Image data are rasters, stored in a rectangular matrix of rows and columns. Radiometric resolution determines how many gradations of brightness can be stored for each cell (pixel) in the matrix; 8-bit resolution, where each pixel contains an integer value from 0 to 255, is most common. Modern sensors often collect data at higher resolution, and advanced image processing software can make use of these values for analysis. The human eye cannot detect very small differences in brightness, and most GIS software can only read an 8-bit value.
In a grayscale image, 0 = black and 255 = white; and there is just one 8-bit value for each pixel. However, in a natural color image, there is an 8-bit value for red, an 8-bit brightness value for green, and an 8-bit value for blue. Therefore, each pixel in a color image requires 3 separate values to be stored in the file. There are three possible ways to organize these values in a raster file.
Orthoimages are delivered in a variety of image formats, compressed and uncompressed. The most common are TIF and JPG. Compression eases data management challenges, as large high-resolution orthophoto projects can easily result in terabytes of uncompressed imagery. Compression can also speed display in GIS systems. The downside is that compression can introduce artifacts and change pixel values, possibly hampering interpretation and analysis, particularly with respect to fine detail. The decision to compress should be driven by end user requirements; it is not uncommon to deliver a set of uncompressed imagery for archival and special applications along with a set of compressed imagery for easy use by large numbers of users. If there is an intention for web-based display or distribution of orthoimagery, a compressed set of orthoimagery is often recommended. In any event, georeferencing information must also be provided. Both TIF and JPG image formats can accommodate georeferencing information, either imbedded in the image file itself, as in the case of GeoTIF, or as a separate file for each image, as in the case of TIF with a TFW (TIF World) file. The georeferencing information tells GIS software 1) the size of a pixel, 2) where to place one corner of the image in the real world, and 3) whether the image is rotated with respect to the ground coordinate system.
Other popular image formats you may encounter are: