interp3 problem

I have a dataset as below, named it as "a" in Matlab.

46.567 -111.28 2423.2 -1.2006

45.883 -110.93 2231.1 0.7962

46.783 -109.52 1844.0 2.898

46.667 -110.33 2316.5 -0.21062

46.800 -110.68 1966.0 0.12335

45.70 -110.45 1456.0 7.9804

45.767 -111.15 1398.0 2.0

46.917 -110.85 2258.6 -0.39873

46.583 -111.27 2026.9 1.20

46.117 -110.47 1981.2 3.4545

46.083 -110.43 2468.9 0.11388

45.867 -110.93 1996.4 1.9115

46.783 -110.62 2468.9 -1.4

46.90 -110.90 1996.4 0.71936

the first column are Lat, Lon, Elevation and temperature. Now I want to use these data to do the interp3 in matlab. I write the code:

% original data

x=a(:,2);

y=a(:,1);

z=a(:,3);

[X,Y,Z]=meshgrid(x,y,z);

[r,m,n]=size(X);

V=zeros(r,m,n);

for i=1:r

V(:,:,i)=a(i,4);

end

% the location for interp3 U_lat=46.958346;

D_lat=46.891312;

L_lon=-110.907516;

R_lon=-110.836179;

xi=L_lon:0.0005:R_lon;

yi=D_lat:0.0005:U_lat;

zi=1950:5:2500;

[Xi,Yi,Zi]=meshgrid(xi,yi,zi);

% do the interp3

Vi=Vi=interp3(X,Y,Z,V,Xi,Yi,Zi);

and error show as:

??? Error using ==> interp3 at 172 X, Y, and Z must be monotonic vectors or arrays produced by MESHGRID.

I don't know if the data organization of X,Y,Z,V,Xi,Yi and Zi is right or not. It always shows this error message. Any help is greatly appreciated! Thanks!

Answer 1

[Honglei Chen]

For interp3 to work, the data must be in a grid. However, from the data you show, it seems that they are not really sampled in a grid, i.e., the data is not always sampled in the same Lat, Lon, and Altitude. In this case, you may want to try TriScatteredInterp, somewhat like this.

F = TriScatteredInterp(x,y,z,v)
Vi = F(Xi,Yi,Zi)

check the doc for details.

doc TriScatteredInterp

HTH

[Liyin]

It did works and finish the interpolation. But I am not sure the results is right or not. I want to display the F with plot3. But it fails. Another question is, which factor determine the accuracy of the results? the number of points? Thanks!