Re: Contouring in GnuPlot

[Theo H. <th...@ph...>]

MartinOShea wrote:
> Hello
> 
> I have a set of sample data as follows:
> 
> CEMI (x) ggbs(y) FLOW MM (z)
> 
> 0.0        90.0     0
> 6.5        58.7     166
> 6.6        59.0     169
> 7.3        65.0     141
> 6.1        54.5     162		
> 8.5        76.2     217
> 7.1        64.2     174
> 6.0        54.2     209 
> 7.0        63.0     198 
> 7.6        68.3     171
> 
> which produces a simple set of points on a 3D graph using splot. However,
> what I would like is to do is to interpolate between points to calculate the
> coordinates for a flow contour of, for example, 175 mm?
> 
> Can anyone advise how I might do this?

Contouring requires that the data be in grid format, i.e., with an equal 
number of 'y' values for every 'x' value. See `help grid_data` for 
details. You can use `set dgrid3d` to convert a non-gridded data set to 
gridded, but the results of the default algorithm are (to my sight) 
generally unsatisfactory. If you're willing to build gnuplot from 
source, there is an option to replace the default dgrid3d algorithm with 
a more sophisticated one that produces results which are (again, to my 
sight) more pleasing. Try './configure --enable-thin-splines'.

In either case, once you've used `set dgrid3d` to produce an 
interpolated surface, this surface can be used to create contours. Since 
contours are a global setting, and not a per-dataset one, you'll likely 
want to use `set table` to output the contours to a file, then plot that 
file with your original data.

THeo