• Spacing,
  
  
• Site, and
  
  
• Association
  

While these signatures were originally developed for conventional photography, many remain relevant for sensor imagery too.


Signatures:

SHAPE

Shape is the general form or outline of an object. Man tends to build things with clearly defined edges and straight lines that intersect at right angles. Nature, on the other hand, produces irregular, meandering lines which seldom intersect at right angles. Compare, for example, the manmade canal with the river. But even when we are discussing things made by man, shape can distinguish between various objects. Look at the differences between a road and a rail line. Roads are marked by sharper curves and intersections that form a right angle. Rail lines must have sweeping curves, no steep grades, and intersections form “Vs”. Similarly, aircraft, for example, may be recognized by the shape of their wings.

SIZE

This aspect considers the dimensions of the object or feature. It can be only the relative size compared with that of known objects nearby, or the image measure and the actual size determined. The importance of size as a signature is clear when you consider that objects may be identified to model within a type based solely on this single consideration. For example, the size of a mortar base provides a clue to the caliber of the weapon. Also, the spacing between gun emplacements can serve to define the type of weapon and caliber. The wing span on an aircraft can serve to identify one specific aircraft type from another.

SPACING

Spacing is the arrangement of objects, especially those repeated. Visually, it might be the random spacing of trees by nature versus that of an orchard. Man’s habits and reluctance to change aid the identification process. The bunkers in an ammunition storage area are a classic example.

SITE

This is the location of an object in relation to its environment. Many objects will stand out simply because they do not belong in the area or place they are in. This particular aspect is very useful to delineate function or role, for example, distinguishing the headquarters buildings from the airfield control tower or a missile site from a storage facility. In some instances, a facility being constructed for a specific weapon systems has a unique pattern, allowing identification of the associated system’s presence even before the actual equipment arrives.

SHADE

This signature component refers to the gray tone of the object as it appears on the imagery. Types of vegetation and foilage are distinguishable on imagery solely based on their tonal response. Imagery shade is a direct result of the objects surface texture. Unpainted metal objects reflect brightly, and show up white on the imagery, for example. Shade is particularly useful in distinguishing camouflage from natural vegetation.

SHADOW

Shadow adds dimension to the length and width of objects and features. It will also serve to profile the structure of an object which is extremely useful in identification. For example, looking straight down on a lattice—mast mounted radar antenna affords a unique perspective with limited information for identification. However, because the sun provides shadows of this item on the ground, thus providing a “side” view, recognition of the radar antenna is often possible.

SURFACE

Surface is the frequency of change within the photographic image. Smooth surfaces render a lighter tone while rough surfaces tend to reflect darker. Trails made across a grass area offer a different tone than the grass that is standing upright and has not been pushed down.

ASSOCIATION

Some objects are commonly associated with other objects so routinely that one indicates or confirms the presence of the other. A combination or arrangement of features may be used to identify almost any military installation. Similarly, the presence of certain radar systems can indicate a specific SAM system. Convergence of Evidence, that is, the combining of certain associated pieces of information, is a very significant interpretive technique that each image analyst must develop and use.

Supporting Factors:

SEASON OF THE YEAR

This factor directly affects the image content. In the fall and winter deciduous vegetation is dormant, with no leaf cover. Snow may cover certain information that is normally present, and reveal other aspects. Winter coverage may be used in some areas to define information otherwise covered by tree canopy. I should mention too that seasonal solar elevation and available light influence the acquisition of imagery.

SCALE

Scale is the relationship of the photographic image size to the ground size of the object. In maps we express the scale as a representative fraction, one unit of the map being equal to a certain number of ground units. In aerial photography a representative fraction is also used. It is called the photo scale reciprocal, or PSR. The scale of imagery is important in establishing what levels and types of exploitation will be possible. Some examples of the scale required for specific levels of interpretation are shown in the table below.

SCALES REQUIRED FOR INTERPRETATION

SEQUENTIAL PHOTOGRAPHS

Imagery of the same area or target taken at different times, often called comparative cover or change detection coverage. Through this type of coverage effort it is frequently possible to detect subtle changes, differences in order of battle, activity levels, and new construction.

STEREOSCOPY

This is the science of viewing illustrations and photography while (mentally) seeing the third dimension. It delineates changes in terrain, buildings, trees, specific facility, and or site features; anything that has height. Structural elements become evident, often even without shadows and other aids. Many bits of important information not discernible on monoscopic coverage are evident in stereo.

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