bug with interp2 ?
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Hello,
I want to interpolate a 2D data "v" from a 2km latitude and longitude grid (xx and yy) to a 10 km grid (Xq and Yq). but with interp2 I get the following error :
>> [Xq,Yq] = ndgrid(lon_extraction,lat_extraction); %
>> whos xx yy v X Y
Name Size Bytes Class Attributes
X 23x83 15272 double
Y 23x83 15272 double
v 407x793 2582008 double
xx 407x793 2582008 double
yy 407x793 2582008 double
>> h_extr_interp = interp2(xx,yy, v, X,Y);
Error using griddedInterpolant
Grid arrays must have NDGRID structure.
Error in interp2>makegriddedinterp (line 226)
F = griddedInterpolant(varargin{:});
Error in interp2 (line 134)
F = makegriddedinterp(X, Y, V, method,extrap);
>>
(I get the same result using meshgrid instead of ndgrid)
any ideas why I get this message??
3 commentaires
Timbro
le 2 Mai 2023
"(I get the same result using meshgrid instead of ndgrid)"
Your code shows that you do not use the outputs of NDGRID, so changing the function to MESHGRID will not change that.
"Problem solved using griddata instead of interp2"
If you have data in ND-grid format, then use INTERPN:
The MATLAB documentation explains how to select the operator based on the data arrangement:
Matt J
le 3 Mai 2023
Problem solved using griddata instead of interp2
griddata is quite a bit slower than interp2. You should not use it unless you are certain your xx,yy locations don't define a regular lattice.
Réponses (1)
Walter Roberson
le 2 Mai 2023
Déplacé(e) : Walter Roberson
le 2 Mai 2023
[Xq,Yq] = ndgrid(lon_extraction,lat_extraction);
h_extr_interp = interp2(xx,yy, v, X,Y);
What do xx and yy or X and Y have to do with Xq and Yq ? You build an ndgrid but you do not use it in the interp2
5 commentaires
Timbro
le 3 Mai 2023
"Maybe interp2 can only be used to switch from a grid to a finer grid, but not the opposite."
No.
Lets try it right now, modifying the first example from the documentation:
[X,Y] = meshgrid(-3:3,-2:2);
V = peaks(X,Y)
Finer grid:
[Xq,Yq] = meshgrid(-3:0.25:3,-2:0.25:2);
Vq = interp2(X,Y,V,Xq,Yq)
Courser grid:
[Xq,Yq] = meshgrid(-3:2:3,-2:2:2);
Vq = interp2(X,Y,V,Xq,Yq)
No errors, no problems, everything works exactly as documented.
"bug with interp2 ?"
I very much doubt that.
Most likely the problem is the format of your data, which so far you have not provided us with. For example, if the data are actually in ND-grid format:
[X,Y] = ndgrid(-3:3,-2:2);
V = peaks(X,Y) % not meshgrid data
[Xq,Yq] = meshgrid(-3:2:3,-2:2:2); % what you use here is irrelevant
Vq = interp2(X,Y,V,Xq,Yq) % ... because X & Y still have the wrong format
If you either uploaded or showed your actual data, then we could check its format. Until you do that, we can safely assume that its format is incorrect for the interpolation function that you have chosen.
Your data are scattered, they are not on a grid:
S = load('test_bug.mat')
S.xx
S.yy
The rows and columns clearly do not consist of identical values. Therefore your data are not gridded.
Compare against the perfectly gridded sample points returned by NDGRID or MESHGRID:
[X,Y] = meshgrid(0:3,4:7)
So you will need to use a scattered interpolant (not a gridded interpolant) e.g. GRIDDATA:
Timbro
le 3 Mai 2023
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