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# gmtspatial

gmtspatial - Do geospatial operations on lines and polygons

## Synopsis

gmtspatial [ table ] [ -A[amin_dist][unit]] [ -C ] [ -D[+ffile][+aamax][+ddmax][+c|Ccmax][+sfact] ] [ -E+|- ] [ -I[e|i] ] [ -Npfile[+a][+pstart][+r][+z] ] [ -Q[+[unit]] ] [ -R[unit]xmin/xmax/ymin/ymax[r] ] [ -Si|u|s|j ] [ -T[clippolygon] ] [ -V[level] ] [ -b[i|o][ncol][type][w][+L|+B] ] [ -f[i|o]colinfo ] [ -g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] ] [ -h[i|o][n][+c][+d][+rremark][+rtitle] ] [ -icols[l][sscale][ooffset][,...] ] [ -ocols[,...] ] [ -:[i|o] ]

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

## Description

gmtspatial 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.

## Required Arguments

None.

## Optional 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.
-A[amin_dist][unit]
  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.)
-C
  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.
-D[+ffile][+aamax][+ddmax][+c|Ccmax][+sfact]
  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 [0] (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 (+). 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).
-E+|- ]
  Reset the handedness of all polygons to match the given + (counter-clockwise) or - (clockwise). Implies -Q+.
-I[e|i]
  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 paris of polygons) crossovers [Default is both].
-Npfile[+a][+pstart][+r][+z]
  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 [0]. 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.
-Q[+[unit]]
  Measure the area of all polygons or length of line segments. Use -Q+ 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.

-Rwest/east/south/north[/zmin/zmax][r]
  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). Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied from the grid. Using -Runit 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.
-Si|j|s|u
  Spatial processing of polygons. Choose from -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.
-T[clippolygon]
  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][type] (more ...)
  Select binary input. [Default is 2 input columns].

-bo[ncols][type] (more ...)
  Select binary output. [Default is same as input].

-f[i|o]colinfo (more ...)
  Specify data types of input and/or output columns.

-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (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][,...] (more ...)
  Select input columns.

-ocols[,...] (more ...)
  Select output columns.

-:[i|o] (more ...)
  Swap 1st and 2nd column on input and/or output.

-^
  Print a short message about the syntax of the command, then exits (NOTE: on Windows use just -).
+
  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
-?
  Print a complete usage (help) message, including the explanation of options, then exits.
--version
  Print GMT version and exit.
--show-datadir
  Print full path to GMT share directory and exit.

## Units

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. Prepend - to a distance (or the unit is no distance is given) to perform “Flat Earth” calculations (quicker but less accurate) or prepend + to perform exact geodesic calculations (slower but more accurate).

## 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, whereas other values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in the 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.

## Example

To compute the area of all geographic polygons in the multisegment file polygons.d, run

>     gmt gmtspatial polygons.d -Q > areas.d

Same data, but now orient all polygons to go counter-clockwise and write their areas to the segment headers, run

>     gmt gmtspatial polygons.d -Q+ -E+ > areas.d

To determine the intersections between the polygons A.d and B.d, run

>     gmt gmtspatial A.d B.d -Ce > crossovers.d

## See Also

gmt, gmtconvert, gmtselect, gmtsimplify

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