Download Visual Interpretation in ARS and more Study notes Civil Engineering in PDF only on Docsity!
VISUAL INTERPRETATION OF SATELLITE IMAGES
3.1 INTRODUCTION
Image interpretation of remote sensing data is to extract qualitative and quantitative information from the photograph or imagery. It involves identification of various objects on the terrain which may be natural or artificial consists of points, lines, or polygons. It depends on the way how different features reflect or emits the incident electromagnetic radiation and their recording by a camera or sensor. In the very beginning, when digital images and computerized classification were not available, the aerial photographs were analyzed only by visual interpretation. Accuracy of the interpretation depends on the training, experience, scale of photograph, geographic location of the study area, associated map, ground observation data etc. After the availability of satellite images, the data were categorized in two processing methods: analogue aerial photographs and digital satellite images. Though satellite images can be visually interpreted and aerial photographs can be processed by computers. In image or photograph, some objects may be readily identifiable while other may not. It depends on individual perceptions and experience. The detail to which an image or photograph can be analyzed depends on the resolution of the image and scale of the photograph. Satellite images are generally have small scale than aerial photographs and cannot be analyzed stereoscopically.
3.2 ELEMENTS OF VISUAL INTERPRETATION
In our daily life we interpret many photos and images, but interpretation of aerial photographs and images are different because of three important aspects: (1) The portrayal of features from an overhead, often unfamiliar, perspective; (2) The frequent use of wavelengths outside of the visible portion of the spectrum; and (3) The depiction of the earth’s surface at unfamiliar scales and. eight fundamental parameters or elements are used in the interpretation of remote sensing images or photographs. These are tone or color, texture, size, shape, pattern, shadow, site and association. In some cases, a single such element is alone sufficient for successful identification; in others, the use of several elements will be required.
Fig. 1. Ordering of image elements in image interpretation.
i) Tone or color : Tone is the relative brightness of grey level on black and white image or color/F.C.C
image. Tone is the measure of the intensity of the reflected or emitted radiation of the objects of the terrain. Lower reflected objects appear relatively dark and higher reflected objects appear bright. Figure 9.1a represents a band imaged in NIR region of the electromagnetic spectrum. Rivers does not reflect in NIR region thus appear black and the vegetation reflects much thus appears bright. Our eyes can discriminate only 16-20 grey levels in the black and white photograph, while more than hundreds of color can be distinguished in a color photograph. In multispectral imaging, optimal three bands are used to generate color composite image. False Color Composite (FCC) using NIR, red and green are most preferred combination for visual interpretation. In a standard FCC, NIR band passes through red channel, red band passes through green channel and green band passes through blue channel. Vegetation reflects in NIR region of the electromagnetic spectrum therefore in standard FCC vegetation appears red (Fig. 2b), much which is more suitable in vegetation identification.
appropriate result more quickly. The most measured parameters are length, width, perimeter, area, and occasionally volume. 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. Fig. 4. Satellite view of a part of a city.
v) Shape: Shape refers to the general form, configuration or outline of an individual object. Shape is one of
the most important single factors for recognizing object from an image. Generally regular shapes, squares, rectangles, circles are signs of man-made objects, e.g., buildings, roads, and cultivated fields, whereas irregular shapes, with no distinct geometrical pattern are signs of a natural environment, e.g., a river, forest. In a general case of misinterpretation in between roads and train line: roads can have sharp turns, joints perpendicularly, but rails line does not. From the shape of the following image, it can be easily said that the dark-blue colored object is a river. Fig. 5. Satellite image of an area
vi) Shadow: Shadow is a helpful element in image interpretation. It also creates difficulties for some objects
in their identification in the image. Knowing the time of photography, we can estimate the solar elevation/illumination, which helps in height estimation of objects. The outline or shape of a shadow affords an impression of the profile view of objects. But objects within shadow become difficult to interpret. Shadow is also useful for enhancing or identifying topography and landforms, particularly in radar imagery. Fig. 6. Shadow of objects used for interpretation.
vii) Association: Association refers to the occurrence of certain features in relation to others objects in the
imagery. In urban area a smooth vegetation pattern generally refers to a play ground or grass land not agricultural land (Fig 7). Fig. 7. Satellite image of an urban area.
viii) Site: Site refers to topographic or geographic location. It is also an important element in image
interpretation when objects are not clearly identified using the previous the elements. A very high reflectance feature in the Himalayan valley may be snow or cloud, but in Kerala one cannot say it as snow.
