Digital image interpretationPattern analysisShape analysis

Texture analysis

By umar musa umar1661310004RS & GIS DEPARTMENTFACULTY OF CIVIL ENG.

IMAGE INTERPRETATIONIn it simplest definition, Image interpretation is define as the act of examining images for the purpose of identifying objects and judging their significanceImage interpretation is a process of identifying what we see on the images and communicate the information obtained from these images to others for evaluating this significance. This process, however, is not restricted to making decisions concerning what objects appear in images, but it also usually includes a determination of their relative locations and extents.

The image interpretation process can involve various levels of complexity from a simple direct recognition of objects in the scene to the inference of site conditions. An example of this can be a

national highway or a major river on the satellite imagery, more particularly on a false colour composite. If the interpreter has some experience, the interpretation of these linear features, road and river, may be straight forward.

On the other hand, the interpreter has to adopt some other image characteristics in order to infer the appearance of the objects on the image.

COMPLEXITY OF IMAGE INTERPRETATION

IMAGE INTERPRETATION KEYS

Keys that provide useful reference of refresher materials and valuable training aids for novice interpreters are called image interpretation keys.They are used for identification of complex categoriesThere are two types of keys: selective key and elimination key

A selective key is also called reference key which contains numerous example images with supporting text. The interpreter selects one example Image that most nearly resembles the feature or condition found on the Image under study.

An elimination key is arranged so thatthe interpretation proceeds step by step from the general to the specific, and leads to the elimination of all features or conditions except the one being identified.

Elimination keys are also called dichotomous keys where the interpreter makes a series of choices between two alternatives and progressively eliminates all but one possible answer.

The elimination key is used and successfully employed for agricultural studies, and forestry applications.

Process of Image InterpretationAs previously mentioned, the

image 'interpretation is a complex process of physical and psychological activities occurring in a sequence of time. The sequence begins with the

detection and Identification of Images andlater by their measurements. The image interpretation has

various aspects which have overlapping functions. These aspects are detection, recognition and identification, analysis, classification, and idealisation.

BASIC ELEMENTS OF INTERPRETATION

The basic elements of image interpretation includes the followingTone:- Tone refers to colour or reflective brightness of objects of the earth surfaces. Tone is express qualitatively as light, medium and dark,.Shape along with texture and shadow help in the interpretation.Difference of soil moisture for instance can easily be identified using tone

A) Grey scale image indicating tonal variation B) coloured image of different shade of colours

Variations in tone also allows the elements of shape, texture, and pattern of objects to be distinguished.

Texture:-This is an expression of roughness or smoothness as exhibited by imagery it is the rate of changes of tonal values,. Mathematically it is given as:

ΔD where D is the densityδX X is the distance measured from

one arbitrary stating point

Because texture is the frequency of tonal difference, it is qualitatively be expressed as course, medium and fine.

Texture variation in an image from smooth in the south to the rough in the north

Pattern:-

Pattern refers to the spatial arrangement of visibly discernible objects.

Typically an orderly repetition of similar tones and textures will produce a distinctive and ultimately recognizable pattern.

Orchards with evenly spaced trees, and urban streets with regularly spaced houses are good examples of pattern.

However, it is important to note that, pattern depend on the scale of an image

An image of a city showing characteristics pattern

Size:-

The size of object is one of its most distinguishing characteristics and one of most important element of image interpretation. Size of object in an image is also a function of scale

Measurement of size e.g. height as by using parallax bar , give clue o the nature of object.

For instance, the measurement of dip and strike of rock formation help in identifying sedimentary formation.

Shape:-

Shape refers to the general form, structure, or outline of individual objects. Shape can be a very distinctive clue for interpretation.

Straight edge shapes typically represent urban or agricultural (field) targets, while natural features, such as forest edges, are generally more irregular in shape, except where man has created a road or clear cuts. Farm or crop landirrigated by rotating sprinkler systems would appear as circular shapes.

An urban image showing different shape

Shadow:-

Shadow is also helpful in interpretation as it may provide an idea of the profile and relative height of a target or targets which may make identification easier.

However, shadows can also reduce or eliminate interpretation in their area of influence, since targets within shadows are much less (or not at all) discernible from their surroundings.

Shadow is also useful for enhancing or identifying topography and landforms, particularly in radar imagery.

A shadow casted by some images in

Site:-

Site refer to topographic or geographic location and is particularly important aid in the identification of vegetation type.

For example, certain tree species would be expected to occur on well drained upland site, whereas the trees species would be expected to occur on poorly drained lowland site.

Similarly sudden appearance or disappearance of vegetation is a good clue to the underlying soil type or drainage

Association:-

Association takes into account the relationship between other recognizable objects or features in proximity to the target of interest. The identification of features that one would expect to associate with other features may provide information to facilitate identification. In the example given above, commercial properties may be associated with proximity to major transportation routes, whereas residential areas would be associated with schools, playgrounds, and sports fields.

In ourexample, a lake is associated with boats, a marina, and adjacent recreational land.

Resolution:-

This depend s on many factors, but always some objects are too small or have too little contrast with their surroundings to be clearly seen on the images.

Thus, a spectral and spatial resolution of an image gives an important clue for interpretation and therefore, have to be considered before in perpetration

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