Satellite images as delivered by image provider agencies typically need to undergo certain processing steps before information retrieval (in later processing) is possible. Some of the pre-processing will have been done by the provider. The degree of this already done processing is called the “processing level” of the product received.
The processing level refers to the amount of processing performed on an image (by the provider) between the time it was received at the ground receiving station and its delivery to the client. Although there are different image platforms and provider agencies, effort has been made to harmonize the description of product processing levels for all satellite images.
About seven processing levels, known as levels 0, 1A, 1B, 2A, 2B, 3A and 3B, are typically recognized. These levels are hierarchical, meaning that level 2 has undergone a higher degree of processing than level 1, and so forth. Definitions of the different processing levels can be found here, while descriptions of processing steps are provided in subsequent sections of this chapter. It is thus, before further processing of RS data, crucial to know and understand the image data processing level as received; this is described in the associated metadata. All delivered images have a metadata file, normally in an .xml or .txt file format.
The steps that are detailed in this section assume a processing level of 2A.