| Re: Interpolating a regular grid [message #47805] |
Tue, 07 March 2006 07:10  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Clive Cook writes:
> It seems that the hardest part in asking for help is knowing what to
> ask. I am not sure if my subject header actually corresponds to my
> question, however i though it was close enough.
>
> I have a regularly gridded array of data (calibration data for an
> instument) and i want to extract values from the array. The x-array is
> pressure and the y-array is temperature. The data i require does not
> match with any points in the array. For example the x-array runs from
> 70 to 1050 mb in steps of 9 mb and the x-array goes from 180-330 K ,
> how do i interpolate the array so that i could look at values which
> fall in between the regularly gridded data. So, clarifying this how
> would i extract a value which corresponded to a pressure of 700.6 mb
> and a temperature of 234.56 K.
You don't say what size your gridded array is, but I am going to
assume it is a 110x150 array, so that we can get even values
in pressure:
p = Scale_Vector(Indgen(110), 70, 1051) ; in units of 9 mb.
and temperature:
t = Scale_Vector(Indgen(150), 180, 329) ; in units of 1 deg.
To interpolate your gridded array, you need to know the *indices*
of pressure and temperature. One way to find the proper indices
is like this:
xindex = Value_Locate(p, 700.6)
xindex = yindex + ((700.6 - p[yindex])/(p[yindex+1] - p[yindex]))
yindex = Value_Locate(t, 234.56)
yindex = yindex + ((234.56- t[yindex])/(t[yindex+1] - t[yindex]))
So, now, the value you are looking for is this:
interpValue = Interpolate(array, xindex, yindex)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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| Re: Interpolating a regular grid [message #47887 is a reply to message #47805] |
Tue, 07 March 2006 17:34  |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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David Fanning wrote:
> To interpolate your gridded array, you need to know the *indices*
> of pressure and temperature. One way to find the proper indices
> is like this:
>
> xindex = Value_Locate(p, 700.6)
> xindex = yindex + ((700.6 - p[yindex])/(p[yindex+1] - p[yindex]))
>
> yindex = Value_Locate(t, 234.56)
> yindex = yindex + ((234.56- t[yindex])/(t[yindex+1] - t[yindex]))
>
> So, now, the value you are looking for is this:
>
> interpValue = Interpolate(array, xindex, yindex)
For what it's worth, the Motley library at
http://www.dfanning.com/hadfield/idl/README.html
has a function called MGH_LOCATE which will calculated one or more
"fractional index" values in the way you have shown. It also has cousins
MGH_LOCATE2 & MGH_LOCATE2A that do the same thing for 2-D curvilinear
arrays.
Note to self: I must update that library.
--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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