Satellite Image Interpretation:

Methods of Image Interpretation:

• Visual

1. Visual image interpretation on a hardcopy image/photograph

2. Visual image interpretation on a digital image

• Digital image processing

Types of interpretation:

• Qualitative

• Quantitative

Basic Principle of Image Interpretation:

• Image is a pictorial representation of pattern of landscape.

• Pattern indicates type of objects and their physical, biological, and cultural relationships

• Similar objects under similar conditions reflect similarly.

• A systematic examination of photos and supporting material.

• Interpretation is made of physical nature of the object.

• Information extracted is proportional to knowledge, skill and experience of analyst; the methods and equipment used.

Factors governing interpretability:

• Training, Experience

• Nature of object or phenomenon

• Quality of photographs

• Equipment and method of interpretation

• Interpretation keys, guides, manuals and other aids

• Prior knowledge of the area.

Methodology depends on basic function:

• Kind of information to be interpreted

• Accuracy of the results to be obtained

• The reference level of the person executing the interpretation

• Kind and type of imagery or photographs available

• Instruments available

• Scale and other requirements of the final map

• External knowledge available and any other sensory surveys that have been or will be made in the near future in the same area.

Activities of Image Interpretation:

• Detection

• Recognition

• Analysis

• Deduction

• Classification

• Idealization

• Convergence of evidence

Elements of Image Interpretation: 

• Recognizing targets is the key to interpretation and information extraction.

• Observing the differences between targets and their backgrounds involves comparing different targets based on any, or all, of the visual elements of tone, shape, size, pattern, texture, shadow, and association.

Tone:

• Tone refers to the relative brightness or color of objects in an image.

• Generally, tone is the fundamental element for distinguishing between different targets or features.

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

Figure 1: Tone (Source: NASA Earth Observatory)

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 land irrigated by rotating sprinkler systems would appear as circular shapes.

Figure 2: Shape (Source: NASA Earth Observatory)

Size:

• Size of objects in an image is a function of scale.

• It is important to assess the size of a target relative to other objects in a scene, as well as the absolute size, to aid in the interpretation of that target.

• A quick approximation of target size can direct interpretation to an appropriate result more quickly. 

For example, if an interpreter had to distinguish zones of land use, and had identified an area with a number of buildings in it, large buildings such as factories or warehouses would suggest commercial property, whereas small buildings would indicate residential use.

Figure 3: Size (Source: ESA)

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.

Figure 4: Pattern (Source: NASA Earth Observatory)

Figure 5: Pattern (Source: NASA Earth Observatory)

Texture:

• Texture refers to the arrangement and frequency of tonal variation in particular areas of an image.

• Texture is one of the most important elements for distinguishing features in radar imagery.

Rough textures would consist of a mottled tone where the grey levels change abruptly in a small area, whereas smooth textures would have very little tonal variation.

Figure 6: Texture (Source: Earth Observation)

Shadow:

• Shadow may provide an idea of the profile and relative height of a target or targets which could 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.

Figure 7: Shadow (Source: GIS Lounge)

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.

Figure 8: Association (Source: National Geographic Society)

Commercial properties may be associated with proximity to major transportation routes, whereas residential areas would be associated with schools, playgrounds, and sports fields.