Greenspline -Sl option

Q&A
1

Hi, the description of the -Sl option says that output data “do not exceed the range of the given data” but I’m finding that to not be true. The below example is modified from one in the documentation. I added a few points at the end and these points cause the output grd to exceed the input range. What am I missing? Many thanks

gmt begin 2D
gmt info Table_5_11.txt
gmt greenspline Table_5_11.txt -R0/6.5/-0.2/6.5 -I0.1 -Sl -V -Z1 -GS1987.nc
gmt plot -R0/6.5/-0.2/6.5 -JX6i -B -Sc0.1 -Gblack Table_5_11.txt
gmt grdimage S1987.nc
gmt grdcontour -C25 -A50 S1987.nc
gmt plot -R0/6.5/-0.2/6.5 -JX6i -B -Sc0.1 -Gblack Table_5_11.txt
gmt grdinfo S1987.nc
gmt end show
Table_5_11.txt (705 Bytes)

2

Don’t know (and the program’s author is no longer with us). Maybe it was meant to say that at each point location the solution does exceed that of the data.

3

Ok, thanks. When I use sphinterpolate with the -Qp option in this example (below) instead of greenspline -Sl, I get the desired effect, but this is a cartesian example - is this a bad idea? It does seem to cause some other non desired effects at the top of the plot. I am looking for an equivalent non-spherical option that uses piecewise linear interpolation or something similar to not exceed the input data. Any advice? thanks

gmt begin 2D
  gmt info Table_5_11.txt
  gmt sphinterpolate Table_5_11.txt -R0/6.5/-0.2/6.5 -I0.1 -Qp -V -GS1987.nc
  gmt plot -R0/6.5/-0.2/6.5 -JX6i -B -Sc0.1 -Gblack Table_5_11.txt
  gmt grdimage S1987.nc
  gmt grdcontour -C25 -A50 S1987.nc
  gmt plot -R0/6.5/-0.2/6.5 -JX6i -B -Sc0.1 -Gblack Table_5_11.txt
  gmt grdinfo S1987.nc
gmt end show