sample1d(1) GMT sample1d(1)
NAME
sample1d - Resample 1-D table data using splines
SYNOPSIS
sample1d [ table ] [ -Af|p|m|r|R[+l] ] [ -Fl|a|c|n[+1|+2] ] [
-Iinc[unit] ] [ -Nknotfile ] [ -Sstart[/stop] ] [ -Tcol ] [
-V[level] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -fflags ] [ -ggaps
] [ -hheaders ] [ -iflags ] [ -oflags ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
sample1d reads a multi-column ASCII [or binary] data set from file [or
standard input] and interpolates the time-series or spatial profile at
locations where the user needs the values. The user must provide the
column number of the independent (monotonically increasing or decreas-
ing) variable, here called time (it may of course be any type of quan-
tity). Equidistant or arbitrary sampling can be selected. All columns
are resampled based on the new sampling interval. Several interpolation
schemes are available. Extrapolation outside the range of the input
data is not supported.
REQUIRED ARGUMENTS
None.
OPTIONAL ARGUMENTS
table This is one or more ASCII [of binary, see -bi] files with one
column containing the independent time variable (which must be
monotonically in/de-creasing) and the remaining columns holding
other data values. If no file is provided, sample1d reads from
standard input.
-Af|p|m|r|R
For track resampling (if -Ta|unit is set) we can select how this
is to be performed. Append f to keep original points, but add
intermediate points if needed; note this selection does not nec-
essarily yield equidistant points [Default], m as f, but first
follow meridian (along y) then parallel (along x), p as f, but
first follow parallel (along y) then meridian (along x), r to
resample at equidistant locations; input points are not neces-
sarily included in the output, and R as r, but adjust given
spacing to fit the track length exactly. Finally, append +l if
distances should be measured along rhumb lines (loxodromes).
-Fl|a|c|n[+1|+2]
Choose from l (Linear), a (Akima spline), c (natural cubic
spline), and n (no interpolation: nearest point) [Default is
-Fa]. You may change the default interpolant; see GMT_INTER-
POLANT in your gmt.conf file. You may optionally evaluate the
first or second derivative of the spline by appending 1 or 2,
respectively.
-Iinc[unit]
inc defines the sampling interval [Default is the separation
between the first and second abscissa point in the table].
Append a distance unit (see UNITS) to indicate that the first
two columns contain longitude, latitude and you wish to resample
this path with a spacing of inc in the chosen units. For sam-
pling of (x, y) Cartesian tracks, specify the unit as c. Use -A
to control how path resampling is performed.
-Nknotfile
knotfile is an optional ASCII file with the time locations where
the data set will be resampled in the first column. Note: If -H
is selected it applies to both table and knotfile. Also note
that -i never applies to knotfile since we always consider the
first column only.
-Sstart[stop]
For equidistant sampling, start indicates the time of the first
output value. [Default is the smallest even multiple of inc
inside the range of table]. Optionally, append /stop to indicate
the time of the last output value [Default is the largest even
multiple of inc inside the range of table].
-Tcol Sets the column number of the independent time variable [Default
is 0 (first)].
-V[level] (more a|)
Select verbosity level [c].
-bi[ncols][t] (more a|)
Select native binary input. [Default is 2 (or at least the num-
ber of columns implied by -T)].
-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).
-^ 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.
CALENDAR TIME SAMPLING
If the abscissa are calendar times then you must use the -f option to
indicate this. Furthermore, -I then expects an increment in the current
TIME_UNIT units. There is not yet support for variable intervals such
as months.
EXAMPLES
To resample the file profiles.tdgmb, which contains (time,dis-
tance,gravity,magnetics,bathymetry) records, at 1km equidistant inter-
vals using Akimaas spline, use
gmt sample1d profiles.tdgmb -I1 -Fa -T1 > profiles_equi_d.tdgmb
To resample the file depths.dt at positions listed in the file
grav_pos.dg, using a cubic spline for the interpolation, use
gmt sample1d depths.dt -Ngrav_pos.dg -Fc > new_depths.dt
To resample the file points.txt every 0.01 from 0-6, using a cubic
spline for the interpolation, but output the first derivative instead
(the slope), try
gmt sample1d points.txt S0/6 -I0.01 -Fc+1 > slopes.txt
To resample the file track.txt which contains lon, lat, depth every 2
nautical miles, use
gmt sample1d track.txt -I2n -AR > new_track.dt
To do approximately the same, but make sure the original points are
included, use
gmt sample1d track.txt -I2n -Af > new_track.dt
To obtain a rhumb line (loxodrome) sampled every 5 km instead, use
gmt sample1d track.txt -I5k -AR+l > new_track.dt
SEE ALSO
gmt(1), gmt.conf(5), greenspline(1), filter1d(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 sample1d(1)
gmt5 5.4.2 - Generated Thu Jun 29 16:17:34 CDT 2017