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5.1.1.3 Calendar time coordinates

Figure 5.3: Linear transformation of calendar coordinates.

Several particular issues arise when we seek to make linear plots using calendar date/time as the input coordinates. As far as setting up the coordinate transformation we must indicate whether our input data have absolute time coordinates or relative time coordinates. For the former we append T after the axis scale (or width), while for the latter we append t at the end of the -Jx (or -JX) option. However, other command line arguments (like the -R option) may already specify whether the time coordinate is absolute or relative. An absolute time entry must be given as [date]T[clock] (with date given as yyyy[-mm[-dd]], yyyy[-jjj], or yyyy[-W ww[-d]], and clock using the hh[:mm[:ss[.xxx]]] 24-hour clock format) whereas the relative time is simply given as the units of time since the epoch followed by t (see TIME_SYSTEM for information on specifying the time unit and the epoch). As a simple example, we will make a plot of a school week calendar (Figure 5.3).

    gmtset PLOT_DATE_FORMAT o TIME_WEEK_START Sunday PLOT_CLOCK_FORMAT -hham TIME_FORMAT_PRIMARY full
    psbasemap -R2001-9-24T/2001-9-29T/T07:0/T15:0 -JX4i/-2i -Ba1Kf1kg1d/a1Hg1hWsNe -P > GMT_linear_cal.ps

When the coordinate ranges provided by the -R option and the projection type given by -JX (including the optional d, g, t or T) conflict, GMT will warn the users about it. In general, the options provided with -JX will prevail.

Next: 5.1.2 Cartesian Logarithmic projection Up: 5.1.1 Cartesian Linear Transformation Previous: 5.1.1.2 Geographic coordinates Contents Index