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Manual Reference Pages  -  GMT2KML (1)


gmt2kml - Convert GMT data tables to KML files for Google Earth


     Specifying Pens
     Specifying Color


gmt2kml [ infile(s) ] [ -Aa|g|s[alt|xscale] ] [ -Ccpt ] [ -Ddescriptfile ] [ -E[altitude] ] [ -Fe|s[cpt]|t|l|p ] [ -Gf|n[-|fill] ] [ -H[i][nrec] ] [ -Iicon ] [ -K] [ -Lcol1:name1,col2:name2,... ] [ -N[+|name_template|name] ] [ -O] [ -Q[s|l|p]transparency ] [ -Ra|w/e/s/n ] [ -Sc|nscale] ] [ -Ttitle[/foldername] ] [ -V ] [ -W-|pen ] [ -Zargs ] [ -:[i|o] ] [ -bi[s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ] [ -m[i|o][flag] ] [ > output.kml ]


gmt2kml reads one or more GMT table file and converts them to a single output file using Google Earth’s KML format. Data may represent points, lines, or polygons, and you may specify additional attributes such as title, altitude mode, colors, pen widths, transparency, regions, and data descriptions. You may also extend the feature down to ground level (assuming it is above it) and use custom icons for point symbols.
The input file should contain the following columns:
lon lat [ alt ] [ timestart [ timestop ] ]
where lon and lat are required for all features, alt is optional for all features (see also -A and -C), and timestart and timestop apply to events and timespan features.
  ASCII (or binary, see -bi) data file(s) to be operated on. If not given, standard input will be read.


No space between the option flag and the associated arguments.
-A Select one of three altitude modes recognized by Google Earth that determines the altitude (in m) of the feature: a absolute altitude, g altitude relative to sea surface or ground, s altitude relative to seafloor. To plot the features at a fixed altitude, append an altitude alt (in m). Use 0 to clamp the features to the chosen reference surface. Append xscale to scale the altitude from the input file by that factor. If no value is appended, the altitude (in m) is read from the 3rd column of the input file. [By default the features are clamped to the sea surface or ground].
-C Use color palette for assigning colors to the symbol, event, or timespan icons, based on the value in the 3rd column of the input file. Ignored when plotting lines or polygons.
-D File with HTML snippets that will be included as part of the main description content for the KML file [no description]. See SEGMENT INFORMATION below for feature-specific descriptions.
-E Extrude feature down to ground level [no extrusion].
-F Sets the feature type. Choose from points (event, symbol, or timespan), line, or polygon [symbol]. The first two columns of the input file should contain (lon, lat). When altitude or value is required (i.e., no altitude value was given with -A, or -C is set), the third column needs to contain the altitude (in m). The event (-Fe) is a symbol that should only be active at a particular time, given in the next column. Timespan (-Ft) is a symbol that should only be active during a particular time period indicated by the next two columns (timestart, timestop). Use NaN to indicate unbounded time limits. If used, times should be in ISO format yyyy-mm-ddThh:mm:ss[.xxx] or in GMT relative time format (see -f).
-G Set fill color for symbols, extrusions and polygons (-Gf) [Default is lightorange] or text labels (-Gn) [Default is white]. Optionally, use -Gf- to turn off polygon fill, and -Gn- to disable labels. (See SPECIFYING FILL below).
-H Input file(s) has header record(s). If used, the default number of header records is N_HEADER_RECS. Use -Hi if only input data should have header records [Default will write out header records if the input data have them]. Blank lines and lines starting with # are always skipped.
-I Specify the URL to an alternative icon that should be used for the symbol [Default is a Google Earth circle]. If the URL starts with + then we will prepend to the name. [Default is a local icon with no directory path].
-K Allow more KML code to be appended to the output later [finalize the KML file].
-L Extended data given. Append one or more strings of the form col:name separated by commas. We will expect the listed data columns to exist in the input and they will be encoded in the KML file as Extended Data sets, whose attributes will be available in the Google Earth balloon when the item is selected.
-N By default, if multisegment headers contain a -L"label string" then we use that for the name of the KML feature (polygon, line segment or set of symbols). Default names for these segments are "Line %d" and "Point Set %d", depending on the feature, where %d is a sequence number of line segments within a file. Each point within a line segment will be named after the line segment plus a sequence number. Default is simply "Point %d".
Alternatively, select one of these options: (1) append + to supply individual symbol labels directly at the end of the data record, (2) append a string that may include %d or a similar integer format to assign unique name IDs for each feature, with the segment number (for lines and polygons) or point number (symbols) appearing where %d is placed, (3) give no arguments to turn symbol labeling off; line segments will still be named.
-O Appended KML code to an existing KML file [initialize a new KML file].
-Q Set the transparency level for the selected feature (e, s, t, l, or p, plus n for name labels). Transparency goes from 0 (fully transparent) to 1 (opaque) [0.75 for polygons, 1 for symbols, lines, and labels].
-R Issue a single Region tag. Append w/e/s/n to set a particular region (will ignore points outside the region), or append a to determine and use the actual domain of the data (single file only) [no region tags issued].
-S Scale icons or labels. Here, -Sc sets a scale for the symbol icon, whereas -Sn sets a scale for the name labels [1 for both].
-T Sets the document title [GMT Data Document]. Optionally, append /FolderName; this allows you, with -O, -K, to group features into folders within the KML document. [The default folder name is "Name Features", where Name is Point, Event, Timespan, Line, or Polygon].
-V Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
-W Set pen attributes for lines or polygon outlines. Append pen attributes to use [Defaults: width = 1p, color = black, texture = solid]. Optionally, use -W- to turn off polygon outline Note that for KML the pen width is given as integer pixel widths so you must specify pen width as np, where n is an integer. (See SPECIFYING PENS below).
-Z Set one or more attributes of the Document and Region tags. Append +aalt_min/alt_max to specify limits on visibility based on altitude. Append +llod_min/lod_max to specify limits on visibility based on Level Of Detail, where lod_max == -1 means it is visible to infinite size. Append +ffade_min/fade_max to fade in and out over a ramp [abrupt]. Append +v to make a feature not visible when loaded [visible]. Append +o to open a folder or document in the sidebar when loaded [closed].
-: Toggles between (longitude,latitude) and (latitude,longitude) input and/or output. [Default is (longitude,latitude)]. Append i to select input only or o to select output only. [Default affects both].
-bi Selects binary input. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of columns in your binary input file if it exceeds the columns needed by the program. Or append c if the input file is netCDF. Optionally, append var1/var2/... to specify the variables to be read. [Default is 2 input columns].
-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 Multiple segment file(s). Segments are separated by a special record. For ASCII files the first character must be flag [Default is ’>’]. For binary files all fields must be NaN and -b must set the number of output columns explicitly. By default the -m setting applies to both input and output. Use -mi and -mo to give separate settings to input and output. The -m option make sure that segment headers in the input files are copied to output, but it has no effect on the data selection. Selection is always done point by point, not by segment.


