triangulate

Delaunay triangulation or Voronoi partitioning and gridding of Cartesian data

Synopsis

gmt triangulate [ table ] [ -A ] [ -Cslpfile ] [ -Dx|y ] [ -Eempty ] [ -Goutgrid ] [ -Iincrement ] [ -Jparameters ] [ -Lindexfile[+b] ] [ -M ] [ -N ] [ -Q[n] ] [ -Rregion ] [ -S[first][+z[a|l|m|p|u]][+n]] [ -T ] [ -V[level] ] [ -Z ] [ -bbinary ] [ -dnodata[+ccol] ] [ -eregexp ] [ -fflags ] [ -hheaders ] [ -iflags ] [ -qiflags ] [ -rreg ] [ -sflags ] [ -wflags ] [ -:[i|o] ] [ --PAR=value ]

Note: No space is allowed between the option flag and the associated arguments.

Description

triangulate reads one or more ASCII [or binary] files (or standard input) containing x, y[, z] and performs Delaunay triangulation, i.e., it finds how the points should be connected to give the most equilateral triangulation possible. If a map projection (give -R and -J) is chosen then it is applied before the triangulation is calculated. By default, the output is triplets of point id numbers that make up each triangle and is written to standard output. The id numbers refer to the points position (line number, starting at 0 for the first line) in the input file. As an option, you may choose to create a multiple segment file that can be piped through plot to draw the triangulation network. If -G -I are set a grid will be calculated based on the surface defined by the planar triangles. The actual algorithm used in the triangulation is either that of Watson [1982] or Shewchuk [1996] [Default] (if installed; type gmt get GMT_TRIANGULATE to see which method is selected). This choice is made during the GMT installation. Furthermore, if the Shewchuk algorithm is installed then you can also perform the calculation of Voronoi polygons and optionally grid your data via the natural nearest neighbor algorithm. Some Linux users may find their distribution does not provide Shewchuk due to it not being released under a GNU License. Note: For geographic data with global or very large extent you should consider sphtriangulate instead since triangulate is a Cartesian or small-geographic area operator and is unaware of periodic or polar boundary conditions.

Required Arguments

table

One or more ASCII (or binary, see -bi[ncols][type]) data table file(s) holding a number of data columns. If no tables are given then we read from standard input.

Optional Arguments

-A

Compute the area of the Cartesian triangles and append the areas in the output segment headers [no areas calculated]. Requires -S and is not compatible with -Q.

-Cslpfile

Read a slope grid (in degrees) and compute the propagated uncertainty in the bathymetry using the CURVE algorithm [Zambo et al., 2016]. Requires the -G option to specify the output grid. Note that the slpgrid sets the domain for the output grid so -R, -I, [-rreg] are not required. Cannot be used in conjunction with -D, -F, -M, -N, -Q, -S and -T.

-Dx|y

Take either the x- or y-derivatives of surface represented by the planar facets (only used when -G is set).

-Eempty

Set the value assigned to empty nodes when -G is set [NaN].

-Goutgrid[=ID][+ddivisor][+ninvalid][+ooffset|a][+sscale|a][:driver[dataType][+coptions]]

Optionally, append =ID for writing a specific file format. The following modifiers are supported:

  • +d - Divide data values by given divisor [Default is 1].

  • +n - Replace data values matching invalid with a NaN.

  • +o - Offset data values by the given offset, or append a for automatic range offset to preserve precision for integer grids [Default is 0].

  • +s - Scale data values by the given scale, or append a for automatic scaling to preserve precision for integer grids [Default is 1].

Note: Any offset is added before any scaling. +sa also sets +oa (unless overridden). To write specific formats via GDAL, use =gd and supply driver (and optionally dataType) and/or one or more concatenated GDAL -co options using +c. See the “Writing grids and images” cookbook section for more details.

-Ix_inc[+e|n][/y_inc[+e|n]]

Set the grid spacing as x_inc [and optionally y_inc].

