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