For agricultural and tree species identification a number of keys have been successfully employed used on a region-by-region and season-by-season basis, as vegetation can widely vary depending on location and season. Besides these, the time the photograph is taken, film type, and photo-scale should be carefully considered while developing interpretation keys. The Table 1 shows an example of interpretation keys for forestry mapping. The keys are specified with respect to the crown’s shape, rim shape of the crown, tone, shadow, projected, tree shape, pattern, texture, and other factors. Table 1 Interpretation keys for forestry mapping Species Crown shape Edge of Crown Tone Pattern Texture Cedar Conical with sharp spear Circular and sharp Dark Spotted grain Hard and coarse Cypress Conical with round crown Circular but not sharp Dark but lighter than cedar Spotted Hard and fine Pine Cylindrical with shapeless crown Circular but unclear Light and unclear Irregularly spotted Soft but coarse Larch Conical with unclear crown Circular with unclear edge Lighter than cypress Spotted Soft and fine Fir/spruce Conical with wide crown Circular with zig-zag edge Dark and clear Irregular Coarse Deciduous Irregular shapes Unclear Lighter Irregular Coarse
(Source: Bhatta, 2008)
3.5 APPLICATION OF PHOTO INTERPRETATION
Aerial photos have been used for several applications. Using the principles of visual photo interpretation, information about the earth's surface features, can be obtained. Some of the areas where aerial photos have been extensively used are: (i) Topographical mapping, (ii) Geology, (iii) Soil mapping, (iv) Forestry, (v) Terrain evaluation, (vi) Land use/Land cover mapping, (vii) Agriculture, (viii) Water resources, and (ix) Environmental studies/flood damage studies.
(i) Topographical Mapping
The basis of topographical mapping using aerial photograph is the common overlap between two successive photos in the forward and lateral direction. The forward direction is the direction of the aircraft flight and the minimum forward overlap required is 60 percent. The lateral overlap, at right angles to the forward direction can vary from 25 to 30 percent. The overlapping photos are placed in the so-called stereo projectors in such a manner that the model of the ground is recreated and the observer sees the three-dimensional view of the ground. For this it is necessary to create geometrical conditions between the two photos in the stereo projector in such a way that the inclination between the two photos in the stereo projector in such a way that the inclination between the two successive photos is exactly the same as at the time of taking pictures by the aerial camera. It is possible to draw not only the planimetry but also the contours at desired scale and interval by suitable selection of ground control points. These ground control points appear on the aerial photos. The biggest advantage of aerial mapping lies in cost and efficiency. Whereas a very large number of such control points are required for mapping by traditional ground survey methods, such as plane tabling, a much less density of control points is required by aerial methods. The time required by photo mapping is also much less compared to ground methods. At present, the accuracy achieved is so much that the aerial mapping has almost become a universal technique of mapping.
different Individual landscape elements on the photos. These physiographic systems may be related more specifically to conditions created by stratigraphy, block faulting, ground water and surface water hydrology, geomorphology, erosion and weathering processes, wind action or sediment logy, but more generally they are related to combinations of these conditions and processes.
(iv) Forestry
The main application of photo interpretation to Forestry involves: (i) Preparation of a Base Map, (ii) Identification of tree species, (iii) Quantitative measurements about the density of trees in a given area, height of trees and crown shapes and volumes. The principles of photo interpretation as explained earlier such as shape, size, pattern, shadow, tone and texture are applied for (i), (ii) mentioned above. Individual tree species have their characteristic shape and size. With some ground knowledge and experience it is possible to identify these on the air photos. Shadows of trees help to know their shape in profile. Changes of tone and texture also help in identification of species. Tress also gives a peculiar pattern depending upon underground moisture condition. Trees along a straight line may be due to underground water channel. Trees also occur in clumps. Knowledge about tree height can easily be obtained using a simple parallax bar with viewing under stereoscope. Physiographic conditions also help in identification of tree types. For example, Sal trees generally grow on low hills up to 1000 meters and Pine trees up to 2000 meters or more of terrain heights above sea level.
