Most remotely sensed images nowadays exist in digital form. Even domestic cameras are now usually digital instruments, and the use of photographic film is becoming rarer and rarer. Analogue images, such as photographs, are continuous, both in their spatial extent (they can be enlarged almost without limit) and radiometrically (there is a continuous range of shades of grey). The word ‘picture’ is usually used for such an image.
On the other hand, a digital image is spatially and radiometrically discrete. A remote sensing sensor detects the reflected radiation of the Earth’s surface and stores it as numbers in a raster. In accordance, each area that has been detected constitutes a cell in a raster. The grey levels increment in a stepwise fashion, and the scene is made up from an array of individual elements called ‘picture elements’, abbreviated to ‘pixels’, each of which is represented by one of the discrete grey levels. A pixel is the smallest addressable element in a raster image.
The spatial resolution of a raster image refers to the size of the ground element represented by an individual pixel. The size of an area represented in a pixel depends of the capability of the sensor to detect details. A pixel cannot be subdivided, and enlargement merely produces larger pixels, which contain no more information than the original ones. We are familiar with this effect on our television or computer screen – the picture we see consists of an array of dots of light, the density of which determines the screen resolution.
The number of distinct grey levels into which the intensity of the signal is divided and that can be represented by a pixel is called radiometric resolution of a digital image, and it depends of the number of bits per pixel (bpp). A 1 bpp image uses 1 bit for each pixel, so each pixel can be either on or off (monochrome). Each additional bit doubles the number of grey levels available, so a 2 bpp image can have 4 grey levels, a 3 bpp image can have 8 grey levels, and so forth. In colour imaging systems, a colour is typically represented by three component intensities such as red, green, and blue; usually their raster images have an 8-bit resolution (256 grey levels), a 16-bit resolution (65,536 grey levels), or a 24-bit resolution (16,777,216 grey levels).