A header file is usually a separate 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. the ESA 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.