1 Introduction

A remotely sensed image is made up of a rectangular matrix of individual pixels, where each pixel represents a specific ground area. The value of each pixel represents the magnitude of upwelling electromagnetic energy from that given ground area (Mather, 1999). Energy occurs in different wavelengths, and by measuring the energy in different bands or spectra of wavelengths, coming from the same ground area, ground features can be distinguished from each other, as they show typical characteristics in the different bands recorded, e.g. water is different form soil or vegetation.

The values for pixels in one waveband typically ranges from 0 to 255, this being the range of values that can be represented by one byte of computer storage. Thus, the detectors on the sensing devices will be set such that the energy will be recorded digitally within that range. For any one image, the majority of the actual data values recorded may be concentrated within only one small part of the 0-255 range.

A consequence of this is that small variations in pixel values, which may have interest and which might distinguish between features, may be difficult to detect visually (Jones, 1998). Viewing remotely-sensed images is therefore concerned with manipulating the pixel values in such a way that features can be clearly discriminated between when the image is displayed. It is therefore clear that the amount of information that can be retrieved from an image lies not in the cell values per se, but in the way these values can be displayed.

References

Mather, P. (1999) Computer Processing of Remotely-Sensed Images, An Itroduction, 2nd ed.

Lillesand, T.M., and Keifer, R. W. (1994) Remote Sensing and Image Interpretation, 3rd ed

Jones, C. (1997) Geographical Information Systems and Computer Cartography