Geospatial operations on points, lines and polygons
gmt spatial [ table ] [ -A[amin_dist][unit]] [ -C ] [ -D[+ffile][+aamax][+ddmax][+c|Ccmax][+sfact] ] [ -E+p|n ] [ -F[l] ] [ -I[e|i] ] [ -Npfile[+a][+pstart][+r][+z] ] [ -Q[unit][+cmin[/max]][+h][+l][+p][+s[a|d]] ] [ -Rregion ] [ -Sh|i|u|s|j ] [ -T[clippolygon] ] [ -V[level] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [ -jflags ] [ -oflags ] [ -:[i|o] ] [ --PAR=value ]
Note: No space is allowed between the option flag and the associated arguments.
spatial reads one or more data files (which may be multisegment files) that contains closed polygons and operates of these polygons in the specified way. Operations include area calculation, handedness reversals, and polygon intersections.
- 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.
- Perform spatial nearest neighbor (NN) analysis: Determine the nearest neighbor of each point and report the NN distances and the point IDs involved in each pair (IDs are the input record numbers starting at 0). Use -Aa to decimate a data set so that no NN distance is lower than the threshold min_dist. In this case we write out the (possibly averaged) coordinates and the updated NN distances and point IDs. A negative point number means the original point was replaced by a weighted average (the absolute ID value gives the ID of the first original point ID to be included in the average.). Note: The input data are assumed to contain (lon, lat) or (x, y), optionally followed by a z and a weight  column. We compute a weighted average of the location and z (if present).
- Clips polygons to the map region, including map boundary to the polygon as needed. The result is a closed polygon (see -T for truncation instead). Requires -R.
- Check for duplicates among the input lines or polygons, or, if file is given via +f, check if the input features already exist among the features in file. We consider the cases of exact (same number and coordinates) and approximate matches (average distance between nearest points of two features is less than a threshold). We also consider that some features may have been reversed. Features are considered approximate matches if their minimum distance is less than dmax  (see Units) and their closeness (defined as the ratio between the average distance between the features divided by their average length) is less than cmax [0.01]. For each duplicate found, the output record begins with the single letter Y (exact match) or ~ (approximate match). If the two matching segments differ in length by more than a factor of 2 then we consider the duplicate to be either a subset (-) or a superset (+). Finally, we also note if two lines are the result of splitting a continuous line across the Dateline (|). For polygons we also consider the fractional difference in areas; duplicates must differ by less than amax [0.01]. By default, we compute the mean line separation. Use +Ccmin to instead compute the median line separation and therefore a robust closeness value. Also by default we consider all distances between points on one line and another. Append +p to limit the comparison to points that project perpendicularly to points on the other line (and not its extension).
- Reset the handedness of all polygons to match the given +p (counter-clockwise; positive) or +n (clockwise; negative). Implies -Q+.
- Force input data to become polygons on output, i.e., close them explicitly if not already closed. Optionally, append l to force line geometry.
- Determine the intersection locations between all pairs of polygons. Append i to only compute internal (i.e., self-intersecting polygons) crossovers or e to only compute external (i.e., between pairs of polygons) crossovers [Default is both].
- Determine if one (or all, with +a) points of each feature in the input data are inside any of the polygons given in the pfile. If inside, then report which polygon it is; the polygon ID is either taken from the aspatial value assigned to Z, the segment header (first -Z, then -L are scanned), or it is assigned the running number that is initialized to start . By default the input segment that are found to be inside a polygon are written to stdout with the polygon ID encoded in the segment header as -ZID. Alternatively, append +r to just report which polygon contains a feature or +z to have the IDs added as an extra data column on output. Segments that fail to be inside a polygon are not written out. If more than one polygon contains the same segment we skip the second (and further) scenario.
- Measure the area of all polygons or length of line segments. Use -Q+h to append the area to each polygons segment header [Default simply writes the area to stdout]. For polygons we also compute the centroid location while for line data we compute the mid-point (half-length) position. Append a distance unit to select the unit used (see Units). Note that the area will depend on the current setting of PROJ_ELLIPSOID; this should be a recent ellipsoid to get accurate results. The centroid is computed using the mean of the 3-D Cartesian vectors making up the polygon vertices, while the area is obtained via an equal-area projection. Normally, all input segments will be be reflected on output. Use c to restrict processing to those whose length (or area for polygons) fall inside the specified range set by min and max. If max is not set it defaults to infinity. To sort the segments based on their lengths or area, use s and append a for ascending and d for descending order [ascending]. By default, we consider open polygons as lines. Append +p to close open polygons and thus consider all input as polygons, or append +l to consider all input as lines, even if closed.
