Four main types of information contained in an optical image are often utilized for image interpretation:
They are illustrated in the following examples.
A panchromatic image extracted from a SPOT panchromatic scene at a ground resolution of 10 m. The ground coverage is about 6.5 km (width) by 5.5 km (height). The urban area at the bottom left and a clearing near the top of the image have high reflected intensity,while the vegetated areas on the right part of the image are generally dark. Roads and blocksof buildings in the urban area are visible. A river flowing through the vegetated area, cutting across the top right corner of the image can be seen. The river appears bright due to sediments while the sea at the bottom edge of the image appears dark.
The following three images show the three bands of a multispectral image extracted from a SPOT multispectral scene at a ground resolution of 20 m. The area covered is the same as that shown in the above panchromatic image. Note that both the XS1 (green) and XS2 (red) bands look almost identical to the panchromatic image shown above. In contrast, the vegetated areas now appear bright in the XS3 (near infrared) band due to high reflectance of leaves in the near infrared wavelength region. Several shades of grey can be identified for the vegetated areas, corresponding to different types of vegetation. Water mass (both the river and the sea) appear dark in the XS3 (near IR) band.
In displaying a colour composite image, three primary colours (red, green and blue) are used. When these three colours are combined in various proportions, they produce different colours in the visible spectrum. Associating each spectral band (not necessarily a visible band) to a separate primary colour results in a colour composite image.
|Many colours can be formed by combining the three primary colours (Red, Green, Blue) in various proportions.|
If a multispectral image consists of the three visual primary colour bands (red, green, blue), the three bands may be combined to produce a "true colour" image. For example, the bands 3 (red band), 2 (green band) and 1 (blue band) of a LANDSAT TM image or an IKONOS multispectral image can be assigned respectively to the R, G, and B colours for display. In this way, the colours of the resulting colour composite image resemble closely what would be observed by the human eyes.
The display colour assignment for any band of a multispectral image can be done in an entirely arbitrary manner. In this case, the colour of a target in the displayed image does not have any resemblance to its actual colour. The resulting product is known as a false colour composite image. There are many possible schemes of producing false colour composite images. However, some scheme may be more suitable for detecting certain objects in the image.
A very common false colour composite scheme for displaying a SPOT multispectral image is shown below:
This false colour composite scheme allows vegetation to be detected readily in the image. In this type of false colour composite images, vegetation appears in different shades of red depending on the types and conditions of the vegetation, since it has a high reflectance in the NIR band (as shown in the graph of spectral reflectance signature).
Clear water appears dark-bluish (higher green band reflectance), while turbid water appears cyan (higher red reflectance due to sediments) compared to clear water. Bare soils, roads and buildings may appear in various shades of blue, yellow or grey, depending on their composition.
Another common false colour composite scheme for displaying an optical image with a short-wave infrared (SWIR) band is shown below:
An example of this false colour composite display is shown below for a SPOT 4 image.
For optical images lacking one or more of the three visual primary colour bands (i.e. red, green and blue), the spectral bands (some of which may not be in the visible region) may be combined in such a way that the appearance of the displayed image resembles a visible colour photograph, i.e. vegetation in green, water in blue, soil in brown or grey, etc. Many people refer to this composite as a "true colour" composite. However, this term is misleading since in many instances the colours are only simulated to look similar to the "true" colours of the targets. The term "natural colour" is preferred.
The SPOT HRV multispectral sensor does not have a blue band. The three bands, XS1, XS2 and XS3 correspond to the green, red, and NIR bands respectively. But a reasonably good natural colour composite can be produced by the following combination of the spectral bands:
Since vegetation has high NIR reflectance but low red reflectance, vegetated areas will have higher RVI values compared to non-vegetated aeras. Another commonly used vegetation index is the Normalised Difference Vegetation Index (NDVI) computed by
In the NDVI map shown above, the bright areas are vegetated while the nonvegetated areas (buildings, clearings, river, sea) are generally dark. Note that the trees lining the roads are clearly visible as grey linear features against the dark background.
The NDVI band may also be combined with other bands of the multispectral image to form a colour composite image which helps to discriminate different types of vegetation. One such example is shown below. In this image, the display colour assignment is:
At least three types of vegetation can be discriminated in this colour composite image: green, bright yellow and golden yellow areas. The green areas consist of dense trees with closed canopy. The bright yellow areas are covered with shrubs or less dense trees. The golden yellow areas are covered with grass. The non vegetated areas appear in dark blue and magenta.
|This is an IKONOS 1-m resolution pan-sharpened color image of an oil palm plantation. The image is 300 m across. Even though the general colour is green throughout, three distinct land cover types can be identified from the image texture. The triangular patch at the bottom left corner is the oil palm plantation with matured palm trees. Individual trees can be seen. The predominant texture is the regular pattern formed by the tree crowns. Near to the top of the image, the trees are closer together, and the tree canopies merge together, forming another distinctive textural pattern. This area is probably inhibated by shrubs or abandoned trees with tall undergrowths and shrubs in between the trees. At the bottom right corner, colour is more homogeneous, indicating that it is probably an open field with short grass.|
Source : http://www.crisp.nus.edu.sg