(v) Terrain Evaluation
The study of terrain is essential and a prerequisite for proper planning and utilization of land resources. The purpose could be a short term military requirement for certain localized zones, a long term peace time need like development and exploitation of mineral resources, availability of construction material, exploration for ground water etc. The aim of terrain study is to gather maximum and systematic information on various aspects of the ground so that proper evaluation of this information can be done to meet the requirements of different users. A system of classification of terrain is, therefore, necessary where a given area can be subdivided into basic units. The question becomes more important for an inaccessible area where the only available tool is the aerial, photo and the concept of 'known to the unknown' will hold good. In fact the aerial photos, when seen under a stereoscope, bring the 'Ground' to the laboratory. The two important units in terrain classification are Pattern is Facet. A Facet is the fundamental unit in the classification. It is desirable as a piece of
ground possessing uniform physical properties for all practical purposes. It is classified on the basis of surface configurations, nature of surficial deposits, surface and subsurface water region, land use and its associations with other terrain units. A landscape pattern is an area or areas of regularly occurring pattern of topography and surficial deposits including soils. The landscape pattern is recognized on the basis of geologic set up, climate and topography. The following information is recorded for each terrain unit: Terrain Unit Pattern Facet Type of Information Card Pattern Card, Pattern Description, Climate Facet Card, Ground Description, Surficial deposit and soils, Land use, Vegetation, Soil Properties and Classification, Engineering Resources and Water Supply. The detailed method of Terrain study IS as follows: Area of study
- Library Work : Collection of basic material and background information
- Laboratory Exercise : Scanning of air photos, checking of quality, scale etc., study of maps and delineation of major units, preliminary list of landscape Patterns and Facets
- Ground Reconnaissance : Initial Field Traverses
- Study of Aerial Photos : Delineation of facets and planning of field work
- Detailed Field Work o Checking of photo-characteristics, recording terrain data, collecting soils samples, field tests for moisture content and soil strengths o Final list of Pattern and Facets
- Laboratory Tests : Soil tests, rock identification etc.
- Completion of Reports : Confirmatory recce, if required finalization of report, typing of Cards and preparation of maps Main Report, Appendices (pattern, Facets and information cards, Facet maps). The process of photo-interpretation can be divided into steps like observation and measurements, logical reasoning and influence. The equipment required is simple and constitutes pocket stereoscope for quick scanning in the field. Mirror stereoscope for detailed work, parallax bar for height measurement and sketch master to transfer the interpreted data to the base map. In the case of terrain studies presenting in progress the details are directly transferred to existing toposheets. The recognition elements used for photo interpretation aids to describe different terrain units are : (i) Position in landscape and association with other terrain units. (ii) Morphology, surface configuration and micro relief. (iii) Drainage pattern, texture and density, internal and external drainage. (iv) Erosional features and gully sections. (v) Land use and cultural features.
(vii) Agricultural
The three major areas in which photo interpretation can help in the discipline of agriculture are : (i) Crop condition assessment, (ii) Crop type classification, (iii) Crop yield estimation. Crop type is characterized by characteristic pattern and texture on an aerial photo whereas crop condition i.e. healthy or diseased are identified by virtue of tonal and grey level changes. The crop can be damaged due to several causes such as insects, moisture excesses, iron deficiency, nitrogen deficiency, soil salinity and air pollution. The crop yields can easily be determined by determining the area under cultivation for a particular variety. The photo interpretation steps involved in agricultural studies relate to determination of drainage pattern and analysis, erosion pattern and analysis, photo tones, textures and vegetative feature and their pattern.
(viii) Water Resources
The application of photo interpretation techniques of water resources involve two types (i) mapping surface water bodies, (ii) Subsurface or ground water potential. The first part is Simple as surface water bodies like streams, rivers, lakes etc., can be easily identified on the aerial photos. Water bodies appear darker in tone compared to rest of the features because of the simple fact that most of the sunlight that enters a clear water body is absorbed within about two meters of the surface. The degree of absorption depends upon the incident wave length. This infrared light in the region of 1 to 2 micrometer wave lengths is absorbed within a length of a meter for the surface. The ground water location on the other hand is not so simple as this is a subsurface feature. The indicators of ground water are topography and vegetations namely presence of springs and wells and other seepages. Present photo interpretation techniques cannot be applied for estimation or determination of ground water depth. The other features of the interpretation techniques are the delineation of watershed and it’s assessment for reservoir selection and snow cover mapping, which help in determination water available from snow melt. The underground or buried stream can also be mapped for vegetation patterns on the surface along their streams and which also help in locating possible ground water zones.
