A header file is a seperate file associated with an image file. The header file can be either a plain ASCII-file or a binary file. It contains information about the image file it is associated with. These information can comprise the number of pixels per row (x-direction in a two dimensional image), also called number of columns, the number of lines or rows (y-direction in a two dimensional image), the number of bands (corresponding to the z-direction), pixel spacing and spatial resolution, geographic reference information, the byte order (e.g. big-endian or little-endian), spectral information for each band, calibration constants and many more. The purpose of a header file is to provide basic information about the properties of the image data either to the user or to a software and enabling a software to correctly load and display the image content. In this way, information contained in a header file can also be called metadata, which is data about the data. The structure and the information contained in a header file of remote sensing imagery can be found in the so-called product information documents. There is also digital imagery used in remote sensing containing the information found in header files not in a separate file but as part of the digital image data itself. In this case this is called header information or a file header, which is usually found at the beginning of the image file. In some cases , image files may contain several header sections, e.g. theESA Envisat ASAR SAR data imagery contains a Main Product Header and a Specific Product Header section. Header information as part of the image file itself may be stored in ASCII or in binary format, or in a mixed binary format, as it was used for the ESA Envisat SAR data.