Most of the time, GMT will know what kind of and coordinates it is reading because you have selected a particular coordinate transformation or map projection. However, there may be times when you must explicitly specify what you are providing as input using the -f switch. When binary data are expected (-b) they must all be floating point numbers, however for ASCII input there are numerous ways to encode data coordinates (which may be separated by white-space or commas). Valid input data are generally of the same form as the arguments to the -R option (see Section 4.4.1), with additional flexibility for calendar data. Geographical coordinates, for example, can be given in decimal degrees (e.g., -123.45417) or in the ddd[:mm[:ss[.xxx]]][WESN] format (e.g., 123:27:15W).
Because of the widespread use of incompatible and ambiguous formats, the processing of input date components is guided by the template INPUT_DATE_FORMAT in your .gmtdefaults4 file; it is by default set to yyyy-mm-dd. Y2K-challenged input data such as 29/05/89 can be processed by setting INPUT_DATE_FORMAT to dd/mm/yy. A complete discription of possible formats is given in the gmtdefaults man page. The clock string is more standardized but issues like 12- or 24-hour clocks complicate matters as well as the presence or absence of delimiters between fields. Thus, the processing of input clock coordinates is guided by the template INPUT_CLOCK_FORMAT which defaults to hh:mm:ss.xxx.
GMT programs that require a map projection argument will implicitly know what kind of data to expect, and the input processing is done accordingly. However, some programs that simply report on minimum and maximum values or just do a reformatting of the data will in general not know what to expect, and furthermore there is no way for the programs to know what kind of data other columns (beyond the leading and columns) contain. In such instances we must explicitly tell GMT that we are feeding it data in the specific geographic or calendar formats (floating point data are assumed by default). We specify the data type via the -f option (which sets both input and output formats; use -fi and -fo to set input and output separately). For instance, to specify that the the first two columns are longitude and latitude, and that the third column (e.g., ) is absolute calendar time, we add -fi0x,1y,2T to the command line. For more details, see the man page for the program you need to use.