(ix) Environmental Studies/Flood Damage Studies
Under thus heading, the areas of study are (i) Water pollution, (ii) Deforestation and denudation, (iii) Industrial pollution. Water pollution is caused by organic waste from domestic sewage and industrial wastes, production of excessive algae and water weeds and sediments brought down by rivers etc. Sometimes pollution can often be seen on aerial photos and delineated due to different tones and textures. The pure water appears with light grey and polluted water with darker tone. Deposits of oil on water also show darker tone. Use of aerial photos for flood damage can also be done by taking aerial photos before and after floods. Areas prone to successive flooding can be marked and preventive measures can be adopted.
3.6 INTEGRATION OF GIS AND REMOTE SENSING
No one definition of integration between GIS and remote sensing exists. Instead, integration has been used to refer indiscriminately to almost any type of connection, ranging from pragmatic computational amalgamation of data to the conceptual understanding of how geographic features are interrelated. Ever since its formalism by the NCGIA Initiative-12 in 1990 (Star et al., 1991), the move towards ‘seamless’ and ‘hybrid’ integration of data, techniques and organization from the geographic information systems (GIS) domain with those from the remote sensing2 sphere has been arduous, sporadic and irresolute.
1. Evolutionary integration
For some, complete or total integration between GIS and remote sensing is the ultimate goal. Ehlers et al.(1989) proposed three stages in the evolution of integration that focused on the degree of interaction between data models, the level of data exchange, the pursuit of close geometric registration, the matching of cartographic representation, a parallel user interface, and the compatibility of geographic abstraction. The three stages of the evolution are as follows: Stage 1 would focus on the separate but equal development of databases from each technology. Data would be exchanged in predominantly vector format (for GIS) and raster models (for remote sensing) but capable of being simultaneously displayed by overlays. Analysis would be limited to the update of GIS coverages by the positional comparison of thematic attributes generated from classified remotely sensed images; or the use of GIS data for facilitating image georegistration. Stage 2 oversees the continuation of separate databases, but each technology would share a user
Scientist combine information provided from all three types of images to forecast the weather, monitor forest fires, ice flows, ocean currents, and long term climate patterns from the unique global perspective that only satellites can provide.
1. VISIBLE SATELLITE IMAGE
In the solar spectrum, or the shortwave, very little absorption occurs in the visible spectral region; however, scattering of light is large. A visible satellite image represents sunlight scattered by objects suspended in the atmosphere or on Earth , obviously visible images are only available during the day. Differences in the reflected radiation of clouds, water, land, and vegetation allow us to distinguish these features in the imagery. Dark areas in a visible satellite image represent geographic regions where only small amounts of visible sunlight are reflected back to space. Areas of white indicate clouds while shades of gray indicate generally clear skies. The oceans are usually dark while snow and thick clouds are bright.
2. INFRARED SATELLITE IMAGE
All objects emit radiation in amounts related to their temperature and their ability to emit radiation. An infrared (IR) instrument provides information on the temperature of land, water, and clouds by measuring the infrared radiation emitted from surfaces below the satellite. The radiant energy measured by infrared radiometers is converted to a temperature. In infrared images, cold objects are white and hot surfaces appear black.
3. WATER VAPOR SATELLITE IMAGE
Water vapor imagery is a valuable tool for weather analysis and forecasting, because it represents flow patterns of the upper troposphere. Water vapor is transparent to radiation at visible and 10-12 micron wavelengths. This is why visible and IR satellite imagery are used to observe surface features and clouds. However, water vapor is a very efficient absorber and emitter of radiation with wavelengths between 6.5 and 6.9 microns. So, satellite radiometers measuring the amount of radiation emitted by the atmosphere at these wavelengths can be used to detect water vapor in the atmosphere. The water vapor satellite image displays the water vapor concentration in the atmospheric layer between 600 and 300 millibars, or approximately 4000 to 9000 meters (2.5 ~ 5.6 miles) above the surface of the earth. This is the middle and upper parts of the troposphere, a key region for storm development and growth. By combining information from visible, IR and water vapor images, scientists can obtain a very thorough