- west, east, south, and north specify the region of interest, 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). Set geographic regions by specifying ISO country codes from the Digital Chart of the World using -Rcode1,code2,…[+r|R[incs]] instead: Append one or more comma-separated countries using the 2-character ISO 3166-1 alpha-2 convention. To select a state of a country (if available), append .state, e.g, US.TX for Texas. To specify a whole continent, prepend = to any of the continent codes AF (Africa), AN (Antarctica), AS (Asia), EU (Europe), OC (Oceania), NA (North America), or SA (South America). Use +r to modify the bounding box coordinates from the polygon(s): Append inc, xinc/yinc, or winc/einc/sinc/ninc to adjust the region to be a multiple of these steps [no adjustment]. Alternatively, use +R to extend the region outward by adding these increments instead [no extension]. Alternatively for grid creation, give Rcodelon/lat/nx/ny, where code is a 2-character combination of L, C, R (for left, center, or right) and T, M, B for top, middle, or bottom. e.g., BL for lower left. This indicates which point on a rectangular region the lon/lat coordinate refers to, and the grid dimensions nx and ny with grid spacings via -I is used to create the corresponding region. Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing and registration, if applicable) are copied from the grid. Appending +uunit expects projected (Cartesian) coordinates compatible with chosen -J and we inversely project to determine actual rectangular geographic region. For perspective view (-p), optionally append /zmin/zmax. In case of perspective view (-p), a z-range (zmin, zmax) can be appended to indicate the third dimension. This needs to be done only when using the -Jz option, not when using only the -p option. In the latter case a perspective view of the plane is plotted, with no third dimension. Clips polygons to the map region, including map boundary to the polygon as needed. The result is a closed polygon.
- Spatial processing of polygons. Choose from -Sh which identifies perimeter and hole polygons (and flags/reverses them), -Si which returns the intersection of polygons (closed), -Su which returns the union of polygons (closed), -Ss which will split polygons that straddle the Dateline, and -Sj which will join polygons that were split by the Dateline. Note: Only -Ss has been implemented.
- Truncate polygons against the specified polygon given, possibly resulting in open polygons. If no argument is given to -T we create a clipping polygon from -R which then is required. Note that when the -R clipping is in effect we will also look for polygons of length 4 or 5 that exactly match the -R clipping polygon.
- -V[level] (more …)
- Select verbosity level [c].
- -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 same as input].
- -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).
- -je|f|g (more …)
- Determine how spherical distances are calculated.
- -ocols[,…][t[word]] (more …)
- Select output columns (0 is first column; t is trailing text, append word to write one word only).
- -:[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.
For map distance unit, append unit d for arc degree, m for arc minute, and s for arc second, or e for meter [Default], f for foot, k for km, M for statute mile, n for nautical mile, and u for US survey foot. By default we compute such distances using a spherical approximation with great circles (-jg) using the authalic radius (see PROJ_MEAN_RADIUS). You can use -jf to perform “Flat Earth” calculations (quicker but less accurate) or -je to perform exact geodesic calculations (slower but more accurate; see PROJ_GEODESIC for method used).
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.
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.
To determine the centroid of the remote GSHHH high-resolution polygon for Australia, as well as the land area in km squared, try:
gmt spatial @GSHHS_h_Australia.txt -fg -Qk
To turn all lines in the multisegment file lines.txt into closed polygons, run
gmt spatial lines.txt -F > polygons.txt
To compute the area of all geographic polygons in the multisegment file polygons.txt, run
gmt spatial polygons.txt -Q > areas.txt
Same data, but now orient all polygons to go counter-clockwise and write their areas to the segment headers, run
gmt spatial polygons.txt -Q+h -E+p > areas.txt
To determine the areas of all the polygon segments in the file janmayen_land_full.txt, add this information to the segment headers, sort the segments from largest to smallest in area but only keep polygons with area larger than 1000 sq. meters, run
gmt spatial -Qe+h+p+c1000+sd -V janmayen_land_full.txt > largest_pols.txt
To determine the intersections between the polygons A.txt and B.txt, run
gmt spatial A.txt B.txt -Ie > crossovers.txt
To truncate polygons A.txt against polygon B.txt, resulting in an open line segment, run
gmt spatial A.txt -TB.txt > line.txt
OGR/GMT files are considered complete datasets and thus you cannot specify more than one at a given time. This causes problems if you want to examine the intersections of two OGR/GMT files. The solution is to convert them to regular datasets via gmtconvert and then run gmt spatial on the converted files.