Project data onto lines or great circles, or generate tracks


gmt project [ table ] -Ccx/cy [ -Aazimuth ] [ -Ebx/by ] [ -Fflags ] [ -Gdist[/colat][+h] ] [ -L[w|l_min/l_max] ] [ -N ] [ -Q ] [ -S ] [ -Tpx/py ] [ -V[level] ] [ -Ww_min/w_max ] [ -Zmajor/minor/azimuth[+e] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [ -oflags ] [ -sflags ] [ -:[i|o] ] [ --PAR=value ]

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


project reads arbitrary (x, y[,z]) data from standard input [or table ] and writes to standard output any combination of (x, y, z, p, q, r, s), where (p, q) are the coordinates in the projection, (r, s) is the position in the (x, y) coordinate system of the point on the profile (q = 0 path) closest to (x, y), and z is all remaining columns in the input (beyond the required x and y columns).

Alternatively, project may be used to generate (r, s, p) triples at equal increments dist along a profile. In this case ( -G option), no input is read.

Projections are defined in any (but only) one of three ways:

(Definition 1) By a Center -C and an Azimuth -A in degrees clockwise from North.

(Definition 2) By a Center -C and end point E of the projection path -E.

(Definition 3) By a Center -C and a roTation pole position -T.

To spherically project data along a great circle path, an oblique coordinate system is created which has its equator along that path, and the zero meridian through the Center. Then the oblique longitude (p) corresponds to the distance from the Center along the great circle, and the oblique latitude (q) corresponds to the distance perpendicular to the great circle path. When moving in the increasing (p) direction, (toward B or in the azimuth direction), the positive (q) direction is to your left. If a Pole has been specified, then the positive (q) direction is toward the pole.

To specify an oblique projection, use the -T option to set the Pole. Then the equator of the projection is already determined and the -C option is used to locate the p = 0 meridian. The Center cx/cy will be taken as a point through which the p = 0 meridian passes. If you do not care to choose a particular point, use the South pole (ox = 0, oy = -90).

Data can be selectively windowed by using the -L and -W options. If -W is used, the projection Width is set to use only points with w_min < q < w_max. If -L is set, then the Length is set to use only those points with l_min < p < l_max. If the -E option has been used to define the projection, then -Lw may be selected to window the length of the projection to exactly the span from O to B.

Flat Earth (Cartesian) coordinate transformations can also be made. Set -N and remember that azimuth is clockwise from North (the y axis), NOT the usual cartesian theta, which is counterclockwise from the x axis. azimuth = 90 - theta.

No assumptions are made regarding the units for x, y, r, s, p, q, dist, l_min, l_max, w_min, w_max. If -Q is selected, map units are assumed and x, y, r, s must be in degrees and p, q, dist, l_min, l_max, w_min, w_max will be in km.

Calculations of specific great-circle and geodesic distances or for back-azimuths or azimuths are better done using mapproject as project is strictly spherical.

project is CASE SENSITIVE. Use UPPER CASE for all one-letter designators which begin optional arguments. Use lower case for the xyzpqrs letters in Fflags.

Required Arguments

cx/cy sets the origin of the projection, in Definition 1 or 2. If Definition 3 is used (-T), then cx/cy are the coordinates of a point through which the oblique zero meridian (p = 0) should pass. The cx/cy is not required to be 90 degrees from the pole.

Optional Arguments

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.
azimuth defines the azimuth of the projection (Definition 1).
bx/by defines the end point of the projection path (Definition 2).
Specify your desired output using any combination of xyzpqrs, in any order [Default is xyzpqrs]. Do not space between the letters. Use lower case. The output will be ASCII (or binary, see -bo) columns of values corresponding to your flags. The z flag is special and refers to all numerical columns beyond the leading x and y in your input record. If output format is ASCII then z also includes any trailing text (which is placed at the end of the record regardless of the order of z in flags). Note: If -G is selected, then the output order is hardwired to be rsp and -F is not allowed.
Generate mode. No input is read. Create (r, s, p) output points every dist units of p. See -Q option. Alternatively, append /colat for a small circle instead [Default is a colatitude of 90, i.e., a great circle]. Use -C and -E to generate a circle that goes through the center and end point. Note, in this case the center and end point cannot be farther apart than 2*|colat|. Finally, if you append +h the we will report the position of the pole as part of the segment header [no header].
Length controls. Project only those points whose p coordinate is within l_min < p < l_max. If -E has been set, then you may alternatively use -Lw to stay within the distance from C to E.
Flat Earth. Make a Cartesian coordinate transformation in the plane. [Default uses spherical trigonometry.]
Map type units, i.e., project assumes x, y, r, s are in degrees while p, q, dist, l_min, l_max, w_min, w_max are in km. If -Q is not set, then all these are assumed to be in the same units.
Sort the output into increasing p order. Useful when projecting random data into a sequential profile.
px/py sets the position of the rotation pole of the projection. (Definition 3).
-V[level] (more …)
Select verbosity level [c].
Width controls. Project only those points whose q coordinate is within w_min < q < w_max.
Used in conjunction with -C (sets its center) and -G (sets the distance increment) to create the coordinates of an ellipse with major and minor axes given in km (unless -N is given) and the azimuth of the major axis in degrees. Append +e to adjust the increment set via -G so that the the ellipse has equal distance increments [Default uses the given increment and closes the ellipse].
-bi[ncols][t] (more …)
Select native binary format for primary input. [Default is 2 input columns].
-bo[ncols][type] (more …)
Select native binary output. [Default is given by -F or -G].
-d[i|o]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.
-g[a]x|y|d|X|Y|D|[col]zgap[u][+n|p] (more …)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more …)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,][,t[word]] (more …)
Select input columns and transformations (0 is first column, t is trailing text, append word to read one word only).
-ocols[,…][t[word]] (more …)
Select output columns (0 is first column; t is trailing text, append word to write one word only).
-s[cols][+a|+r] (more …)
Set handling of NaN records.
-:[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 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.
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.


