Digital Image Processing Tutorial- Satellite Image Classification-Interpretation Keys
Image Interpretation Elements
As
discussed in last article about image interpretation and also defined image
interpretation elements/keys.
Elements/Keys
are below-
Figure-Use of Elements/Keys for Image Interpretation
- Size
- Shape
- Shadow
- Tone/Color
- Texture
- Pattern
- Height & depth
- Site
- Association
Color
Color is primary element and convenient to identify and recognize earth surface feature on multi band satellite image. Color images can be obtained by combination of three bands and two type of combinations can be prepared, first Natural Color composite (NCC) and the second one is False color composite (FCC).
In both type of combination color may vary for the same feature, such as vegetation appears green in NCC whereas it will appear red in FCC.
Figure- Color variation in FCC and NCC
Tone
It refers to the average brightness of an area or, in the case of color imagery, to the dominant color of the region. It depends on the nature of the surface and illumination. Smooth surfaces behave like specular reflectors, they tend to reflect radiation in a single direction. These features may appear bright or dark.
Whereas Rough surfaces behave as diffuse reflectors and Scatter radiation in all directions. Tone is also very important element for interpretation in panchromatic images (Black and White satellite image).
Figure- Tone variation
Size
Size
can be defined as dimension of a feature. The object size is most significant
distinguishable characteristic and image element for image interpretation. Size
can be represented by absolute size and relative size of object.
- Relative size determined by comparing the object with familiar nearby features. Relative size is important in differentiating between objects.
- For example during the interpretation of land use map than differentiating residential, commercial and industrial features becomes very easy by deploying relative size concept as industry and commercial building features are comparatively bigger in size with respect to residential buildings.
- Absolute size refers to derive measurements by using
satellite images/aerial photographs. In this process absolute size of object is
measured by multiplying length. But accuracy of measurement depends upon the
accuracy of geometric correction.
Figure- Showing variation in Size of features
Shape
It
can explain as the structure or outline of a feature. The shape of an object is
described as the geometric form of object appeared on image.
Shape can be in
different regular form such as square, rectangles and circles of man-made
features. For this examples can be agriculture fields in rectangle shape,
stadium in circle shape etc.
whereas natural features have irregular shape with
no distinct pattern visible such as water body, river and forest etc.
It
is Important to note that shape depends on perspective and overhead
perspective, introduces scale effects.
Figure- Type of Feature Shapes
Shadow
It
is very important elements as it help to identify object as well as create
obstacle for visual image interpretation. Shadow cast by object depends on the
nature of the object, angle of illumination, perspective, and slope of the
ground surface.
Shadow also gives height information of tall buildings as well
as shape details of object. Shadow also helpful in interpretation as it provides
information object height and its profile. So it may make identification of
feature much easier.
Shadow
at same time hide the information as many objects not appeared clearly under
the shadow and create problem in identification and reorganization of features.
Figure- Role of Shadow in Image Interpretation
Texture
It
states to the variation in tone over a surface or the apparent roughness or
smoothness of the surface in an image. Homogeneous surface will appear smooth
such as grassland/agriculture whereas non uniform surface will appear rough
such as forest area/scattered vegetation.
Figure- Texture variation of rough and smooth
Pattern
Patterns
refer to the distinctive arrangement of features. Both man made and natural
feature may arrange systematically or randomly on the earth surface. Pattern can
help to identify agriculture and urban features, such as Orchards have trees
/plant rows and Residential areas/agriculture fields have rectangular rows.
Height and Depth
Relative
heights can be used to interpret objects. In a similar way, relative depths can
often be interpreted. Height can describe as include tall, intermediate and
short; whereas depth can be defined as deep, intermediate, and shallow.
This
element facilitate to measure height/elevation and depth of feature, which can
be used to identify and discriminate an object.
Figure- Height Element to discriminate objects
Site
Site
is important element as it describe feature based on its unique physical or
socioeconomic characteristics. Some features
occupy a distinctive topographic position because of their function.
Such
as sewage treatment facilities at the lowest feasible topographic position and
power plants located adjacent to water bodies for cooling.
Figure- Importance of Site Element for Object Recognition
Association
Association
refers to the distinctive spatial interrelationships between features such as
Schools often associated with athletic fields/ground whereas large parking lots
often associated with malls/commercial/industrial/stadium buildings. This is
very important when trying to interpret an object or activity.
Association refers to the fact that
certain features and activities are almost always related to the presence of
certain other features and activities.
Figure- Showing association of features
All the above elements can be used together or in combination to identify and recognize objects in image interpretation process.
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