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Re: GRIDDATA woes [message #59111 is a reply to message #59040]

Tue, 04 March 2008 19:02

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

> Thanks Bill and Ken,
>
> I had scoured the c.l.i-p archives but never used "regridding" keyword
> which, in hindsight, seems the perfect keyword. The perils of keyword
> searches...
>
> The discussion on INTERPOLATE that you reference (see http://tinyurl.com/38mr7k)
> is the first time I have ever "gotten" INTERPOLATE. Thank you! The
> function has always felt so awkward because the units x and y are in
> dimensions - it always left me feeling a little disconnected from the
> physical meaning. I'll get over it.
>
> For my purposes the INTERPOLATE method is probably just the ticket,
> but I do have this lingering question about the fact that the input
> values are drawn from the surface of a sphere. What are the
> conditions under which I do need to worry about it? Is it the spacing
> between the input values? The extend over the sphere? Some
> combination?

Remember, interpolation is an approximation. You make assumptions about the
behavior of a function between known (tabulated) points. Bilinear
interpolation is the crudest possible interpolation scheme.

Doing bilinear interpolation on a sphere is also an approximation, as
the bilinear interpolator is strictly defined in Cartesian coordinates.

If I remember correctly, though, your grid is rather finely spaced. On
that scale it is probably a very good approximation to assume that the
world is flat. If you are worried about errors at that level, or you have a
better idea of the actual shape of your function, then you could use a higher-order
interpolator. That would depend on your purpose. It is a good idea to
think about these things, but then I'm not doing geodesy. :-)

Ken

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[Message index]

		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 13:14
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 11:49
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 09:54
		Re: GRIDDATA woes By: wgallery on Tue, 04 March 2008 09:43
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 09:14
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 09:05
		Re: GRIDDATA woes By: Brian Larsen on Tue, 04 March 2008 08:23
		Re: GRIDDATA woes By: ben.bighair on Tue, 04 March 2008 08:05
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 08:03
		Re: GRIDDATA woes By: David Fanning on Tue, 04 March 2008 07:35
		Re: GRIDDATA woes By: Kenneth P. Bowman on Mon, 03 March 2008 06:24
		Re: GRIDDATA woes By: wgallery on Mon, 03 March 2008 11:00
		Re: GRIDDATA woes By: Kenneth P. Bowman on Tue, 04 March 2008 19:04
		Re: GRIDDATA woes By: Kenneth P. Bowman on Tue, 04 March 2008 19:02
		Re: GRIDDATA woes By: wgallery on Tue, 04 March 2008 14:17
		Re: GRIDDATA woes By: Kenneth Bowman on Wed, 05 March 2008 06:14
		Re: GRIDDATA woes By: Kenneth P. Bowman on Tue, 04 March 2008 18:55
		Re: GRIDDATA woes By: ben.bighair on Tue, 04 March 2008 14:46
		Re: GRIDDATA woes By: pgrigis on Tue, 04 March 2008 14:39

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