The interpreter should always keep
in mind the basic qualities of the imagery they are dealing with, e.g.
film filter combination, the season and time of day of acquisition,
and the image scale, etc. In addition the interpreter should always
remember to examine all the titling information on an image.
2. Interpretation should begin
with the general and proceed to the specific. After gaining an
overall impression of the photograph the interpreter should begin to
examine the physical features (e.g. water bodies, mountains, forests,
etc.) and cultural features (e.g. urban areas, farms, road networks
etc.). The interpreter should then move to more specific questions
e.g. what type of trees make up the forest? What types of roads
are present?
3. Interpretation should be conducted
logically one step at a time. Following from 2 above it is good
to go from a detailed examination of landforms to vegetation, to hydrology
and so on. Then address cultural features in the same fashion. What
types of urban features are present single family residences, multi-family
residences, industries, retail districts and so on.
F. Convergence of Evidence
1. Image interpretation is basically
a deductive process. Features that can be detected and identified
lead the interpreter to the location and identification of other features. This
is convergence, and for many applications of air photo interpretation
this involves the activities of one or two individuals synthesizing
a large amount of information.
2. Deductive interpretation requires
either the conscious or unconscious consideration of all of the elements
of image interpretation. The completeness and accuracy of an interpretation
is in some measure proportional to the interpreters understanding of
the "how and the why" of the elements, techniques and methods
of interpretation.
*
THE MORE IMAGES YOU INTERPRET THE BETTER INTERPRETER YOU BECOME.
Sources - References:
Jensen, J.R. , 2000, Remote Sensing of
the Environment: An Earth Resource Perspective, Upper Saddle River, NJ:
Prentice Hall, 544 pages. [Ch 5 pages119-135]
Avery T. E. and G. L Berlin, 1992,
Fundamentals of remote Sensing and Airphoto Interpretation, Fifth Edition,
New York, Macmillan Publishing Company, 472 p. [pages 51-67]
Estes, J.E. , E.J. Hajic, and L.R.
Tinney (Author-editors), “Fundamentals of Image Analysis: Analysis of
Visible and Thermal Infrared Data”, Chapter
24, in Manual of Remote Sensing, 2nd. ed. Falls Church, Va. American Society
of Photogrammetry, pp. 987-1124.
Paine, D. E, 1981, Aerial Photography and
Image Interpretation for Resource Management, New York, John Wiley and
Sons, 571 p.
John E. Estes Slide
Collection, spaceimaging.com,
fas.org
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