explain_-f_full.rst_

The **-f** option
^^^^^^^^^^^^^^^^^

**Syntax**

**-f**\ [**i**\|\ **o**]\ *colinfo*
    Specify the data types of input and/or output columns (time or geographical data).

**Description**

The **-f** option specifies what kind of data each input or output column contains when map projections are not
required. Optionally, append **i** or **o** to make this apply only to input or output, respectively [Default applies
to both]. Append a text string with information about each column (or range of columns) separated by commas. Each group
starts with the column number (0 is the first column) followed by either **x** (longitude), **y** (latitude), **T**
(absolute calendar time), **t** (relative time in chosen :term:`TIME_UNIT` since :term:`TIME_EPOCH`), **d** (dimension)
or **s** (trailing text). If several consecutive columns have the same format you may specify a range of columns rather
than a single column. Column ranges must be given in the format *start*\ [:*inc*]:*stop*, where *inc* defaults to 1 if
not specified.  For example, if our input file has geographic coordinates (latitude, longitude) with absolute calendar
coordinates in the columns 3 and 4, we would specify **fi**\ 0\ **y**,1\ **x**,3:4\ **T**. All other columns are assumed
to have the default (**f**\ loating point) format and need not be set individually. **Notes**: (1) You can also indicate that
all items from the given column and to the end of the record should be considered *trailing text* by giving the code
**s** (for string). Only the last group may set **s**. (2) Types **T** and **s** can only be used with ASCII data sets.

Three shorthand options for common selections are available. The shorthand **-f**\ [**i**\|\ **o**]\ **g** means
**-f**\ [**i**\|\ **o**]0x,1y (i.e., geographic coordinates), while **-f**\ [**i**\|\ **o**]\ **c** means
**-f**\ [**i**\|\ **o**]0:1\ **f** (i.e., Cartesian coordinates). A first special use of **-f** is to select
**-fp**\ [*unit*], which *requires* **-J -R** and lets you use *projected* map coordinates (e.g., UTM meters) as data input.
Such coordinates are automatically inverted to longitude, latitude during the data import. Optionally, append a length
*unit* (see table :ref:`distance units <tbl-distunits>`) [default is meter]. The second special case is to use **-fa**.
We use this when reading a file whose first column is plain text (and NO date or time) followed by numeric records.
Normally, such a file would result in a _"Selection led to no output columns"_ error because the file was interpreted
to be made only of text (_i.,e.,_ no numbers). Or even worse, only part of the file is read. The **-fa** option
forces the first column to be ignored (output as NaN) and the remaining records parsed normally. User may add the
**-i1:** to fully drop that first column in reading. For more information, see Sections :ref:`input-data-formats`
and :ref:`output-data-formats`.