# GRDCONTOUR

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
ASCII FORMAT PRECISION
FILE FORMATS
EXAMPLES
SEE ALSO

## NAME

grdcontour − Contouring of 2-D gridded data sets

## SYNOPSIS

grdcontour grdfile −C cont_int −J parameters [ −A[-|annot_int][labelinfo] ] [ −B[p|s]parameters ] [ −D dumpfile ] [ −E azimuth/elevation ] [ −F[l|r] ] [ −G[d|f|n|l|L|x|X]params ] [ −K ] [ −L low/high ] [ −O ] [ −P ] [ −Q cut ] [ −R west/east/south/north[r] ] [ −S smoothfactor ] [ −T[+|-][gap/length][:LH] ] [ −U[just/dx/dy/][c|label] ] [ −V ] [ −W[+][type]pen ] [ −X[a|c|r][x-shift[u]] ] [ −Y[a|c|r][y-shift[u]] ] [ −Z[factor[/shift]][p] ] [ −c copies ] [ −bo[s|S|d|D[ncol]|c[var1/...]] ] [ −m[flag] ]

## DESCRIPTION

grdcontour reads a 2-D grid file and produces a contour map by tracing each contour through the grid. As an option, the x/y/z positions of the contour lines may be dumped to a single multisegment file or many separate files. PostScript code is generated and sent to standard output. Various options that affect the plotting are available.
grdfile

2-D gridded data set to be contoured. (See GRID FILE FORMATS below).

| −C |  | The contours to be drawn may be specified in one of three possible ways:

(1) If cont_int has the suffix ".cpt" and can be opened as a file, it is assumed to be a color palette table. The color boundaries are then used as contour levels. If the cpt-file has annotation flags in the last column then those contours will be annotated. By default all contours are labeled; use −A− to disable all annotations.
(2) If cont_int is a file but not a cpt-file, it is expected to contain contour levels in column 1 and a C(ontour) OR A(nnotate) in col 2. The levels marked C (or c) are contoured, the levels marked A (or a) are contoured and annotated. Optionally, a third column may be present and contain the fixed annotation angle for this contour level.
(3) If no file is found, then cont_int is interpreted as a constant contour interval. If −A is set and −C is not, then the contour interval is set equal to the specified annotation interval.
If a file is given and −T is set, then only contours marked with upper case C or A will have tickmarks. In all cases the contour values have the same units as the grid.

−J

Selects the map projection. Scale is UNIT/degree, 1:xxxxx, or width in UNIT (upper case modifier). UNIT is cm, inch, or m, depending on the MEASURE_UNIT setting in .gmtdefaults4, but this can be overridden on the command line by appending c, i, or m to the scale/width value. When central meridian is optional, default is center of longitude range on −R option. Default standard parallel is the equator. For map height, max dimension, or min dimension, append h, +, or - to the width, respectively.

More details can be found in the psbasemap man pages.

CYLINDRICAL PROJECTIONS:

−Jc lon0/lat0/scale (Cassini)
−Jcyl_stere/[lon0/[lat0/]]scale (Cylindrical Stereographic)
−Jj[lon0/]scale (Miller)
−Jm[lon0/[lat0/]]scale (Mercator)
−Jm lon0/lat0/scale (Mercator - Give meridian and standard parallel)
−Jo[a]lon0/lat0/azimuth/scale (Oblique Mercator - point and azimuth)
−Jo[b]lon0/lat0/lon1/lat1/scale (Oblique Mercator - two points)
−Joc lon0/lat0/lonp/latp/scale (Oblique Mercator - point and pole)
−Jq[lon0/[lat0/]]scale (Cylindrical Equidistant)
−Jt lon0/[lat0/]scale (TM - Transverse Mercator)
−Ju zone/scale (UTM - Universal Transverse Mercator)
−Jy[lon0/[lat0/]]scale (Cylindrical Equal-Area)

CONIC PROJECTIONS:

−Jb lon0/lat0/lat1/lat2/scale (Albers)
−Jd lon0/lat0/lat1/lat2/scale (Conic Equidistant)
−Jl lon0/lat0/lat1/lat2/scale (Lambert Conic Conformal)
−Jpoly/[lon0/[lat0/]]scale ((American) Polyconic)

AZIMUTHAL PROJECTIONS:

