When map projections are not required we must explicitly state what kind of data each input or output column contains. This is accomplished with the -f option. Following an optional i (for input only) or o (for output only), we append a text string with information about each column (or range of columns) separated by commas. Each string starts with the column number (0 is first column) followed by either x (longitude), y (latitude), T (absolute calendar time) or t (relative time). If several consecutive columns have the same format you may specify a range of columns rather than a single column, i.e., 0-4 for the first 5 columns. For example, if our input file has geographic coordinates (latitude, longitude) with absolute calendar coordinates in the columns 3 and 4, we would specify fi0y,1x,3-4T. All other columns are assumed to have the default, floating point format and need not be set individually. The shorthand -f[io]g means -f[io]0x,1y (geographic coordinates). For more information, see Section 4.10.