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While the previous GMT verions 1-3 allowed for a single algorithm that
determined where labels would be placed, GMT 4 allows for five different
algorithms. Futhermore, a new ``symbol'' option (-Sq for ``quoted line'')
has been added to psxy and psxyz and hence the new label
placement mechanisms apply to those programs as well. The contouring programs
expect the algorithm to be specified as arguments to -G while the line plotting
programs expect the same arguments to follow -Sq. The information appended
to these options is the same in both cases and is of the form [code]info.
The five algorithms correspond to the five codes below (some codes will appear in both
upper and lower case; they share the same algorithm but differ in some other ways).
In what follows, the phrase ``line segment'' is taken to mean either a contour or a line
to be labelled. The codes are:
- d:
- Full syntax is ddist[cimp][/frac].
Place labels according to the distance measured along the projected line on the map. Append the
unit you want to measure distances in [Default is taken from MEASURE_UNIT].
Starting at the beginning of a line, place labels every dist increment of
distance along the line. To ensure that closed lines whose total length is less
than dist get annotated, we may append frac which will place the first
label at the distance dist frac from the start of a closed
line (and every dist thereafter). If not given, frac defaults to 0.25.
- D:
- Full syntax is Ddist[dekmn][/frac].
This option is similar to d except the original data must be referred to geographic
coordinates (and a map projection must have been chosen) and actual EarthO.1 surface distances along the lines are considered. Append the
unit you want to measure distances in; choose among degree, meter [Default],
kilometer, statute miles, or nautical miles. Other aspects are similar to code d.
- f:
- Full syntax is ffix.d[/slop[cimp]]. Here, an ASCII file fix.d is
given which must contain records whose first two columns hold the coordinates of points along
the lines at which locations the labels should be placed. Labels will only be placed if the coordinates
match the line coordinates to within a distance of slop (append unit or we use MEASURE_UNIT).
The default slop is zero, meaning only exact coordinate matches will do.
- l:
- Full syntax is lline1[,line2[, ...]]. One or more straight line segments
are specified separated by commas, and labels will be placed at the intersections between these lines and
our line segments. Each line specification implies a start and stop point, each
corresponding to a coordinate pair. These pairs can be regular coordinate pairs (i.e., longitude/latitude separated by a slash), or they
can be two-character codes that refer to predetermined points relative to the map region. These codes
are taken from the pstext justification keys [LCR][BMT] so that the first
character determines the -coordinate and the second determines the -coordinate. In grdcontour,
you can also use the two codes Z+ and Z- as shorthands for the location of the grid's global
maximum and minimum, respectively. For example, the line LT/RB is a diagonal from the
upper left to the lower right map corner, while Z-/135W/15S is a line from the grid minimum to the point
(135W, 15S).
- L:
- Same as l except we will treat the lines given as great circle start/stop coordinates and fill in
the points between before looking for intersections.
- n:
- Full syntax is nnumber[/minlength[cimp]]. Place
number of labels along each line regardless of total line length. The line is divided into number
segments and the labels are placed at the centers of these segments. Optionally, you may give a minlength
distance to ensure that no labels are placed closer than this distance to its neighbors.
- N:
- Full syntax is Nnumber[/minlength[cimp]]. Similar to
code n but here labels are placed at the ends of each segment (for number ). A special
case arises for number when a single label will be placed according to the sign of number:
places one label justified at the start of the line, while places one label justified at the end of the line.
- x:
- Full syntax is xcross.d. Here, an ASCII file cross.d is
a multi-segment file whose lines will intersect our segment lines; labels will be placed at these intersections.
- X:
- Same as x except we treat the lines given as great circle start/stop coordinates and fill in the
points between before looking for intersections.
Only one algorithm can be specified at any given time.
Next: O.2 Label Attributes
Up: O. Annotation of Contours
Previous: O. Annotation of Contours
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Paul Wessel
2004-10-01