NAME
grdtrack - Sampling of a 2-D grdfile along 1-D trackline (a sequence of
x,y points)
SYNOPSIS
grdtrack xyfile -Ggrdfile [ -H[nrec] ] [ -Lflag ] [ -M[flag] ] [ -N ] [
-Q[value] [ -Rwest/east/south/north ] [ -S ] [ -V ] [ -Z ] [ -: ] [
-bi[s][n] ] [ -f[i|o]colinfo ][ -bo[s][n] ]
DESCRIPTION
grdtrack reads a grdfile and a table (from file or standard input) with
(x,y) positions in the first two columns (more columns may be present).
It interpolates the grid at the positions in the table and writes out
the table with the interpolated values added as a new column. A bicubic
[Default] or bilinear [-Q] interpolation is used, requiring boundary
conditions at the limits of the region (see -L).
xyfile This is an ASCII [or binary, see -b] file where the first 2
columns hold the (x,y) positions where the user wants to sample
the 2-D data set.
-G grdfile is a 2-D binary grd file with the function f(x,y).
OPTIONS
No space between the option flag and the associated arguments.
-H Input file(s) has Header record(s). Number of header records can
be changed by editing your .gmtdefaults4 file. If used, GMT
default is 1 header record. Use -Hi if only input data should
have header records [Default will write out header records if
the input data have them].
-L Boundary condition flag may be x or y or xy indicating data is
periodic in range of x or y or both set by -R, or flag may be g
indicating geographical conditions (x and y are lon and lat).
[Default uses "natural" conditions (second partial derivative
normal to edge is zero).]
-M Multiple segment file. Segment separator is a record beginning
with flag. [Default is ’>’].
-N Node value only: Return the value at the grid node closest to
the point, no interpolation needed.
-Q Quick mode, use bilinear rather than bicubic interpolation.
optionally, append value in the 0 < value <= 1 range. This
parameter controls how close to nodes with NaN values the inter-
polation will go. E.g., a value of 0.5 will interpolate about
1/2-way from a non-NaN to a NaN node, whereas 0.1 will go about
90% of the way, etc. [Default is 1, which means none of the four
nearby nodes may be NaN].
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north 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 wesn. The
two shorthands -Rg -Rd stand for global domain (0/360 or
-180/+180 in longitude respectively, with -90/+90 in latitude).
For calendar time coordinates you may either give relative time
(relative to the selected TIME_EPOCH and in the selected
TIME_UNIT; append t to -JX|x), or absolute time of the form
[date]T[clock] (append T to -JX|x). At least one of date and
clock must be present; the T is always required. The date string
must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
yyyy[-Www[-d]] (ISO week calendar), while the clock string must
be of the form hh:mm:ss[.xxx]. The use of delimiters and their
type and positions must be as indicated (however, input/output
and plotting formats are flexible).
-S Suppress the output of interpolated points that result in NaN
values.
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-Z Only write out the sampled z-values [Default writes all
columns].
-: Toggles between (longitude,latitude) and (latitude,longitude)
input/output. [Default is (longitude,latitude)].
-bi Selects binary input. Append s for single precision [Default is
double]. Append n for the number of columns in the binary
file(s).
[Default is 2 input columns].
-bo Selects binary output. Append s for single precision [Default is
double]. Append n for the number of columns in the binary
file(s).
-f Special formatting of input and output columns (time or geo-
graphical data) Specify i(nput) or o(utput) [Default is both
input and output]. Give one or more columns (or column ranges)
separated by commas. Append T (Absolute calendar time), t (time
relative to chosen TIME_EPOCH), x (longitude), y (latitude), g
(geographic coordinate), or f (floating point) to each column or
column range item.
HINTS
If an interpolation point is not on a node of the input grid, then a
NaN at any node in the neighborhood surrounding the point will yield an
interpolated NaN. Bicubic interpolation [default] yields continuous
first derivatives but requires a neighborhood of 4 nodes by 4 nodes.
Bilinear interpolation [-Q] uses only a 2 by 2 neighborhood, but yields
only zeroth-order continuity. Use bicubic when smoothness is important.
Use bilinear to minimize the propagation of NaNs.
EXAMPLES
To sample the file hawaii_topo.grd along the SEASAT track track_4.xyg
(An ASCII table containing longitude, latitude, and SEASAT-derived
gravity, preceded by one header record):
grdtrack track_4.xyg -Ghawaii_topo.grd -H > track_4.xygt
SEE ALSO
gmt(l), surface(l), sample1d(l)
GMT4.0 1 Oct 2004 GRDTRACK(l)
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