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## 4.4.1 Data domain or map region: The -R option

Figure 4.4: The plot region can be specified in two different ways. (a) Extreme values for each dimension, or (b) coordinates of lower left and upper right corners.

The -R option defines the map region or data domain of interest. It may be specified in one of two ways (Figure 4.4):

1. -R xmin/xmax/ymin/ymax. This is the standard way to specify Cartesian data domains and geographical regions when using map projections where meridians and parallels are rectilinear.
2. -R xlleft/ylleft/xuright/yuright r. This form is used with map projections that are oblique, making meridians and parallels poor choices for map boundaries. Here, we instead specify the lower left corner and upper right corner geographic coordinates, followed by the suffix r.

For rectilinear projections the two forms give identical results. Depending on the selected map projection (or the kind of expected input data), the boundary coordinates may take on three different formats:

Geographic coordinates:
  These are longitudes and latitudes and may be given in decimal degrees (e.g., -123.45417) or in the [±]ddd[:mm[:ss[.xxx]]][WESN] format (e.g., 123:27:15W). Note that -Rg and -Rd are shorthands for ``global domain'' -R 0/360/-90/90 and -R-180/180/-90/90, respectively.

  When used in conjunction with the Cartesian Linear Transformation (-Jx or -JX) --which can be used to map floating point data, geographical coordinates, as well as time coordinates-- it is prudent to indicate that you are using geographical coordinates in one of the following ways:

  - Use -Rg or -Rd to indicate the global domain.
  - Use -Rg xmin/xmax/ymin/ymax to indicate a limited geographic domain.
  - Add W, E, S, or N to the coordinate limits or add the generic D or G. Example: -R 0/360G/-90/90N.

Calendar time coordinates:
  These are absolute time coordinates referring to a Gregorian or ISO calendar. The general format is [date]T[clock], where date must be in the yyyy[-mm[-dd]] (year, month, day-of-month) or yyyy[-jjj] (year and day-of-year) for Gregorian calendars and yyyy[-W ww[-d]] (year, week, and day-of-week) for the ISO calendar. If no date is given we assume the present day. Following the [optional] date string we require the T flag.

  The optional clock string is a 24-hour clock in hh[:mm[:ss[.xxx]]] format. If no clock is given it implies 00:00:00, i.e., the start of the specified day. Note that not all of the specified entities need be present in the data. All calendar date-clock strings are internally represented as double precision seconds since proleptic Gregorian date Mon Jan 1 00:00:00 0001. Proleptic means we assume that the modern calendar can be extrapolated forward and backward; a year zero is used, and Gregory's reforms4.4 are extrapolated backward. Note that this is not historical.

Relative time coordinates:
  These are coordinates which count seconds, hours, days or years relative to a given epoch. A combination of the parameters TIME_EPOCH and TIME_UNIT define the epoch and time unit. The parameter TIME_SYSTEM provides a few shorthands for common combinations of epoch and unit, like j2000 for days since noon of 1 Jan 2000. Denote relative time coordinates by appending the optional lower case t after the value. When it is otherwise apparent that the coordinate is relative time (for example by using the -f switch), the t can be omitted.

Other coordinates:
  These are simply any coordinates that are not related to geographic or calendar time or relative time and are expected to be simple floating point values such as [±]xxx.xxx[EeDd[±]xx], i.e., regular or exponential notations, with the enhancement to understand FORTRAN double precision output which may use D instead of E for exponents. These values are simply converted as they are to internal representation.4.5

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