Image Formats

Satellite images of the earth have been easily and commercially available for some time now. Originally, they were the purview of the military. Some of the better known satellites that are used for this purpose are IKONOS which captures 1- meter resolution images (that is, the picture shows distinct objects of one-meter size), the INSATs, the US Landsat, Canadas RADARSAT and the European Space Agency’s ERS satellites.

These images are archived and distributed in various formats. We may be familiar with some of these formats, while some are specifically developed to handle extra information. 

For browsing, common formats–GIF and JPEG–are used. But, for archiving the data, the files are stored on Digital Linear Tapes (DLT) in the Framed Raw Expanded Data format (FRED). This raw image format is considered a true ‘digital negative’, which records data over a wider bit range (typically 10 or 12 bits) than JPEG or 8-bit TIFF.

Satellite images are used for a variety of end uses, and to suit their requirements, images are stored in formats that can also provide lots of additional information. 

Superstructure Format: This is one such format, developed by the Landsat Ground Station Operators Working Group (LGSOWG). The Committee for Earth Observation Satellite (CEOS) has adopted it to exchange data between different users. The Superstructure Format has the provision to include ancillary data pertaining to the image file, like mapping or other geographic information, and hence is most suitable for applications where further processing has to be performed on the images. 

GeoTIFF: This enhanced version of TIFF contains all the information necessary for converting the image co-ordinates to geographic and cartographic co-ordinates. 

DIMAP with GeoTIFF: Digital Image MAPping is a metadata format designed to document digital image maps using XML. The underlying image format is GeoTIFF. Spot Image, Satellus and CNES (the French National Space Agency) have developed DIMAP. It is an open initiative. Any one can contribute modifications to the format. 

Band Interleaved by Line (BIL): This is one of the oldest formats used for space imaging. It treats each line as separate storage units. The brightness values for each line is stored one after another. It is practical to use this format if all bands in an image are to be used.

National Imagery Transmission Format (NITF): This is a US standard for digital imagery used by the Intelligence Community, the Department of Defense, and elated departments and agencies of the Government. 

Flexible Image Transport System (FITS) is an archive and interchange format for astronomical data files. Originally designed for transfer of images, FITS is now widely used as an astronomical data transfer format. 

Shruti Pareek