comp.lang.idl-pvwave archive: archive » Re: Regrid / Interpolation Question

Home » Public Forums » archive » Re: Regrid / Interpolation Question

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Regrid / Interpolation Question [message #79670]

Fri, 23 March 2012 11:53

Sean[1]
Messages: 11
Registered: July 2005

Junior Member

> INTERPOLATE uses the concept of 'fractional coordinates', which you can think
> of as floating-point indices into the array.
>
> If your tabulated points are not evenly spaced, you need to first reverse
> interpolate the desired output coordinates onto the unevenly spaced grid to get
> the fractional coordinates. That is, think of your unevenly spaced x's as a
> function of array index.

I understand the concept of fractional coordinates, but I still don't understand how to reverse interpolate without either a) using interpol(), or b) using a loop. Perhaps a more concrete example would help with this discussion:

Lets say I have 3 temperature vs. height profiles. Each profile has 6 points in the vertical, so the arrays are (6,3).

temp = [ [270, 224.3, 200., 190., 210, 230.], [284,231, 206.5, 208,200.,190.,110],$ [300,280,230,220.,185.,200.]]
height=[ [0.5,1,2.3,2.7,3.2,4], [0.,1.3,3.4,,3.6,3.8,5.3], [1.,1.2,2.7,3.6,4.4,6]]
nx = 6
ny = 3

I want to interpolate to interpolate the temperature to 2 new heights:

heightout = [1.5, 4]
nout = 2

So my output array should be (nout=2,ny=3). One looped way, using both INTERPOL and INTERPOLATE, would be

xinterpolates = fltarr(nout, ny)

for j =0, ny-1 do xinterpolates[*,j] = interpol( indgen(6), height[*,j], heightout)

yinterpolates = lindgen(nout,ny) / nout

tempout = interpolate( temp, xinterpolates, yinterpolates)

As I said earlier, the yinterpolates part is trivial, but I don't see how to reverse interpolate to get the xinterpolates without using a loop + INTERPOL().

In this particular example, I also don't see why it wouldn't just be faster to do

tempout = fltarr(nout, ny)
for j=0, ny-1 do tempout[*,j] = interpol( temp[*,j], height[*,j], heightout)

Report message to a moderator

[Message index]

		Re: Regrid / Interpolation Question By: Sean[1] on Fri, 23 March 2012 11:53
		Re: Regrid / Interpolation Question By: Kenneth P. Bowman on Fri, 23 March 2012 11:20
		Re: Regrid / Interpolation Question By: Sean[1] on Fri, 23 March 2012 11:04
		Re: Regrid / Interpolation Question By: Kenneth P. Bowman on Fri, 23 March 2012 07:59
		Re: Regrid / Interpolation Question By: Sean[1] on Mon, 26 March 2012 11:32

Previous Topic:	New Coyote Graphics Program: cgPolygon
Next Topic:	Time axis labelling (1 major label every 7 days with 6 minor ticks)

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Thu Oct 09 19:59:45 PDT 2025

Total time taken to generate the page: 1.43681 seconds