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Re: 3D congrid without interpolation [message #53405]

Sat, 14 April 2007 10:59

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

JD Smith wrote:
> On Thu, 12 Apr 2007 15:40:07 -0700, mgalloy@gmail.com wrote:
>
>> On Apr 12, 3:19 pm, David Fanning <n...@dfanning.com> wrote:
>>> Humm. Hard for me to imagine what you are using to
>>> do this that is interpolating anything for you.
>>> CONGRID is normally used, but that won't interpolate
>>> unless you explicitly tell it to.
>>
>> CONGRID interpolates 3-dimensional arrays by default. From the online
>> help for the INTERP keyword for CONGRID:
>>
>> INTERP
>> Set this keyword to force CONGRID to use linear interpolation when
>> resizing a 1- or 2-dimensional array. CONGRID automatically uses
>> linear interpolation if the input array is 3-dimensional. When the
>> input array is 1- or 2-dimensional, the default is to employ nearest-
>> neighbor sampling.
>
> How is "nearest neighbor sampling" not interpolation? Does it
> explicitly avoid knowledge of how the new array cell is positioned
> w.r.t. the old one, and simply grab averages of nearby neighbors? ...

No. It grabs the value of the one nearest neighbor, with appropriate
rules for breaking ties. No averaging of any kind is done on that
value, which is why it's inappropriate to call this 'interpolation'.

> ... Why
> would this ever be preferable to a linear interpolation?

Well, for one thing, it's a lot faster.

However, another good reason is if you're re-binning categorical
data, where the codes representing each category are arbitrary, and
it's simply not meaningful to take the average of the category codes.
If category 1 means 'corn' and category 3 means 'wheat', you don't
want a thin barrier line of category 2 (meaning 'barley') to ocurr at
the boundaries between wheat fields and corn fields. when you rebin
your data. Nearest neighbor interpolation will always generate either
1 or 3 along that boundary.

A third case that I'm very familiar with is mainly of use for
debugging purposes. I'm responsible for programs which calibrate and
geolocate satellite images. Occasionally I want to create a plot with
a resolution much higher than the resolution of our images, using
nearest negghbor interpolation. For any given low-resolution pixel,
there's multiple high-resolution pixels for which it is the nearest
neighbor, and they all get assigned the same color. As a result, I can
very clearly where the boundaries are between the low-resolution
pixels. That helps me decide whether or not we've geolocated those
pixels correctly.

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[Message index]

		Re: 3D congrid without interpolation By: James Kuyper on Sat, 14 April 2007 10:59
		Re: 3D congrid without interpolation By: JD Smith on Fri, 13 April 2007 18:00
		Re: 3D congrid without interpolation By: David Fanning on Fri, 13 April 2007 17:33
		Re: 3D congrid without interpolation By: JD Smith on Fri, 13 April 2007 15:57
		Re: 3D congrid without interpolation By: Mike[2] on Fri, 13 April 2007 08:21
		Re: 3D congrid without interpolation By: Qing on Thu, 12 April 2007 19:45
		Re: 3D congrid without interpolation By: David Fanning on Thu, 12 April 2007 17:30
		Re: 3D congrid without interpolation By: Michael Galloy on Thu, 12 April 2007 15:40
		Re: 3D congrid without interpolation By: David Fanning on Thu, 12 April 2007 14:19
		Re: 3D congrid without interpolation By: mxhamidi on Mon, 16 April 2007 11:43
		Re: 3D congrid without interpolation By: JD Smith on Mon, 23 April 2007 09:53

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