Provided by: gmt-common_5.4.3+dfsg-1_all 
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 decreasing) variable, here called time
(it may of course be any type of quantity). 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 -T…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
necessarily 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 necessarily 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_INTERPOLANT 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 sampling 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 …)
Select verbosity level [c].
-bi[ncols][t] (more …)
Select native binary input. [Default is 2 (or at least the number of columns implied by -T)].
-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).
-^ 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.
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,distance,gravity,magnetics,bathymetry) records,
at 1km equidistant intervals using Akima’s 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, gmt.conf, greenspline, filter1d
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.3 Jan 03, 2018 SAMPLE1D(1gmt)