Re: interpolation techniques

Hi David,

On Nov 16, 2007, at 8:41 AM, David B. Reusch wrote:

> Hello,
>
> For some time now, I have successfully been using
> gsn_csm_contour_map to create contour maps of my irregularly spaced
> ice core data from Greenland. Now I would like to create
> interpolated data equivalent to what one would get by picking
> points from the maps created by gsn_csm_contour_map (effectively
> what I imagine gsn_csm_contour_map is creating internally before
> plotting the contours). I've seen that natgrid appears to be the
> recommended function for creating regular grids from irregularly
> spaced data. Assuming natgrid really is the appropriate function
> for this task, what I'd like to know before I invest more time on
> this:
> (a) If I were to contour the new output grid, would it produce a
> map reasonably close to what I get now using gsn_csm_contour_map
> and my original data? I'm not expecting them to be *identical* but
> I do want them to pass a reasonable "those look similar" test.

It should produce satisfactory results.

> (b) The natgrid output grid is rectangular so I'm anticipating the
> possibility of extrapolation outside the spatial range of my input
> data -- is this going to happen and is there any advice for dealing
> with this situation?

Of course extrapolation is always problematic. The way that Natgrid
does its
extrapolation is described at:

   http://www.ncarg.ucar.edu/ngmath/natgrid/extrapolation.html

> (c) I'm working with lat/long in a polar region, so I'm not quite
> sure how to create the output points so that they are equally
> spaced -- do I need to be thinking about converting from lat/long
> to an equal area grid?

I am not sure exactly what you are wanting to do here. Do you mean
that you want the physical distance
between output longitudes at different latitudes to represent the
same physical distances? If so,
why do you want to do this instead of just interpolating at the usual
output lan/lon grid?

Be warned that Natgrid, while a very good general-purpose
interpolator, is very slow.
>
> By the way, the link to further docs on Natgrid on the page for the
> natgrid function appears to be broken, i.e., the link w/in the text
> " This function is part of the Natgrid package...". See this page:
> http://www.ncl.ucar.edu/Document/Functions/Built-in/natgrid.shtml

Thanks for reporting this. We will fix it today. The URL in
question is:

   http://www.ncarg.ucar.edu/ngmath/natgrid/nnhome.html

Fred Clare

>
> Thanks,
> Dave
> -- 517 Deike Building Earth & Environmental Systems Institute Penn
> State University University Park, PA 16802
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Received on Fri Nov 16 2007 - 12:31:49 MST