Digital Image

## Analog and Digital Images

An image is a two-dimensional representation of objects in a real scene. Remote sensing images are representations of parts of the earth surface as seen from space. The images may be analog or digital. Aerial photographs are examples of analog images while satellite images acquired using electronic sensors are examples of digital images.

| A digital image is a two-dimensional array of pixels. Each pixel has an intensity value (represented by a digital number) and a location address (referenced by its row and column numbers).

## Pixels

A digital image comprises of a two dimensional array of individual picture elements called pixels arranged in columns and rows. Each pixel represents an area on the Earth's surface. A pixel has an intensity value and a location address in the two dimensional image.

The intensity value represents the measured physical quantity such as the solar radiance in a given wavelength band reflected from the ground, emitted infrared radiation or backscattered radar intensity. This value is normally the average value for the whole ground area covered by the pixel.

The intensity of a pixel is digitised and recorded as a digital number. Due to the finite storage capacity, a digital number is stored with a finite number of bits (binary digits). The number of bits determine the radiometric resolution of the image. For example, an 8-bit digital number will range from 0 to 255 (i.e. 28 - 1). The detected intensity value needs to be scaled and quantized to fit within this range of value. In a Radiometric Calibrated image, the actual intensity value can be derived from the pixel digital number.

The address of a pixel is denoted by its row and column coordinates in the two-dimensional image. In order to be useful, the exact geographical location of each pixel on the ground must be derivable from its row and column indices, given the imaging geometry and the satellite orbit parameters.

| "A Push-Broom" Scanner: This type of imaging system is commonly used in optical remote sensing satellites such as SPOT. The imaging system has a linear detector array (usually of the CCD type) consisting of a number of detector elements (6000 elements in SPOT HRV). Each detector element projects an "instantaneous field of view (IFOV)" on the ground. The signal recorded by a detector element is proportional to the total radiation collected within its IFOV. At any instant, a row of pixels are formed. As the detector array flies along its track, the row of pixels sweeps along to generate a two-dimensional image.

## Multilayer Image

Several types of measurement may be made from the ground area covered by a single pixel. Each type of measurement forms an image which carry some specific information about the area. By "stacking" these images from the same area together, a multilayer image is formed. Each component image is a layer in the multilayer image. For example, the SPOT HRV sensor operating in the multispectral mode detects radiations in three wavelength bands: the green (500 - 590 nm), red (610 - 680 nm) and near infrared (790 - 890 nm) bands. A single SPOT multispectral scene consists of three intensity images in the three wavelength bands. In this case, each pixel of the scene has three intensity values corresponding to the three bands.

Multilayer images can also be formed by combining images obtained from different sensors, and other subsidiary data. For example, a multilayer image may consist of three layers from a SPOT multispectral image, a layer of ERS synthetic aperture radar image, and perhaps a layer consisting of the digital elevation map of the area being studied.

| An illustration of a multilayer image consisting of five component layers.

## Spatial Resolution

Spatial resolution refers to the size of the smallest object that can be resolved on the ground. In a digital image, the resolution is limited by the pixel size, i.e. the smallest resolvable object cannot be smaller than the pixel size. The intrinsic resolution of an imaging system is determined primarily by the instantaneous field of view (IFOV) of the sensor, which is a measure of the ground area viewed by a single detector element in a given instant in time. However this intrinsic resolution can often be degraded by other factors which introduce blurring of the image, such as improper focusing, atmospheric scattering and target motion. The pixel size is determined by the sampling distance.

A "High Resolution" image refers to one with a small resolution size. Fine details can be seen in a high resolution image. On the other hand, a "Low Resolution" image is one with a large resolution size, i.e. only coarse features can be observed in the image.

The following three images illustrate the point discussed above. The first image is a SPOT image of 10 m pixel size. It was derived by merging a SPOT panchromatic image of 10 m resolution with a SPOT multispectral image of 20 m resolution. The merging procedure "colours" the panchromtic image using the colours derived from the multispectral image. The effective resolution is thus determined by the resolution of the panchromatic image, which is 10 m. This image is processed to degrade the resolution while maintaining the same pixel size. The next two images are the blurred versions of the image with larger resolution size, but still digitized at the same pixel size of 10 m.

10 m resolution, 10 m pixel size

30 m resolution, 10 m pixel size | 80 m resolution, 10 m pixel size
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The following images illustrate the effect of pixel size on the visual appearance of an area. The first image is a SPOT image of 10 m pixel size derived by merging a SPOT panchromatic image with a SPOT multispectral image. The subsequent images show the effects of digitizing the same area with larger pixel sizes.

Pixel Size = 10 m Image Width = 160 pixels, Height = 160 pixels | Pixel Size = 20 m Image Width = 80 pixels, Height = 80 pixels
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Pixel Size = 40 m Image Width = 40 pixels, Height = 40 pixels | Pixel Size = 80 m Image Width = 20 pixels, Height = 20 pixels

## Radiometric Resolution

Radiometric Resolution refers to the smallest change in intensity level that can be detected by the sensing system. The intrinsic radiometric resolution of a sensing system depends on the signal to noise ratio of the detector. In a digital image, the radiometric resolution is limited by the number of discrete quantization levels used to digitize the continuous intensity value.

The following images illustrate the effects of the number of quantization levels on the digital image. The first image is a SPOT panchromatic image quantized at 8 bits (i.e. 256 levels) per pixel. The subsequent images show the effects of degrading the radiometric resolution by using fewer quantization levels.

8-bit quantization (256 levels) | 6-bit quantization (64 levels)
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| 4-bit quantization (16 levels) | 3-bit quantization (8 levels)
| 2-bit quantization (4 levels) | 1-bit quantization (2 levels)

It can be seen that, digitization using a small number of quantization levels does not affect very much the visual quality of the image. Even 4-bit quantization (16 levels) seems acceptable. However, if the image is to be subjected to numerical analysis, the accuracy of analysis will be compromised if few quantization levels are used.

## Data Volume

The volume of the digital data can potentially be large for multispectral data, as a given area is covered in many different wavelength bands. For example, a 3-band multispectral SPOT image covers an area of about 60 x 60 km2 on the ground with a pixel separation of 20 m. So there are about 3000 x 3000 pixels per image. Each pixel intensity in each band is coded using an 8-bit (i.e. 1 byte) digital number, giving a total of about 27 million bytes per image.

In comparison, the panchromatic data has only one band. Thus, panchromatic systems are normally designed to give a higher spatial resolution than the multispectral system. For example, a SPOT panchromatic scene has the same coverage of about 60 x 60 km2 but the pixel size is 10 m, giving about 6000 x 6000 pixels and a total of about 36 million bytes per image. If a multispectral SPOT scene is digitized also at 10 m pixel size, the data volume will be 108 million bytes.

The images taken by a remote sensing satellite is transmitted to Earth through telecommunication. The bandwidth of the telecommunication channel sets a limit to the data volume for a scene taken by the imaging system. Depending on the intended application, spatial resolution may have to be compromised to accommodate a larger number of spectral bands, or a small number of spectral bands may be accepted to allow high resolution imaging.

Spaceborne Remote Sensing Optical Remote Sensing

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