NAME¶
grdtrack - Sampling of a 2-D grdfile along 1-D trackline (a sequence of x,y
points)
SYNOPSIS¶
grdtrack xyfile -Ggrdfile [
-H[
nrec] ]
[
-L[
flag] ] [
-M[
flag] ] [
-Q ] [
-R west/east/south/north ] [
-S ] [
-V ] [
-Z ] [
-: ] [
-bi[
s][
n] ] [
-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
-Lflag option).
- 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 .gmtdefaults file. If used, GMT default is
1 header record.
- -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).] If no flag is supplied, it is
assumed that the x column contains longitudes, which may differ from the
region in -R by [multiples of] 360 degrees [Default assumes no
periodicity].
- -M
- Multiple segment file. Segment separator is a record beginning with
flag. [Default is '>'].
- -Q
- Quick mode. Use bilinear rather than bicubic interpolation.
- -R
- west, east, south, and north specify the Region of interest.
To specify boundaries in degrees and minutes [and seconds], use the
dd:mm[:ss] format. Append r if lower left and upper right map
coordinates are given instead of wesn.
- -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 (lon,lat) and (lat,lon) input/output. [Default is
(lon,lat)]
- -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].
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, preceeded by
one header record), try
grdtrack track_4.xyg
-Ghawaii_topo.grd
-H > track_4.xygt
SEE ALSO¶
gmt(1gmt),
surface(1gmt),
sample1d(1gmt)