Visual Image Interpretation PDF

IMAGE INTERPRETATION AND DATA ANALYSIS VISUAL IMAGE INTERPRETATION ❖ In order to take advantage of and make good use of remote sensing data, we must be able to extract meaningful information from the imagery - interpretation and analysis ❖ Interpretation and analysis of remote sensin...

IMAGE INTERPRETATION AND DATA ANALYSIS VISUAL IMAGE INTERPRETATION ❖ In order to take advantage of and make good use of remote sensing data, we must be able to extract meaningful information from the imagery - interpretation and analysis ❖ Interpretation and analysis of remote sensing imagery involves the identification and/or measurement of various targets in an image in order to extract useful information about them. ❖ Targets in remote sensing images may be any feature or object which can be observed in an image, and have the following characteristics: ❖ Targets may be a point, line, or area feature. ❖ They can have any form, from a bus in a parking lot or plane on a runway, to a bridge or roadway, to a large expanse of water or a field. ❖ The target must be distinguishable; it must contrast with other features around it in the image. ❖Visual interpretation may also be performed by examining digital imagery displayed on a computer screen. ❖Both analog and digital imagery can be displayed as black and white (also called monochrome) images, or as colour images by combining different channels or bands representing different wavelengths. ❖When remote sensing data are available in digital format, digital processing and analysis may be performed using a computer. ❖Digital processing may be used to enhance data as a prelude to visual interpretation. ❖Digital processing and analysis may also be carried out to automatically identify targets and extract information completely without manual intervention by a human interpreter. However, rarely is digital processing and analysis carried out as a complete replacement for manual interpretation. ❖Often, it is done to supplement and assist the human analyst. ❖Manual interpretation and analysis dates back to the early beginnings of remote sensing for air photo interpretation. ❖Digital processing and analysis is more recent with the advent of digital recording of remote sensing data and the development of computers. ❖Both manual and digital techniques for interpretation of remote sensing data have their respective advantages and disadvantages. ❖Generally, manual interpretation requires little, if any, specialized equipment, while digital analysis requires specialized, and often expensive, equipment. ❖ Manual interpretation is often limited to analyzing only a single channel of data or a single image at a time due to the difficulty in performing visual interpretation with multiple images. ❖ The computer environment is more amenable to handling complex images of several or many channels or from several dates. In this sense, digital analysis is useful for simultaneous analysis of many spectral bands and can process large data sets much faster than a human interpreter. ❖ Manual interpretation is a subjective process, meaning that the results will vary with different interpreters. ❖ Digital analysis is based on the manipulation of digital numbers in a computer and is thus more objective, generally resulting in more consistent results. ❖ However, determining the validity and accuracy of the results from digital processing can be difficult. ❖ It is important to reiterate that visual and digital analyses of remote sensing imagery are not mutually exclusive. Both methods have their merits. ❖ In most cases, a mix of both methods is usually employed when analyzing imagery. ❖Ultimate decision of the utility and relevance of the information extracted at the end of the analysis process, still must be made by humans. ❖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. ELEMENTS OF IMAGE INTERPRETATION VISUAL IMAGE INTERPRETATION ELEMENTS OF IMAGE INTERPRETATION SHAPE: ❖ Refers to the general form, structure, or outline of individual objects. ❖ Can be a very distinctive clue for interpretation. ❖ Regular geometric shapes are usually indicators of human presence and use. ❖ Some objects can be identified almost solely on the basis of their shapes: for example - the Pentagon Building, (American) football fields, cloverleaf highway interchanges ❖ Straight edge shapes typically represent urban or agricultural (field) targets. ❖ 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. SIZE: ❖ Must be considered in the context of the scale of a photograph. ❖ Thus, size is a function of scale. ❖ The scale will help to determine if an object is a stock pond or a lake ❖ 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. TONE: ❖ Refers to the relative brightness or color of elements on a photograph. ❖ It is, perhaps, the most basic of the interpretive elements because without tonal differences none of the other elements could be discerned. ❖ Thus, it is a 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. Three aspects of tone used in photointerpretation are: 1. Relative tonality (white, light gray, dull gray, dark gray or black) 2. Uniformity of tone (uniform, mottled, banded, scrabbled) 3. Degree of sharpness of tonal variations (sharp, gradual) TEXTURE: ❖ The impression of "smoothness" or "roughness" of image features is caused by the frequency of change of tone in photographs. ❖ Produced by a set of features too small to identify individually. Refers to the arrangement and frequency of tonal variation in particular areas of an image. ❖ 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. ❖ Smooth textures are most often the result of uniform, even surfaces, such as fields, asphalt, or grasslands. ❖A target with a rough surface and irregular structure, such as a forest canopy, results in a rough textured appearance. ❖Texture is one of the most important elements for distinguishing features in radar imagery. ❖Grass, cement, and water generally appear "smooth", while a forest canopy may appear "rough". SHADOW: ❖ Aid interpreters in determining the height of objects in aerial photographs. ❖ 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. ASSOCIATION: ❖ Some objects are always found in association with other objects. ❖ The context of an object can provide insight into what it is. For instance, a nuclear power plant is not (generally) going to be found in the midst of single-family housing. ❖ 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, commercial properties may be associated with proximity to major transportation routes, whereas residential areas would be associated with schools, playgrounds, and sports fields. In our example, a lake is associated with boats, a marina, and adjacent recreational land. PATTERN: ❖ Refers to the spatial arrangement of visibly discernible objects. ❖ The patterns formed by objects in a photo can be diagnostic. ❖ 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. Patterns resulting from particular distribution of gently curved or straight lines are common and are of geological significance. They may represent faults, joints, dykes or bedding. A single line or lineation is also an illustration of pattern and may result from an orderly arrangement of stream segments, trees, depressions or other features. Drainage patterns are important in the geologic interpretation of aerial photographs; they may reflect underlying structure or lithology. Vegetation patterns may reflect structural features or lithologic character of the rock types. Soil pattern used in engineering geology refers to the combination of surface expressions, such as landforms, drainage characteristics, and vegetation, that are used in the interpretation of ground conditions SITE: ❖ Refers to topographic or geographic location. ❖ This characteristic of photographs is especially important in identifying vegetation types and landforms. ❖ For example, large circular depressions in the ground are readily identified as sinkholes in central Florida, where the bedrock consists of limestone. ELEMENTS OF IMAGE INTERPRETATION  IMAGE INTERPRETATION STRATEGIES ❖ Field Observations – Identification in the field by observation, photography, GPS, etc. ❖ Direct Recognition – Direct recognition intuitiveness ❖ Inference – inference based on knowledge and possible surrogates ❖ Interpretive Overlays – Utilizing addition data in the form of overlays to reveal relationships ❖ Photomorphic Regions – Areas of relatively uniform tone and texture ❖ Interpretation depends on the interpretation keys which an experienced interpreter has established from prior knowledge and the study of the current images. ❖ The eight interpretation elements as well as the time the photograph taken, season, film type and photo-scale should be carefully considered when developing interpretation keys. ❖ Keys usually include both a written and image component. ❖Much interpretation and identification of targets in remote sensing imagery is performed manually or visually, i.e. by a human interpreter. ❖In many cases this is done using imagery displayed in a pictorial or photograph-type format, independent of what type of sensor was used to collect the data and how the data were collected - In this case data as being referred in analog format. ❖Remote sensing images can also be represented in a computer as arrays of pixels, with each pixel corresponding to a digital number, representing the brightness level of that pixel in the image - Digital data