grd2xyz - Convert grid file to data table


grd2xyz grid [ -C[f|i] ] [ -Rregion ] [ -V[level] ] [ -W[a|weight] ] [ -Z[flags] ] [ -bobinary ] [ -dnodata ] [ -fflags ] [ -ho[n] ] [ -oflags ] [ -sflags ]

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


grd2xyz reads one or more binary 2-D grid files and writes out xyz-triplets in ASCII [or binary] format to standard output. Modify the precision of the ASCII output format by editing the FORMAT_FLOAT_OUT parameter in your gmt.conf file or use --D_FORMAT=format on the command line, or choose binary output using single or double precision storage. As an option you may output z-values without the (x,y) coordinates; see -Z below.

Required Arguments

Names of 2-D binary grid files to be converted. (See GRID FILE FORMATS below.)

Optional Arguments

Replace the x- and y-coordinates on output with the corresponding column and row numbers. These start at 0 (C-style counting); append f to start at 1 (Fortran-style counting). Alternatively, append i to write just the two columns index and z, where index is the 1-D indexing that GMT uses when referring to grid nodes.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
Specify the region of interest. Using the -R option will select a subsection of the grid. If this subsection exceeds the boundaries of the grid, only the common region will be output.
-V[level] (more ...)
Select verbosity level [c].
Write out x,y,z,w, where w is the supplied weight (or 1 if not supplied) [Default writes x,y,z only]. Choose -Wa to compute weights equal to the area each node represents.

Write a 1-column ASCII [or binary] table. Output will be organized according to the specified ordering convention contained in flags. If data should be written by rows, make flags start with T (op) if first row is y = ymax or B (ottom) if first row is y = ymin. Then, append L or R to indicate that first element should start at left or right end of row. Likewise for column formats: start with L or R to position first column, and then append T or B to position first element in a row. For gridline registered grids: If grid is periodic in x but the written data should not contain the (redundant) column at x = xmax, append x. For grid periodic in y, skip writing the redundant row at y = ymax by appending y. If the byte-order needs to be swapped, append w. Select one of several data types (all binary except a):

  • a ASCII representation of a single item per record
  • c int8_t, signed 1-byte character
  • u uint8_t, unsigned 1-byte character
  • h int16_t, short 2-byte integer
  • H uint16_t, unsigned short 2-byte integer
  • i int32_t, 4-byte integer
  • I uint32_t, unsigned 4-byte integer
  • l int64_t, long (8-byte) integer
  • L uint64_t, unsigned long (8-byte) integer
  • f 4-byte floating point single precision
  • d 8-byte floating point double precision

Default format is scanline orientation of ASCII numbers: -ZTLa. Note that -Z only applies to 1-column output.

-bo[ncols][type] (more ...)
Select native binary output. [Default is 3]. This option only applies to xyz output; see -Z for z table output.
-d[i|o]nodata (more ...)
Replace input columns that equal nodata with NaN and do the reverse on output.
-f[i|o]colinfo (more ...)
Specify data types of input and/or output columns. See also TIME COORDINATES below. -h Output 1 header record based on information in the first grid file header. Ignored if binary output is selected. [Default is no header].
-ocols[,...] (more ...)
Select output columns (0 is first column).
-s[cols][a|r] (more ...)
Set handling of NaN records.
-^ or just -
Print a short message about the syntax of the command, then exits (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 exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.

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 File Formats

By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format. However, GMT is able to produce grid files in many other commonly used grid file formats and also facilitates so called “packing” of grids, writing out floating point data as 1- or 2-byte integers. (more ...)

Time Coordinates

Time coordinates in netCDF grids, be it the x, y, or z coordinate, will be recognized as such. The variable’s unit attribute is parsed to determine the unit and epoch of the time coordinate in the grid. Values are then converted to the internal time system specified by TIME_UNIT and TIME_EPOCH in the gmt.conf file or on the command line. The default output is relative time in that time system, or absolute time when using the option -f0T, -f1T, or -f2T for x, y, or z coordinate, respectively.


To edit individual values in the 5’ by 5’ file, dump the .nc to ASCII:

gmt grd2xyz >

To write a single precision binary file without the x,y positions from the file file, using scanline orientation, run

gmt grd2xyz -ZTLf > hawaii_grv.b