Digital Image Processing
Digital image processing represents a set of specialized techniques as well as computer processing tools used to enhance the visual appearance of as well as extract information from remotely sensed images. Moreover, it is applied in rectifying imagery to match the selected map base. The digital image processing have 5 broad categories:
- Image Restoration and Rectification
- Image Enhancement
- Information Extraction
- Biophysical modelling
- GIS Integration
Digital Image Processing
Image Restoration and Rectification
The operations of image restoration and rectification adjust the image values as well as geometry for purposes of correcting for errors and limitations in the sensor system. Evidently, this help in minimizing the atmospheric effect. Furthermore, it change the image geometry to match a convenient geographic reference system.
In image restoration and rectification uses two techniques:
- Radiometric corrections
- Geometric corrections.
Digital Image Processing
Radiometric Corrections.
The radiometric corrections are considered necessary because the sensors are usually affected by factors such as:
- Variation in scene illumination.
- Atmospheric conditions
- Sensor characteristics and viewing geometry.
The radiometric corrections are also considered critical in removing the periodic malfunctioning in the component of the sensing system. Commonly referred to as image noise, it may represents the image as;
- Missing scan lines
- Line dropouts
- Salt and pepper e.t.c.
The image correction techniques play a considerable role in eliminating the appearance of these errors. It does this through estimating the replacement values based on the neighboring pixel. Atmospheric correction usually represents a name given to a group of image processing techniques applied in reducing the degradation of image quality which is caused by atmospheric interference.
Lastly, the correction approach range from physical modelling of the energy transmission through the atmosphere to empirical approximations of spectral changes.
Digital Image Processing
Geometric Corrections
Before the remotely sensed images can be brought into a GIS system, the images must be geo-rectified, a process which enables the geographic coordinates of each pixel to be determined at a level of accuracy suited to the intended use. In image analysis, this process is usually referred to as geometric rectification. The geometric distortions usually arise from the following:
- Variation in altitude
- Variation in attitude and velocity of the sensor.
- Variation in characteristic of scanning mechanisms.
- Relief displacement.
- Skew distortions.
The geometric distortions are taken care of by applying geometric corrections in a two step process:
- Mathematical transformation function
- Resampling
The most common approach applied for georeferencing is the ground control points approach. This mainly involve the identification of numerous control points and determining their location in the image and their geographic coordinates from field observation or from referenced source such as topographical map of a suitable level of accuracy. Evidently, the choice of the ground control points is usually features or points which can accurately be determined. The number of points needed depends on the terrain and image characteristics as well as the level of geometric accuracy needed.
The translation applied are usually first and second order polynomial functions which are automatically generated in software from a set of user specified control points. Once an acceptable set of transformation equations have been derived, the rectified image is generated by resampling which represents the process of generating a new image with different geometric traits.
Digital Image Processing
Resampling Algorithms
Based on the application, one of the three resampling algorithms can be used. The three algorithms are;
- Nearest neighbor
- Bilinear interpolation
- Cubic convolution
The nearest neighbor method calculate the center of each pixel in the outer image, finds the corresponding location in the original image as well as uses the value of pixels whose centre is nearest. Bilinear interpolation, one calculates the distance weighted mean from the four nearest neighbors. Additionally, the cubic convolution calculate the distance-weighted-mean from a block of 16 pixels in the input image surrounding the output pixel.
Note:
- The nearest neighbor resampling does not change the original pixel values. On the other hand, bilinear interpolation produces a smoother image when compared to the nearest neighbor method. Additionally, the cubic convolution yields the most visually appealing results. It remains the most preferred for visually interpretation of images.
Digital Image Processing
Image Enhancements
Refers to a number of processes which involves visual interpretability of an image through applying algorithms to brightness, contrast and color radiations of features in the image. Evidently, enhancements facilitate the visual interpretation as well as enhance image data to its preferred values. Some of the common image enhancement techniques include:
- Brightness and Contrast Enhancement.
- Edge Enhancement.
- Information Extraction.
Digital Image Processing
Edge Enhancement
This is usually applied to improve the appearance of spatial patterns present in the data as well as edge in any place where there is an abrupt change in pixel. Edge enhancements play an important role in making light tone lighter and dark tone darker. Furthermore, edge enhancement is very useful for a variety of applications including cartographic displaying, urban planning and geologic mapping.
Information Extraction
There are different methods of extracting information from remotely sensed images such as:
- Extraction of indices, physical quantities and specific features.
- Image classification
- Change detection
Digital Image Processing
Extraction of indices
Vegetation, its type, condition and presence is of key importance to human activities. The vegetative indices usually provide the structure, amount as well as the condition of vegetation. Moreover, they are also used in deriving physical parameters such as Leaf Area Index. Furthermore, they find great usage in deriving percentage of Grain Matter and Biomass.
Additionally, the vegetative indices are valuable for monitoring changes in vegetation conditions and seasonal development over type and facilitate comparison of vegetation in different regions. Evidently, they find wide range of regional and global scale physical studies including. Furthermore, climate, biochemical and hydrologic modelling and natural resources inventory and monitoring. Moreover, land use planning , agricultural crop monitoring and forecasting and deforestation studies. Moreover, there is assessment of rangelands, condition and grazing capacity, desertification and drought forecasting, population studies as well as public health issues.
The normalized differences vegetative index calculated from sensor visible red and near-infrared bands represents one of the broadly applied vegetation indices
Digital Image Processing
Digital Image Classification
It represents categorization of data into useful groupings. The process of image classification involves the following stages:
- Definition of classes
- Obtaining the training data.
- Generating spectral signatures.
- Actual classification.
- Accuracy assessment.
It is imperative to come up with a detailed classification which will help in distinguishing characteristics necessary for the application.
Digital Image Processing
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