2D Interpolation of data

Hi..I was trying to perform a 2D-interpolation for 2 data files. The file AOA0_u600 contains data of nodenumber, x-coordinate, y-coordinate, z-coordinate and pressure.

The file Y-600 contains data of entity id, x-coordinate, y-coordinate and z-coordinate. Both the files contain the data for the same Y-coordinate.

Basically the idea was to interpolate the value of pressure at the values of x and z coordinate (sample points of 1st file) to the values of x and z-coordinate (query points of the 2nd file)

However,when using the interp2 function to interpolate the values, i was recieving the following error

%Error using griddedInterpolant

The grid vectors must be strictly monotonically increasing.

%Error in interp2>makegriddedinterp (line 229)

F = griddedInterpolant(varargin{:});

%Error in interp2 (line 129)

F = makegriddedinterp({X, Y}, V, method,extrap);

Could anyone please guide me as to how to resolve this issue?

clear all;
clc;
fid=fopen('AOA0_u600.txt');
a=textscan(fid, '%f %f %f %f %f', 'HeaderLines', 1);
b=cell2mat(a);
c=sortrows(b,2);
x=c(:,2);
x=x+0.275;
y=c(:,4);
z=c(:,5);
fid =fopen('Y-600.txt');
A=textscan(fid, '%f %f %f %f', 'HeaderLines', 12);
B=cell2mat(A);
C=sortrows(B,2);
X=C(:,2);
X1=X/1000;
Y=C(:,4);
Y1=Y/1000;
D=interp2(x,y,z,X1,Y1);

Answer 1

[Bruno Luong]

You have scattered data, you should use scatteredInterpolant or similar, not interp2.

[Akarsh Shetty]

Thanks! this function worked..so I'm assuming interp1 or interp2 functions can only be used for monotonously increasing/decreasing data. is this correct?

[Bruno Luong]

No it is NOT totally correct in 2D it requires Gridded data which is "A set of points that are axis-aligned and ordered".

Such grid data can be generated by meshgrid or ndgrid command with both linear grid points strickly monotonics.

[Akarsh Shetty]

Alright..I'll keep that in mind!

The scatteredInterpolation Function worked for interpolating the scattered data from the file Y-2010.txt to the file Y_2010. However, i couldnt figure out the logic for the for loop for interpolating similar set of files.

Could you please guide me as how to proceed forward

clear all;
clc;
fid=fopen('Y_2010.txt');
a=textscan(fid, '%f %f %f %f %f', 'HeaderLines', 1);
b=cell2mat(a);
a_u=b(1:68,:);
a_l=b(69:end,:);
x_u=a_u(:,2);
x_u=x_u+0.275;
y_u=a_u(:,4);
z_u=a_u(:,5);
x_l=a_l(:,2);
x_l=x_l+0.275;
y_l=a_l(:,4);
z_l=a_l(:,5);


fid =fopen('Y-2010');
A=textscan(fid, '%f %f %f %f', 'HeaderLines', 12);
B=cell2mat(A);
au=B(B(:,4) >= -440.026398,:);
al=B(B(:,4) < -440.026398,:);
X_U=au(:,2);
X1=X_U/1000;
Y_U=au(:,4);
Y1=Y_U/1000;
M=[X1,Y1];
N=sortrows(M,1);

X_L=al(:,2);
X2=X_L/1000;
Y_L=al(1:63,4);
Y2=Y_L/1000;
O=[X2,Y2];
P=sortrows(O,1);
F=scatteredInterpolant(x_u,y_u,z_u);
G=scatteredInterpolant(x_l,y_l,z_l);
vq=F(N(:,1),N(:,2));
wq=G(P(:,1),P(:,2));
plot(x_u,z_u,'-o');
hold on
plot(x_l,z_l,'-r');
hold on
plot(N(:,1),vq);
hold on
plot(P(:,1),wq);
Q=[N,vq];
R=[P,wq];
dlmwrite('Y_2010_I',[Q;zeros(1,3);R],'delimiter','\t','precision','%f');