How to interpolate ‘z’ for one value of ‘x’ but multiple values of y?

I have an array x of size 1x56 An array y of size 1x20 And an array z of size (20x56)

I want to interpolate to find ‘z’ for a specific value of x but multiple y.

I am trying to plot Interp2(x,y,z,1400,y)

Z =f(x,y,z)

But it doesn’t seem to be working right. Anyone have any ideas?

[Matt J]

What is 'a'? Do you mean 'z'?

[JD]

Yeah I meant ‘z’. I changed it above. Also z is a function of x,y and z.

Is that where the problem is coming from?

[Matt J]

What problem? You still haven't told us what it is. This works fine for me:

>> [x,y,z]=deal(1:56,1:20,rand(20,56));
>> interp2(x,y,z,14,y)

ans =

    0.9969
    0.5535
    0.5155
    0.3307
    0.4300
    0.4918
    0.0710
    0.8877
    0.0646
    0.4362
    0.8266
    0.3945
    0.6135
    0.8186
    0.8862
    0.9311
    0.1908
    0.2586
    0.8979
    0.5934

[JD]

ICE.map_w = [850:100:6400]*2*pi/60; ICE.map_T = [10:10:200]';
ICE_m_f_dot_map_temp_w = [850*ones(1,5) 1400*ones(1,12) 1900*ones(1,13)... 
        2500*ones(1,13) 3000*ones(1,14) 3600*ones(1,14) 4200*ones(1,14)... 
        4700*ones(1,14) 5300*ones(1,14) 5800*ones(1,14) 6400*ones(1,13)]*2*pi/60;
%2.134 2.372 2.563 2.943... 0.022 0.620 0.799 1.019 1.267 1.540 1.817 2.084 2.354 2.628 2.905 3.155 3.519 4.031... 0.028 0.782 0.996 1.249 1.546 1.866 2.189 2.513 2.841 3.169 3.459 3.758 4.175 4.763... 0.034 0.967 1.220 1.513 1.860 2.230 2.597 2.970 3.351 3.732 4.055 4.412 5.039 6.133... 0.042 1.184 1.474 1.804 2.199 2.623 3.039 3.455 3.905 4.341 4.749 5.229 6.035 7.247... 0.054 1.463 1.757 2.108 2.547 3.033 3.518 4.017 4.567 5.125 5.670 6.298 7.207 8.345... 0.069 1.795 2.061 2.417 2.897 3.450 4.027 4.648 5.336 6.076 6.788 7.563 8.609 9.876... 0.084 2.129 2.379 2.739 3.246 3.862 4.545 5.301 6.146 7.045 7.943 8.934 10.209]./1000;
% engine torque [Nm]
ICE_m_f_dot_map_temp_T = [0.00	4.74	19.22	33.71	48.19...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05 178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05	178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05	178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05	178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05	178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05	178.53...
0.00	4.74	19.22	33.71	48.19	62.67	77.15	91.64	106.12	120.60	135.08	149.57	164.05
]; 

% fuel rate [kg/s]
ICE_m_f_dot_map_temp = [0.000	0.082	0.133	0.185	0.228...
 0.000	0.240	0.335	0.443	0.556	0.681	0.811	0.937	1.069	1.210	1.346	1.536...
 0.012	0.349	0.474	0.618	0.775	0.956	1.137	1.305	1.478	1.666	1.851	2.029	2.399...
 0.016	0.475	0.630	0.820	1.021	1.247	1.479	1.692	1.907	2.134	2.372	2.563	2.943...
 0.022	0.620	0.799	1.019	1.267	1.540	1.817	2.084	2.354	2.628	2.905	3.155	3.519	4.031...
 0.028	0.782	0.996	1.249	1.546	1.866	2.189	2.513	2.841	3.169	3.459	3.758	4.175	4.763...
 0.034	0.967	1.220	1.513	1.860	2.230	2.597	2.970	3.351	3.732	4.055	4.412	5.039	6.133...
 0.042	1.184	1.474	1.804	2.199	2.623	3.039	3.455	3.905	4.341	4.749	5.229	6.035	7.247...
 0.054	1.463	1.757	2.108	2.547	3.033	3.518	4.017	4.567	5.125	5.670	6.298	7.207	8.345...
 0.069	1.795	2.061	2.417	2.897	3.450	4.027	4.648	5.336	6.076	6.788	7.563	8.609	9.876...
 0.084	2.129	2.379	2.739	3.246	3.862	4.545	5.301	6.146	7.045	7.943	8.934	10.209]./1000; 

% speed and torque lists for the griddata 
ICE.map_w = [850:100:6400]*2*pi/60;
ICE.map_T = [10:10:200]'; 

ICE.map_m_dot = griddata(ICE_m_f_dot_map_temp_w, ICE_m_f_dot_map_temp_T, ICE_m_f_dot_map_temp, ICE.map_w, ICE.map_T)';
pma = (ICE.map_m_dot'*4*pi*43.5*10e6)./(ICE.map_w.*2e-3);
mfdot_6400c = (pma.*ICE.map_w*2e-3)/(4*pi*43.5*10e6); mfdot_6400 = interp2(ICE.map_w,ICE.map_T,mfdot_6400c',6400*2*pi/60,ICE.map_T); pma_6400 = (mfdot_6400'*4*pi*43.5*10e6)./((6400*2*pi/(60))*2e-3)

The pma_6400 gives me NaN

[Walter Roberson]

By default, interp2() requesting a location outside the defined range, gives a NaN result.

[JD]

Isn't it within my defined range? 6400 is in the defined range, correct?

[Walter Roberson]

Unrecognized function or variable 'ICE_m_f_dot_map_temp_T'.

And therefore we as outside observers do not know what your data range is.

[JD]

It was showing up as a comment/text line

I fixed the code above for those variables

[Walter Roberson]

Error using griddedInterpolant
The grid vectors do not define a grid of points that match the given values.

That is reported inside interp2(). The fix for that is to change

mfdot_6400 = interp2(ICE.map_w,ICE.map_T,mfdot_6400c',6400*2*pi/60,ICE.map_T);

to

mfdot_6400 = interp2(ICE.map_w,ICE.map_T,mfdot_6400c,6400*2*pi/60,ICE.map_T);

That is, remove the complex conjugate transpose.

However you will still get NaN. And that is because:

>> max(ICE.map_w)
ans =
           664.97044500984
>> 6400*2*pi/60
ans =
          670.206432765823

Your probe x is outside the range of any of the data you define.

[JD]

That makes sense, thank you both for the help!

Answer 1

[Matt J]

If an interpolation result is giving you NaN, it normally means you are interpolating at a location which is outside the area covered by your points - hence there is no data there to interpolate between.