−Ja lon0/lat0[/horizon]/scale (Lambert Azimuthal Equal-Area)
−Je lon0/lat0[/horizon]/scale (Azimuthal Equidistant)
−Jf lon0/lat0[/horizon]/scale (Gnomonic)
−Jg lon0/lat0[/horizon]/scale (Orthographic)
−Jg lon0/lat0/altitude/azimuth/tilt/twist/Width/Height/scale (General Perspective).
−Js lon0/lat0[/horizon]/scale (General Stereographic)

MISCELLANEOUS PROJECTIONS:

−Jh[lon0/]scale (Hammer)
−Ji[lon0/]scale (Sinusoidal)
−Jkf[lon0/]scale (Eckert IV)
−Jk[s][lon0/]scale (Eckert VI)
−Jn[lon0/]scale (Robinson)
−Jr[lon0/]scale (Winkel Tripel)
−Jv[lon0/]scale (Van der Grinten)
−Jw[lon0/]scale (Mollweide)

NON-GEOGRAPHICAL PROJECTIONS:

−Jp[a]scale[/origin][r|z] (Polar coordinates (theta,r))
−Jx x-scale[d|l|p pow|t|T][/y-scale[d|l|p pow|t|T]] (Linear, log, and power scaling)

## OPTIONS

No space between the option flag and the associated arguments.

| −A |  | annot_int is annotation interval in data units; it is ignored if contour levels are given in a file. [Default is no annotations]. Append - to disable all annotations implied by −C. The optional labelinfo controls the specifics of the label formatting and consists of a concatenated string made up of any of the following control arguments:

+a angle

For annotations at a fixed angle, +an for line-normal, or +ap for line-parallel [Default]. By appending the u or d we get annotations whose top face the next upper or lower annotation, respectively.

+c dx[/dy]

Sets the clearance between label and optional text box. Append c|i|m|p to specify the unit or % to indicate a percentage of the label font size [15%].

| +d |  | Turns on debug which will draw helper points and lines to illustrate the workings of the quoted line setup.
| +f font |  | Sets the desired font [Default ANNOT_FONT_PRIMARY].

+g[color]

Selects opaque text boxes [Default is transparent]; optionally specify the color [Default is PAGE_COLOR]. (See SPECIFYING COLOR below).

| +j just |  | Sets label justification [Default is MC]. Ignored when -SqN|n+|-1 is used.

+k color

Sets color of text labels [Default is COLOR_BACKGROUND]. (See SPECIFYING COLOR below).

+n dx[/dy]

Nudges the placement of labels by the specified amount (append c|i|m|p to specify the units). Increments are considered in the coordinate system defined by the orientation of the line; use +N to force increments in the plot x/y coordinates system [no nudging].

| +o |  | Selects rounded rectangular text box [Default is rectangular]. Not applicable for curved text (+v) and only makes sense for opaque text boxes.

+p[pen]

Draws the outline of text boxsets [Default is no outline]; optionally specify pen for outline [Default is width = 0.25p, color = black, texture = solid]. (See SPECIFYING PENS below).

+r min_rad

Will not place labels where the line’s radius of curvature is less than min_rad [Default is 0].

| +s size |  | Sets the desired font size in points [Default is 9].
| +u unit |  | Appends unit to all line labels. If unit starts with a leading hyphen (-) then there will be no space between label value and the unit. If z is appended we use the unit specified in the grid file. [Default is no unit].
| +v |  | Specifies curved labels following the path [Default is straight labels].
| +w |  | Specifies how many (x, y) points will be used to estimate label angles [Default is 10].

+=prefix

Prepends prefix to all line labels. If prefix starts with a leading hyphen (-) then there will be no space between label value and the prefix. [Default is no prefix].

| −B |  | Sets map boundary annotation and tickmark intervals; see the psbasemap man page for all the details.

−D

Dump the (x,y,z) coordinates of each contour to separate files, one for each contour segment. The files will be named dumpfile_cont_segment[_i].xyz (or .b is −b is selected), where cont is the contour value and segment is a running segment number for each contour interval (for closed contours we append _i.) If the prefix is given as ’-’ the file names are instead C#_i (interior) or C#_e (external) plus extension, and # is just a running number. This allows us to make short file names that will work with GNU utilities under DOS. However, when −m is used in conjunction with −D a single multisegment file is created instead.

| −E |  | Sets the viewpoint’s azimuth and elevation (for perspective view) [180/90]. For frames used for animation, you may want to append + to fix the center of your data domain (or specify a particular world coordinate point with +w lon0/lat[/z]) which will project to the center of your page size (or specify the coordinates of the projected view point with +v x0/y0).
| −F |  | Force dumped contours to be oriented so that higher z-values are to the left (−Fl [Default]) or right (-Fr) as we move along the contour [Default is arbitrary orientation]. Requires −D.
| −G |  | Controls the placement of labels along the contours. Choose among five controlling algorithms:

−Gd dist[c|i|m|p] or −GD dist[d|e|k|m|n]

For lower case d, give distances between labels on the plot in your preferred measurement unit c (cm), i (inch), m (meter), or p (points), while for upper case D, specify distances in map units and append the unit; choose among e (m), k (km), m (mile), n (nautical mile), or d (spherical degree). [Default is 10c or 4i].

