| Re: How to do surface fit? [message #10782] |
Wed, 28 January 1998 00:00 |
Gang Chen
Messages: 4
Registered: January 1998
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Junior Member |
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> Use the KX keyword to SFIT, it will return an array of coefficients, in
> your case:
>
> IDL> zfit = SFIT(z,2,KX=k)>
> IDL> help,k
> COEFF FLOAT = Array[3, 3]
>
> They are calculated with the convention X= 0,...,NX and Y=0,...,NY
> (so you must to make the needed conversion).
Hi Evilio,
I really appreciate your help on this. It seems that it almost works the
way Iwanted. But I have another question: Since the default grids are
set withX= 0,...,NX and Y=0,...,NY in the source code for SFIT, how can
I do for anon-uniform grids (e.g., X=0, 1, 4, 5, 9, ..., and Y=3, 4, 7,
11, ..)? Do I have to resort to interpolation so that values at uniform
grids can be distributed?
Many thanks,
Gang Chen == gang@cochise.biosci.arizona.edu
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| Re: How to do surface fit? [message #10788 is a reply to message #10782] |
Wed, 28 January 1998 00:00  |
Evilio del Rio
Messages: 17
Registered: December 1997
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Junior Member |
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On Tue, 27 Jan 1998, Gang Chen wrote:
> Hi,
>
> I am trying to fit a surface (probably 2nd degree polynomial:
> z=a+b*x+c*y+d*x*x+e*xy+f*y*y) on a two dimensional grids with IDL. But it
> seems to me that SFIT and SVDFIT would not serve my purpose: SFIT only
> generates the values on those grids without giving the coefficients, and
> it uses different format of polynomial (z=sum of k_i,j * x^i * y^j); While
> SVDFIT works only for curve fit. Does anybody have any idea on how to do
> this by simply calling some existing functions instead of writing more
> complicate code?
>
Use the KX keyword to SFIT, it will return an array of coefficients, in
your case:
IDL> zfit = SFIT(z,2,KX=k)
IDL> help,k
COEFF FLOAT = Array[3, 3]
They are calculated with the convention X= 0,...,NX and Y=0,...,NY
(so you must to make the needed conversion). Also it seems that they are
reversed and the degree of X increases with the 2nd dimension on k and
the degree of Y with the first (so the actual matrix should be
TRANSPOSE(k), or so I see it):
IDL> help,x,y,z
X DOUBLE = Array[5, 6]
Y DOUBLE = Array[5, 6]
Z DOUBLE = Array[5, 6]
IDL> print,x,F='(5(F4.1))'
0.0 1.0 2.0 3.0 4.0
0.0 1.0 2.0 3.0 4.0
(...)
IDL> print,y,F='(5(F4.1))'
0.0 0.0 0.0 0.0 0.0
1.0 1.0 1.0 1.0 1.0
(...)
5.0 5.0 5.0 5.0 5.0
IDL> z = x + 3.0*y*y + 5.0*x*y
IDL> zfit = SFIT(z,2L,KX=k)
IDL> help,k
K FLOAT = Array[3, 3]
IDL> print,k,F='(3(F4.1))'
0.0 0.0 3.0
1.0 5.0 0.0
0.0 0.0 0.0
IDL> print,k[0,1] ; This should be coefficient for the term x^0*y^1 => 0.0
; in our case
1.0
IDL> print,k[1,0]
0.0
> Many thanks,
> Gang Chen == gang@cochise.biosci.arizona.edu
>
Cheers,
____________________________________________________________ ________
Evilio Jose del Rio Silvan Institut de Ciencies del Mar
E-mail: edelrio@icm.csic.es URL: http://www.ieec.fcr.es/~evilio/
"Anywhere you choose,/ Anyway, you're gonna lose"- Mike Oldfield
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