gmtspatial(1) GMT gmtspatial(1)
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 loca-
tion 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 dis-
tance 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 Date-
line (|). For polygons we also consider the fractional differ-
ence 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 dis-
tances 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 poly-
gons. Append i to only compute internal (i.e., self-intersect-
ing 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 con-
tains 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 seg-
ment 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 com-
puted 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
[A+-]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 (Carte-
sian) 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 poly-
gon.
-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 imple-
mented.
-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 a|)
Select verbosity level [c].
-bi[ncols][t] (more a|)
Select native binary input. [Default is 2 input columns].
-bo[ncols][type] (more a|)
Select native binary output. [Default is same as input].
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] (more a|)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-ocols[,a|] (more a|)
Select output columns (0 is first column).
-:[i|o] (more a|)
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 explana-
tion 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 aFlat Eartha 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 FOR-
MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val-
ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for-
mat 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 set-
ting.
EXAMPLE
To turn all lines in the multisegment file lines.txt into closed poly-
gons, 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 jan-
mayen_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(1), gmtconvert(1), gmtselect(1), gmtsimplify(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 gmtspatial(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:03:17 CDT 2017