−Gf ffile.d

Reads the ascii file ffile.d and places labels at locations in the file that matches locations along the contours. Inexact matches and points outside the region are skipped.

−Gl|L line1[,line2,...]

Give start and stop coordinates for one or more comma-separated straight line segments. Labels will be placed where these lines intersect the contours. The format of each line specification is start/stop, where start and stop are either a specified point lon/lat or a 2-character XY key that uses the justification format employed in pstext to indicate a point on the map, given as [LCR][BMT]. In addition, you may use Z+ and Z- which correspond to the locations of the global max and min locations in the grid, respectively. −GL will interpret the point pairs as defining great circles [Default is straight line].

−Gn n_label

Specifies the number of equidistant labels for contours line [1]. Upper case −GN starts labeling exactly at the start of the line [Default centers them along the line]. −GN-1 places one justified label at start, while −GN+1 places one justified label at the end of contours. Optionally, append /min_dist[c|i|m|p] to enforce that a minimum distance separation between successive labels is enforced.

−Gx|X xfile.d

Reads the multi-segment file xfile.d and places labels at the intersections between the contours and the lines inxfile.d. −GX will resample the lines first along great-circle arcs.

In addition, you may optionally append +r radius[c|i|m|p] to set a minimum label separation in the x-y plane [no limitation].

| −K |  | More PostScript code will be appended later [Default terminates the plot system].
| −L |  | Limit range: Do not draw contours for data values below low or above high.
| −O |  | Selects Overlay plot mode [Default initializes a new plot system].
| −P |  | Selects Portrait plotting mode [Default is Landscape, see gmtdefaults to change this].
| −Q |  | Do not draw contours with less than cut number of points [Draw all contours].

−R

xmin, xmax, ymin, and ymax specify the Region of interest. For geographic regions, these limits correspond to west, east, south, and north and you may specify them in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and upper right map coordinates are given instead of w/e/s/n. The two shorthands −Rg and −Rd stand for global domain (0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude). Alternatively, specify the name of an existing grid file and the −R settings (and grid spacing, if applicable) are copied from the grid. For calendar time coordinates you may either give (a) relative time (relative to the selected TIME_EPOCH and in the selected TIME_UNIT; append t to −JX|x), or (b) absolute time of the form [date]T[clock] (append T to −JX|x). At least one of date and clock must be present; the T is always required. The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock string must be of the form hh:mm:ss[.xxx]. The use of delimiters and their type and positions must be exactly as indicated (however, input, output and plot formats are customizable; see gmtdefaults). [Default is region defined in the grid file].

| −S |  | Used to resample the contour lines at roughly every (gridbox_size/smoothfactor) interval.

−T

Will draw tickmarks pointing in the downward direction every gap along the innermost closed contours. Append gap and tickmark length or use defaults [0.5c/0.1c or 0.2i/0.04i]. User may choose to tick only local highs or local lows by specifying −T+ or −T−, respectively. Appending :LH will plot the characters L and H at the center of closed innermost contours (local lows and highs). L and H can be any single character (e.g., LH, -+, etc.) If a file is given by −C and −T is set, then only contours marked with upper case C or A will have tickmarks [and annotation].

−U

Draw Unix System time stamp on plot. By adding just/dx/dy/, the user may specify the justification of the stamp and where the stamp should fall on the page relative to lower left corner of the plot. For example, BL/0/0 will align the lower left corner of the time stamp with the lower left corner of the plot. Optionally, append a label, or c (which will plot the command string.). The GMT parameters UNIX_TIME, UNIX_TIME_POS, and UNIX_TIME_FORMAT can affect the appearance; see the gmtdefaults man page for details. The time string will be in the locale set by the environment variable TZ (generally local time).

| −V |  | Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].

−W

type, if present, can be a for annotated contours or c for regular contours [Default]. pen sets the attributes for the particular line. Default values for annotated contours: width = 0.75p, color = black, texture = solid. Regular contours have default width = 0.25p. (See SPECIFYING PENS below). If the + flag is specified then the color of the contour lines are taken from the cpt file (see −C).

