Summary: Visualization of images
- Satellite or airborne digital images are composed of a two-dimensional array of
discrete picture elements or pixels. The intensity of each pixel corresponds to the
average brightness, or radiance, measured electronically over the ground area
corresponding to each pixel.
- A single band image can be visualized in terms of its gray shades, ranging from
black (0) to white (255).
- Pixels with a weak spectral response are dark toned (black) and pixels representing
a strong spectral response are bright toned (white). The digital numbers are thus
represented by intensities from black to white.
- To compare bands and understand the relationship between the digital numbers of
satellite images and the display, and to be able to display several images, you can
scroll through and zoom in/out on the images and retrieve the DNs of the displayed
images.
- In one map window, a raster image can be displayed together with point, segment
or polygon maps. It is not possible in ILWIS to display two raster maps in one map
window.
- An image is stored in row and column geometry in raster format. When you obtain
an image there is no relationship between the rows/columns and real world
coordinates (UTM, geographic coordinates, or any other reference map projection)
yet. In a process called geo-referencing, the relationship between row and column
number and real world coordinates can be established (see section 6.4).