Geographical (degrees) coordinates: Optionally, append an increment unit. Choose among:

  • d - Indicate arc degrees

  • m - Indicate arc minutes

  • s - Indicate arc seconds

If one of e (meter), f (foot), k (km), M (mile), n (nautical mile) or u (US survey foot), the increment will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on PROJ_ELLIPSOID). If y_inc is not given or given but set to 0 it will be reset equal to x_inc; otherwise it will be converted to degrees latitude.

All coordinates: The following modifiers are supported:

  • +e - Slightly adjust the max x (east) or y (north) to fit exactly the given increment if needed [Default is to slightly adjust the increment to fit the given domain].

  • +n - Define the number of nodes rather than the increment, in which case the increment is recalculated from the number of nodes, the registration (see GMT File Formats), and the domain. Note: If -Rgrdfile is used then the grid spacing and the registration have already been initialized; use -I and -R to override these values.

-Jparameters

Specify the projection. (See full description) (See technical reference) (See projections table).

-Lindexfile[+b]

Give name of file with previously computed Delaunay information. Each record must contain triplets of node numbers for a triangle in the input table [Default computes these using Delaunay triangulation]. If the indexfile is binary and can be read the same way as the binary input table then you can append +b to spead up the reading [Default reads nodes as ASCII].

-M

Output triangulation network as multiple line segments separated by a segment header record.

-N

Used in conjunction with -G to also write the triplets of the ids of all the Delaunay vertices [Default only writes the grid].

-Q[n]

Output the edges of the Voronoi cells instead [Default is Delaunay triangle edges]. Requires -R and is only available if linked with the Shewchuk [1996] library. Note that -Z is ignored on output. Optionally, append directive n for combining the edges into closed Voronoi polygons.

-Rxmin/xmax/ymin/ymax[+r][+uunit]

Specify the region of interest. (See full description) (See technical reference).

-S[first][+z[a|l|m|p|u]][+n]

Output triangles as polygon segments separated by a segment header record which contains node numbers a-b-c and -Zpolynumber. However, if the +n flag is used, we skip writing the polynumber and the node numbers. Optionally, append first, where first is an integer, to report the polygon numbers by counting from first [Default starts at zero]. Incompatible with -Q. Optionally, add modifier

  • +z - Request that -Zzvalue is placed in the segment headers, where zvalue is a representable value for each triangle. Note:

Modifier +z implies -Z. Append directives to select that value:

  • a - Select the mean value [Default].

  • l - Select the lowest value.

  • m - Select the median.

  • p - Select the mode.

  • u - Select the upper value.

-T

Output edges or polygons even if gridding has been selected with the -G option [Default will not output the triangulation or Voronoi polygons if gridding is selected].

-V[level]

Select verbosity level [w]. (See full description) (See technical reference).

-Z

Controls whether we read (x, y) or (x, y, z) data and if z should be output when -M or -S (without +z) are used [Read (x, y) only].

-birecord[+b|l] (more …)

Select native binary format for primary table input. [Default is 2 input columns].

-borecord[+b|l] (more …)

Select native binary format for table output. [Default is same as input]. Node ids are stored as double triplets.

-d[i|o][+ccol]nodata (more …)

Replace input columns that equal nodata with NaN and do the reverse on output.

-e[~]“pattern” | -e[~]/regexp/[i] (more …)

Only accept data records that match the given pattern.

-f[i|o]colinfo (more …)

Specify data types of input and/or output columns.

-h[i|o][n][+c][+d][+msegheader][+rremark][+ttitle] (more …)

Skip or produce header record(s).

-icols[+l][+ddivisor][+sscale|d|k][+ooffset][,…][,t[word]] (more …)

Select input columns and transformations (0 is first column, t is trailing text, append word to read one word only).

-qi[~]rows|limits[+ccol][+a|t|s] (more …)

Select input rows or data limit(s) [default is all rows].

-r[g|p] (more …)

Set node registration [gridline]. (Only valid with -G).

-s[cols][+a][+r] (more …)

Set handling of NaN records for output.

-wy|a|w|d|h|m|s|cperiod[/phase][+ccol] (more …)

Convert an input coordinate to a cyclical coordinate.

-:[i|o] (more …)

Swap 1st and 2nd column on input and/or output.

