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2.4 Aids to and Techniques of Image Interpretation

2.4.1 Collateral Material

A review of existing source material concerning a given area, process; type of facility, or object, can aid in the interpretation of remotely sensed data. The use of collateral material may also result in a better definition of the scope, objectives, and problems associated with a given project. Collateral material can be data of many types and can exist in either analog or digital form. Collateral material has also been called in the literature ancillary data, and site scientific literature. Collateral material may come in the form of text, tables, maps, graphs, or image data/information (metadata). Census data, a map of flora of a given area, a land use map of an area, meteorological statistics, or agricultural crop reports can all be used in support of a given interpretation. Basically, collateral material represents data/information that an intepreter may use to aid in his/her accomplishment of a given analysis task. Material contained within a Geographic Information System (GIS) that is used to assist an interpreter in an analysis can be considered collateral data. Two classes of collateral data deserve special mention here. These are photo/image interpretation keys and field verification.

2.4.1.1 Image Analysis Keys - A photo/image interpretation key is a set of guidelines used to assist interpreters in rrapidly identifying photo/image features. Determination of the type of key and the method of presentation to be employed will depend upon: 1.) The number of objects or conditions to be identified; and 2.) 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. That is, a key can be developed to aid the interpreter as he/she analyzes any type of imagery. 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 be very difficult and a reason why people develop their own keys). All keys are, however, based upon the diagnostic characteristics of objects or conditions to be identified. Depending upon the manner in which the diagnostic features are organized, two types of keys are generally recognized.

Selective keys

Elimination keys (Paine 387) are arranged so that the interpreter follows a precise step-wise process that leads to the elimination of all items except the one (ones) that he/she is trying to identify. Dichotomous keys are essentially a class of elimination key. Most interpreters prefer to use elimination keys in their analyses, although studies have revealed no significant difference between the results achieved from the use of the two types of keys as long as the material within each key is well organized.

2.4.1.2 Field Verification - Field verification can be considered a form of collateral material. Field verification is typically conducted to assist in the analysis of the data to be analyzed. Essentially, this is familiarizing the interpreter with the area or type of feature or object to be interpreted. This type of verification is done prior to the interpretation. After an interpretation, field verification can be accomplished to verify the accuracy of the interpretation conducted. Field work can be very expensive, thus this type of activity must be carefully planned. Field work 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? Will demographic information be collected and will a human subjects release form be necessary? Will photos from light aircraft or higher resolution air or satellite photos be used to verify analyses (e.g. Thematic mapper to verify AVHRR or Airphotos to verify TM). The amount and type of field work required for a given project may vary greatly and is generally dependent upon:

The type of analysis involved;
Image quality, including scale resolution and information to be interpreted;
The accuracy requirements for both classification, and boundary delineation;
The experience of the interpreter and his/her knowledge of the sensor, area, and subject to be interpreted; and,
The terrain conditions and the accessibility of the study area.

For various reasons, an area may be inaccessible and the existence and/or availability of other source material limited.

2.4.2 Handling of Imagery

Although a good deal of photo interpretation is still accomplished using paper prints, the use of 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. Basically prints typically are numbered and should be kept in order in so far as practical. Flight lines might be kept separate. Different dates should be kept separate, etc. When transparencies are on rolls, they are easy to keep in order. If individual frames are cut from the roll, the task of keeping things in order becomes more difficult. Try to keep track of where frames come from and who has them. Any time transparencies are used, surfaces should be as clean as possible. The interpreter should either wear cotton gloves or be sure not to touch the emulsion surface as skin oils can cause image deterioration.

2.4.3 Stereoscopic Viewing

Binocular vision is an important part of most of our daily lives. It needs to be fully understood and consciously exploited by the image interpreter. Although many remote sensor systems can be employed to acquire stereo data; most stereoscopic viewing for interpretation purposes is done from vertical or near vertical aerial photography acquired by conventional aerial camera systems. When learning to interpret stereo data, the interpreter should:

Make certain that the photos are properly aligned at all times, preferably with the shadows falling toward the viewer.
Be careful to keep the eye base and the long axis of the stereoscope parallel to the flight line at all times.
Maintain an even glare free illumination on the prints or transparencies being studied.
Arrange for comfortable seating.
Keep the lenses of the stereoscope clean, properly focused and separated to your interpupillary distance.
The novice interpreter should not work with stereo more than thirty minutes out of any hour period.

Interpreters who have difficulty with stereo should be aware of the following:

A person's eyes may be of unequal strength. If a person normally wears glasses for reading or close-up work, one should also wear glasses when using the stereoscope.
Poor illumination, misaligned prints or uncomfortable viewing positions may result in eye fatigue.
Illness or severe emotional distress may create sensations of dizziness in one using a stereoscope.
Reversal of prints may cause pseudo-stereo. A similar problem may occur if prints are aligned with the shadows falling away from rather than towards the interpreter.
Objects that change positions between exposures cannot be viewed in stereo.
In areas of steep topography, scale differences in adjacent photographs may make it difficult to obtain a three dimensional image.
Dark shadows or clouds may prohibit stereo viewing of an area by obscuring an object on one photo.
Individuals who have continued difficulties using stereoscopes should not attempt to master the art of stereoscopic vision with the unaided eye.

2.4.4 Use of Multiple Images

Multi-Station - Not to be confused with multi-stage. This is the successive overlapping of photographs taken along a given flight line as being flown by an aircraft or a satellite.

Multi-Band - Often seen to have an overlapping meaning with the next term, multispectral. As used by here, multiband 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).

Multi-Spectral - The use of images from various regions of the EM spectrum (e.g. visible, infrared, and microwave).

Multi-Date - The use of images taken over time of a given area.

Multi-Stage - This typically means the acquisition of images from platforms flying at different altitudes (e.g. the use of data flown by low flying aircraft, high flying aircraft, and satellite data in a given study). It 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 subsamples of the area being studied.

Multi-Polarization - Objects in the environment exhibit different qualities with respect to the ability to rotate a signal returned to a sensor.

Multi-Direction - Most sensors employed in remote sensing are aimed vertically (SAR being the major exception). There are times when more information can be obtained using viewing angles other than vertical.

Multi-Enhancement - There are many types of enhancement available to the image analyst today (e.g. optical, electronic, computer assisted, multi-date, multi-band, multi-spectral).

Multi-Disciplinary - Basically, no one interpreter can know everything about the Earth system. By using teams of interpreters with expertise in different disciplinary areas, more information may often be gained for a given application.

Multi-Thematic - Remote sensing images are a one time write, many time read. Many different themes (e.g. hydrology, vegetation, transportation, urban areas, etc.) can be extracted from a single set of images.

Multi-Use - Many types of individuals from environmental planners and resource managers to public policy decision makers can use the output of an image analysis task.

2.4.5 Methods of Search

The interpreter should always keep in mind the basic qualities of the imagery he/she is dealing with, e.g. film filter combinations; season of acquisition; and time of day of acquisition; scale, etc. In addition, the interpreter should always remember to examine all the title information (peripheral information) on an image.

Interpretation should be conducted logically one step at a time.
Interpretation should begin with the general and proceed to the specific.
Interpretation should proceed from the known to the unknown.

2.4.6 Convergence of Evidence

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. Deductive interpretation requires either the conscious or unconscious consideration of all of the elements of image interpretation that we have been discussing. The completeness and accuracy of an interpretation is to some measure, proportional to the interpreter's understanding of the how and the why of the elements, techniques, and methods of interpretation that we have been discussing.

2.3 Elements of Image Interpretation | 2.4 Aids to and Techniques of Image Interpretation | References