5 5 Digital Image Digital Image

Processing Processing

Digital Image Processing Digital Image Processing

Digital Image Processing undergoes Digital Image Processing undergoes three general steps: three general steps:

PrePre--processing processing

Display and enhancement Display and enhancement

Information extraction Information extraction

FlowchartFlowchart

PrePre--Processing of the Remotely Processing of the Remotely

Sensed ImagesSensed Images

When remotely sensed data is received from the When remotely sensed data is received from the imaging sensors on the satellite platforms it imaging sensors on the satellite platforms it containscontains flaws and deficienciesflaws and deficiencies. Pre. Pre--processing processing refers to those operations that are preliminary to refers to those operations that are preliminary to the main analysis. Preprocessing includes a wide the main analysis. Preprocessing includes a wide range of operations from the very simple to range of operations from the very simple to extremes of abstractness and complexity. These extremes of abstractness and complexity. These categorized as follow: categorized as follow:

–– Feature Extraction Feature Extraction

–– Radiometric Corrections Radiometric Corrections

–– Geometric Corrections Geometric Corrections

–– Atmospheric Correction Atmospheric Correction

The techniques involved in The techniques involved in removal of unwanted removal of unwanted and and distracting elements distracting elements such as image/system such as image/system noisenoise, atmospheric interference and sensor , atmospheric interference and sensor motion from an image data occurred due to motion from an image data occurred due to limitations in the sensing of signal digitization, or limitations in the sensing of signal digitization, or data recording or transmission process. Removal data recording or transmission process. Removal of these effects from the digital data are said to of these effects from the digital data are said to be "restored" to their correct or original be "restored" to their correct or original condition, although we can, of course never know condition, although we can, of course never know what are the correct values might be and must what are the correct values might be and must always remember that attempts to correct data always remember that attempts to correct data what may themselves introduce errors. Thus what may themselves introduce errors. Thus image restoration includes the efforts to correct image restoration includes the efforts to correct for both radiometric and geometric errors. for both radiometric and geometric errors.

PrePre--processingprocessing consists of those operations that prepare data for consists of those operations that prepare data for

subsequent analysis that attempts to correct or subsequent analysis that attempts to correct or compensate for systematic errors.compensate for systematic errors.

The digital imageries are subjected to several The digital imageries are subjected to several corrections such as corrections such as geometricgeometric, , radiometricradiometric and and atmosphericatmospheric, though all these correction might , though all these correction might not be necessarily be applied in all cases.not be necessarily be applied in all cases.

These errors are systematic and can be removed These errors are systematic and can be removed before they reach the user.before they reach the user.

geometricgeometric

RRadiometric correctionadiometric correction Data normally involves the processing of digital Data normally involves the processing of digital

images to improve the fidelity of the images to improve the fidelity of the brightness brightness value magnitudesvalue magnitudes (as opposed to geometric (as opposed to geometric correction which involves improving the fidelity of correction which involves improving the fidelity of relative spatial or absolute relative spatial or absolute locationallocational aspects of aspects of image brightness values). image brightness values).

The main purpose for applying radiometric The main purpose for applying radiometric corrections is to reduce the influence of errors or corrections is to reduce the influence of errors or inconsistencies in inconsistencies in image brightness image brightness values that values that may limit one's ability to interpret or quantitatively may limit one's ability to interpret or quantitatively process and analyze digital remotely sensed images.process and analyze digital remotely sensed images.

Throughout this section, radiometric errors and Throughout this section, radiometric errors and inconsistencies will be referred to as "noise", which inconsistencies will be referred to as "noise", which could be considered any undesirable spatial or could be considered any undesirable spatial or temporal variations in image brightness not temporal variations in image brightness not associated with variations in the imaged surface.associated with variations in the imaged surface.

Atmospheric CorrectionAtmospheric Correction

objective of objective of atmospheric correction atmospheric correction is to is to retrieve retrieve the surface reflectance the surface reflectance (that characterizes the (that characterizes the surface properties) from remotely sensed imagery surface properties) from remotely sensed imagery by removing the atmospheric effects. Atmospheric by removing the atmospheric effects. Atmospheric correction has been shown to significantly improve correction has been shown to significantly improve the accuracy of image classification.the accuracy of image classification.

• The investigator should decide which pre-processing techniques are relevant on the basis of the nature of the information to be extracted from remotely sensed data.

• After pre-processing is complete, the analyst may use feature extraction to reduce the dimensionality of the data.

• Thus feature extraction is the process of isolating the most useful components of the data for further study while discarding the less useful aspects (errors, noise etc).

• Feature extraction reduces the number of variables that must be examined, thereby saving time and resources.

Image EnhancementImage Enhancement operations are carried out to improve the operations are carried out to improve the

interpretability of the image by increasing interpretability of the image by increasing apparent apparent contrastcontrast among various features in the scene.among various features in the scene.

The enhancement techniques depend upon two factors The enhancement techniques depend upon two factors mainly:mainly:

1.1. The digital dataThe digital data (i.e. spectral bands and resolution) (i.e. spectral bands and resolution)

2.2. The The objectives of interpretation objectives of interpretation

As an image enhancement technique often As an image enhancement technique often drasticallydrastically alters the alters the original numeric dataoriginal numeric data, it is normally used only for , it is normally used only for visual visual (manual) interpretation and not for further numeric (manual) interpretation and not for further numeric analysisanalysis..

CommonCommon enhancements include enhancements include image reductionimage reduction, , image image rectificationrectification, , image magnificationimage magnification, , transect extractiontransect extraction, , contrast adjustmentscontrast adjustments, , band band ratioingratioing, , spatial filteringspatial filtering, , Fourier transformationsFourier transformations, principal component analysis and , principal component analysis and texture transformationtexture transformation

Information ExtractionInformation Extraction is the last step toward the final output of the image is the last step toward the final output of the image

analysis. analysis.

After preAfter pre--processing and image enhancement the processing and image enhancement the remotely sensed data is subjected to quantitative remotely sensed data is subjected to quantitative analysis to assign individual pixels to specific classes.analysis to assign individual pixels to specific classes.

Classification of the image is based on the known and Classification of the image is based on the known and unknown identity to classify the remainder of the unknown identity to classify the remainder of the image consisting of those pixels of unknown identity. image consisting of those pixels of unknown identity.

After classification is complete, it is necessary to After classification is complete, it is necessary to evaluate its accuracy by comparing the categories on evaluate its accuracy by comparing the categories on the classified images with the areas of known identity the classified images with the areas of known identity on the ground.on the ground.

The final result of the analysis consists of The final result of the analysis consists of mapsmaps(or images), (or images), datadata and a and a reportreport. .

These three components of the result provide the These three components of the result provide the user with full information concerning the source user with full information concerning the source data, the method of analysis and the outcome data, the method of analysis and the outcome and its reliability. and its reliability.