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.
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.
- -R[unit]west/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 (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 use just -).
- -+ 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
options, then exits.
- --version
- Print GMT version and exit.
- --show-datadir
- Print full path to GMT share directory and exit.
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).
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