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 project the remote data sets ship_03.txt (lon,lat,depth) onto a great circle specified by the two points (330,-18) and (53,21) and sort the records on the projected distances along that circle and only output the distance and the depths, try:

gmt project @ship_03.txt -C330/-18 -T53/21 -S -Fpz -Q > ship_proj.txt

To generate points every 10km along a great circle from 10N,50W to 30N,10W:

gmt project -C-50/10 -E-10/30 -G10 -Q > great_circle_points.xyp

(Note that great_circle_points.xyp could now be used as input for grdtrack, etc. ).

To generate points every 1 degree along a great circle from 30N,10W with azimuth 30 and covering a full 360, try:

gmt project -C10W/30N -A30 -G1 -L-180/180 > great_circle.txt

To generate points every 10km along a small circle of colatitude 60 from 10N,50W to 30N,10W:

gmt project -C-50/10 -E-10/30 -G10/60 -Q > small_circle_points.xyp

To create a partial small circle of colatitude 80 about a pole at 40E,85N, with extent of 45 degrees to either side of the meridian defined by the great circle from the pole to a point 15E,15N, try

gmt project -C15/15 -T40/85 -G1/80 -L-45/45 > some_circle.xyp

To generate points approximately every 10km along a an ellipse centered on (30W,70N) with major axis of 1500 km with azimuth of 30 degree and a minor axis of 600 km, try

gmt project -C-30/70 -G10 -Z1500/600/30+e > ellipse.xyp

To project the shiptrack gravity, magnetics, and bathymetry in c2610.xygmb along a great circle through an origin at 30S, 30W, the great circle having an azimuth of N20W at the origin, keeping only the data from NE of the profile and within ±500 km of the origin, run:

gmt project c2610.xygmb -C-30/-30 -A-20 -W-10000/0 -L-500/500 -Fpz -Q > c2610_projected.pgmb

(Note in this example that -W-10000/0 is used to admit any value with a large negative q coordinate. This will take those points which are on our right as we walk along the great circle path, or to the NE in this example.)

To make a Cartesian coordinate transformation of mydata.xy so that the new origin is at 5,3 and the new x axis (p) makes an angle of 20 degrees with the old x axis, use:

gmt project mydata.xy -C5/3 -A70 -Fpq > mydata.pq

To take data in the file pacific.lonlat and transform it into oblique coordinates using a pole from the hotspot reference frame and placing the oblique zero meridian (p = 0 line) through Tahiti, run:

gmt project pacific.lonlat -T-75/68 -C-149:26/-17:37 -Fpq > pacific.pq

Suppose that is a grid file of bathymetry, and you want to make a file of flowlines in the hotspot reference frame. If you run:

gmt grd2xyz | gmt project -T-75/68 -C0/-90 -Fxyq | gmt xyz2grd -Retc -Ietc

then is a file in the same area as, but flow contains the latitudes about the pole of the projection. You now can use grdcontour on to draw lines of constant oblique latitude, which are flow lines in the hotspot frame.

If you have an arbitrarily rotation pole px/py and you would like to draw an oblique small circle on a map, you will first need to make a file with the oblique coordinates for the small circle (i.e., lon = 0-360, lat is constant), then create a file with two records: the north pole (0/90) and the origin (0/0), and find what their oblique coordinates are using your rotation pole. Now, use the projected North pole and origin coordinates as the rotation pole and center, respectively, and project your file as in the pacific example above. This gives coordinates for an oblique small circle.