Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
gmtspatial - Do geospatial operations on lines and polygons
SYNOPSIS
gmtspatial [ table ] [ -A[amin_dist][unit]] [ -C ] [ -D[+ffile][+aamax][+ddmax][+c|Ccmax][+sfact] ] [
-E+|- ] [ -F[l] ] [ -I[e|i] ] [ -Npfile[+a][+pstart][+r][+z] ] [
-Q[[-|+]unit][+cmin[/max]][+h][+l][+p][+s[a|d]] ] [ -Rregion ] [ -Si|u|s|j ] [ -T[clippolygon] ] [
-V[level] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps ] [ -hheaders ] [ -iflags ] [
-oflags ] [ -:[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.). Note: The input data are assumed to contain (lon, lat) or (x, y), optionally
followed by a z and a weight [1] column. We compute a weighted average of the location and z (if
present).
-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 (+). 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).
-E+|- ]
Reset the handedness of all polygons to match the given + (counter-clockwise) or - (clockwise).
Implies -Q+.
-F[l] Force input data to become polygons on output, i.e., close them explicitly if not already closed.
Optionally, append l to force line geometry.
-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][+cmin[/max]][+h][+l][+p][+s[a|d]]
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. For line lengths you may prepend -|+ to the unit
and the calculation will use Flat Earth or Geodesic algorithms, respectively [Default is great
circle distances]. 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.
-Rwest/east/south/north[/zmin/zmax][+r][+uunit]
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 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, 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.
-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][t] (more …)
Select native binary 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]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 and transformations (0 is first column).
-ocols[,…] (more …)
Select output columns (0 is first column).
-:[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.
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, 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.
EXAMPLE
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+ > 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 gmtspatial A.txt -TB.txt > line.txt
NOTES
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 gmtspatial on the converted files.
SEE ALSO
gmt, gmtconvert, gmtselect, gmtsimplify
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.3 Jan 03, 2018 GMTSPATIAL(1gmt)