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 ‘.’.


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.


To convert a file with point locations (lon, lat) into a KML file with red circle symbols, try

gmt2kml mypoints.txt -Gf red -Fs > mypoints.kml

To convert a multisegment file with lines (lon, lat) separated by multisegment headers that contain a -L labelstring with the feature name, selecting a thick white pen, and title the document, try

gmt2kml mylines.txt -W thick,white -Fl -T"Lines from here to there" > mylines.kml

To convert a multisegment file with polygons (lon, lat) separated by multisegment headers that contain a -L labelstring with the feature name, selecting a thick black pen and semi-transparent yellow fill, giving a title to the document, and prescribing a particular region limit, try

gmt2kml mypolygons.txt -Gf yellow -Qp 0.5 -Fp -T"My polygons" -R 30/90/-20/40 > mypolygons.kml

To convert a file with point locations (lon, lat, time) into a KML file with green circle symbols that will go active at the specified time and stay active going forward, try

awk ’{print $1, $2, $3, "NaN"}’ mypoints.txt | gmt2kml -Gf green -Ft > mytimepoints.kml


Google Earth has trouble displaying filled polygons across the Dateline. For now you must manually break any polygon crossing the dateline into a west and east polygon and plot them separately.


Using the KMZ format is preferred as it takes less space. KMZ is simply a KML file and any data files, icons, or images referenced by the KML, contained in a zip archive. One way to organize large data sets is to split them into groups called Folders. A Document can contain any number of folders. Using scripts you can create a composite KML file using the -K, -O options just like you do with GMT plots. See -T for switching between folders and documents.


GMT stores the different features in hierarchical folders, by feature type (when using -O, -K or -T/foldername), by input file (if not standard input), and by line segment (using the name from the segment header, or -N). This makes it more easy in Google Earth to switch on or off parts of the contents of the Document. The following is a crude example:

[ KML header information - not present if -O was given ]
<Document><name>GMT Data Document</name>
<Folder><name>Point Features</name>
<!--This level of folder is inserted only when using -O, -K>
<!--One folder for each input file (not when standard input)>
<Folder><name>Point Set 0</name>
<!--One folder per line segment>
<!--Points from the first line segment in file file1.dat go here>
<Folder><name>Point Set 1</name>
<!--Points from the second line segment in file file1.dat go here>
<Folder><name>Line Features</name>
<!--One folder for each input file (not when standard input)>
<Placemark><name>Line 0</name>
<!--Here goes the first line segment>
<Placemark><name>Line 1</name>
<!--Here goes the second line segment>
[ KML trailer information - not present if -K was given ]


gmt2kml will scan the segment headers for substrings of the form -L"some label" [also see -N discussion] and -D"some description". If present, these are parsed to supply name and description tags for the current feature.


gmtdefaults(1), GMT(1), img2google(1), kml2gmt(1), ps2raster(1)
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GMT 4.5.14 GMT2KML (1) 1 Nov 2015

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