-^ or just -

Print a short message about the syntax of the command, then exit (Note: on Windows just use -).

-+ or just +

Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.

-? or no arguments

Print a complete usage (help) message, including the explanation of all options, then exit.

--PAR=value

Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

ASCII Format Precision

The ASCII output formats of numerical data are controlled by parameters in your gmt.conf file. Longitude and latitude are formatted according to FORMAT_GEO_OUT, absolute time is under the control of FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in ASCII 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 FORMAT_FLOAT_OUT setting.

Grid Values Precision

Regardless of the precision of the input data, GMT programs that create grid files will internally hold the grids in 4-byte floating point arrays. This is done to conserve memory and furthermore most if not all real data can be stored using 4-byte floating point values. Data with higher precision (i.e., double precision values) will lose that precision once GMT operates on the grid or writes out new grids. To limit loss of precision when processing data you should always consider normalizing the data prior to processing.

Inside/outside Status

To determine if a point is inside, outside, or exactly on the boundary of a polygon we need to balance the complexity (and execution time) of the algorithm with the type of data and shape of the polygons. For any Cartesian data we use a non-zero winding algorithm, which is quite fast. For geographic data we will also use this algorithm as long as (1) the polygons do not include a geographic pole, and (2) the longitude extent of the polygons is less than 360. If this is the situation we also carefully adjust the test point longitude for any 360 degree offsets, if appropriate. Otherwise, we employ a full spherical ray-shooting method to determine a points status.

Examples

Note: Below are some examples of valid syntax for this module. The examples that use remote files (file names starting with @) can be cut and pasted into your terminal for testing. Other commands requiring input files are just dummy examples of the types of uses that are common but cannot be run verbatim as written.

To triangulate the points in the file samples.xyz, store the triangle information in a binary file, and make a grid for the given area and spacing, use

gmt triangulate samples.xyz -bo -R0/30/0/30 -I2 -Gsurf.nc > samples.ijk

To draw the optimal Delaunay triangulation network based on the same file using a 15-cm-wide Mercator map, use

gmt triangulate samples.xyz -M -R-100/-90/30/34 -JM15c | gmt plot -R-100/-90/30/34 -JM15c -W0.5p -B1 -pdf network

To instead plot the Voronoi cell outlines, try

gmt triangulate samples.xyz -M -Q -R-100/-90/30/34 -JM15c | gmt plot -R-100/-90/30/34 -JM15c -W0.5p -B1 -pdf cells

To combine the Voronoi outlines into polygons and paint them according to their ID, try

gmt triangulate samples.xyz -M -Qn -R-100/-90/30/34 -JM15c | \
    gmt plot -R-100/-90/30/34 -JM15c -W0.5p+cf -B1 -Ccolors.cpt -pdf polygons

To grid the data using the natural nearest neighbor algorithm, try

gmt triangulate samples.xyz -Gnnn.nc -Qn -R-100/-90/30/34 -I0.5

Notes

The uncertainty propagation for bathymetric grids requires both horizontal and vertical uncertainties and these are weighted given the local slope. See the Zambo et al. [2014] and Zhou and Liu [2004] references for more details.

See Also

gmt, greenspline, nearneighbor, contour, sphdistance, sphinterpolate, sphtriangulate, surface

References

Shewchuk, J. R., 1996, Triangle: Engineering a 2D Quality Mesh Generator and Delaunay Triangulator, First Workshop on Applied Computational Geometry (Philadelphia, PA), 124-133, ACM, May 1996.

Watson, D. F., 1982, Acord: Automatic contouring of raw data, Comp. & Geosci., 8, 97-101.

Zambo, S., Elmore, P. A., Bourgeois, B. S., and Perkins, A. L., 2016, Uncertainty estimation for sparse data gridding algorithms, Proceedings of the U.S. Hydro Conference,National Harbor, MD, 16-19 March 2015.

Zhou, Q., and Liu, X., 2004, Error analysis on grid-based slope and aspect algorithms, Photogrammetric Eng. & Remote Sensing, 70 (8), 957-962.

Shewchuk’s Homepage