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. -Rxmin/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. -Rxlleft/ylleft/xuright/yurightr. 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 [$\pm$]ddd[:mm[:ss[.xxx]]][W$\vert$E$\vert$S$\vert$N] format (e.g., 123:27:15W). Note that -Rg and -Rd are shorthands for ``global domain'' -R0/360/-90/90 and -R-180/180/-90/90, respectively.

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[-Www[-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 enteties 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 reforms^4.4 are extrapolated backward. Note that this is not historical.

Other coordinates:

These are simply anything that is neither geographic nor calendar time related and are expected to be simple floating point values such as [$\pm$]xxx.xxx[E$\vert$e$\vert$D$\vert$d[$\pm$][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. One exception is the concept of relative time which is read as a floating point offset from an absolute time reference point (epoch). The unit and the epoch are specified with the TIME_SYSTEM parameter. Relative time coordinates are expected when a coordinate transformation involving relative time has been selected or when the -f switch has been used to indicate relative time coordinates.^4.5