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A digital image is a two dimensional array of digital numbers (DNs) each number representing a physical parameter eg. reflectance or emittance values. The digital numbers are sometimes also referred to as pixel values or brightness values. The smallest cell, unit or building block in this array is a picture element or pixel.

Pixels are most often equidimensional (squares) and are arranged in a regular grid made of rows and columns. Each pixel in an image can be uniquely identified by the combination of its row number (sometimes also known as line number, or scan line number) and its column number (sometimes also known as sample number, or pixel number).

Georeferenced digital images can be stacked over each other in several layers so that the corresponding cell/unit/pixel lies directly over each other in the layer stack.