Access an existing file to read it is generally quite simple. Just indicate the name of the file or dataset on the command line. However, creating a file is more complicated. It may be necessary to indicate the the format to create, various creation options affecting how it will be created and perhaps a coordinate system to be assigned. Many of these options are handled similarly by different GDAL utilities, and are introduced here. Many utilities default to creating GeoTIFF files if a format is not specified. File extensions are not used to guess output format, nor are extensions generally added by GDAL if not indicated in the filename by the user. The creation options available vary by format driver, and some simple formats have no creation options at all. A list of options supported for a format can be listed with the ‘–format <,format>,’ command line option but the web page for the format is the definitive source of information on driver creation options. These utilities allow the coordinate system (SRS = spatial reference system) to be assigned in a variety of formats. Source.