−X −Y

Shift plot origin relative to the current origin by (x-shift,y-shift) and optionally append the length unit (c, i, m, p). You can prepend a to shift the origin back to the original position after plotting, or prepend r [Default] to reset the current origin to the new location. If −O is used then the default (x-shift,y-shift) is (0,0), otherwise it is (r1i, r1i) or (r2.5c, r2.5c). Alternatively, give c to align the center coordinate (x or y) of the plot with the center of the page based on current page size.

−Z

Use to subtract shift from the data and multiply the results by factor before contouring starts [1/0]. (Numbers in −A, −C, −L refer to values after scaling and translation have occurred.) Append p to indicate that this grid file contains z-values that are periodic in 360 degrees (e.g., phase data, angular distributions) and that special precautions must be taken when determining 0-contours.

| −bo |  | Selects binary output. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of desired columns in your binary output file.
| −c |  | Specifies the number of plot copies. [Default is 1].

−f

Special formatting of input and/or output columns (time or geographical data). Specify i or o to make this apply only to input or output [Default applies to both]. Give one or more columns (or column ranges) separated by commas. Append T (absolute calendar time), t (relative time in chosen TIME_UNIT since TIME_EPOCH), x (longitude), y (latitude), or f (floating point) to each column or column range item. Shorthand −f[i|o]g means −f[i|o]0x,1y (geographic coordinates).

| −m |  | When used in conjunction with −D a single multisegment file is created, and each contour section is preceded by a header record whose first column is flag followed by the contour level.

SPECIFYING PENS

pen

The attributes of lines and symbol outlines as defined by pen is a comma delimetered list of width, color and texture, each of which is optional. width can be indicated as a measure (points, centimeters, inches) or as faint, thin[ner|nest], thick[er|est], fat[ter|test], or obese. color specifies a gray shade or color (see SPECIFYING COLOR below). texture is a combination of dashes ‘-’ and dots ‘.’.

SPECIFYING COLOR

| color |  | The color of lines, areas and patterns can be specified by a valid color name; by a gray shade (in the range 0−255); by a decimal color code (r/g/b, each in range 0−255; h-s-v, ranges 0−360, 0−1, 0−1; or c/m/y/k, each in range 0−1); or by a hexadecimal color code (#rrggbb, as used in HTML). See the gmtcolors manpage for more information and a full list of color names.

## ASCII FORMAT PRECISION

The ASCII output formats of numerical data are controlled by parameters in your .gmtdefaults4 file. Longitude and latitude are formatted according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to D_FORMAT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (−bo if available) or specify more decimals using the D_FORMAT setting.

## FILE FORMATS

GMT is able to recognize many of the commonly used grid file formats, as well as the precision, scale and offset of the values contained in the grid file. When GMT needs a little help with that, you can add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid type and precision, and scale and offset are optional scale factor and offset to be applied to all grid values, and nan is the value used to indicate missing data. See grdreformat(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the first 2-dimensional grid that can find in that file. To coax GMT into reading another multi-dimensional variable in the grid file, append ?varname to the file name, where varname is the name of the variable. Note that you may need to escape the special meaning of ? in your shell program by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes. See grdreformat(1) and Section 4.18 of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.

## EXAMPLES

To contour the file hawaii_grav.grd every 25 mGal on a Mercator map at 0.5 inch/degree, annotate every 50 mGal (using fontsize = 10), using 1 degree tickmarks, and draw 30 minute gridlines:

grdcontour hawaii_grav.grd −Jm 0.5i −C 25 −A 50+s10 −B 1g30m > hawaii_grav.ps

To contour the file image.grd using the levels in the file cont.d on a linear projection at 0.1 cm/x-unit and 50 cm/y-unit, using 20 (x) and 0.1 (y) tickmarks, smooth the contours a bit, use "RMS Misfit" as plot-title, use a thick red pen for annotated contours, and a thin, dashed, blue pen for the rest, and send the output to the default printer:

grdcontour image.grd −Jx 0.1c/50.0c −C cont.d −S 4 −B 20/0.1:."RMS Misfit": −Wa thick,red −Wc thinnest,blue,- | lp

The labeling of local highs and lows may plot outside the innermost contour since only the mean value of the contour coordinates is used to position the label.

## SEE ALSO

GMT(1), gmtdefaults(1), gmtcolors(5), psbasemap(1), grdimage(1), grdview(1), pscontour(1)