Matlab error using imagesc()

I am trying to plot frequency-wavenumber from Das strain data. Error is found when I use imagesc to get the graph.

Error using image

Image XData and YData must be vectors.

Error in imagesc (line 52)

hh = image(varargin{:}, 'CDataMapping', 'scaled');

Error in a6 (line 52)

imagesc(k,f,abs(st));

The Matlab script is given below. Any help would be appreciated.

Dasdata_folder = 'C:\Users\dan24\Documents\MATLAB\Flow Loop Fiber Strain Readings\DAS Data\2.5 Lpm Step Test for 2 inch pipe with 0 insulation/';
testName = '2.5 Lpm';
file_start = []; 
plot_title = '2.5 Lpm flow';
save_plot = true; %saves plots as png files if true
channels = 55:67; %define specific channels you want plots of 
folder_path = [Dasdata_folder testName '/'];
figure_save_name = [testName '_' file_start];
Dasdata = ReadDasLogV4(folder_path, file_start);
Fs = 15625; %sampling frequency
i = 1:length(channels);
y = Dasdata.Strain(channels(i),:);
L = length(y); data=abs(y/L);
T=Dasdata.Time(channels);
dt=T(2:end)-T(1:end-1);
Nt=length(T);
X=Dasdata.Position(channels);
dx=X(2:end)-X(1:end-1);
Nx=length(X);
fn=1./2./dt;
kn=1./2./dx;
df=1./Nt./dt; 
dk=1./Nx./dx;
f=[-fliplr(1:(Nt/2)) 0 (1:(Nt/2-1))].*df;
k=[-fliplr(1:(Nx/2)) 0 (1:(Nx/2-1))].*dk;
st=fftshift(fft2(data))./Nx./Nt;
figure(1);
%xticks = get(gca,'XTick')/Fs;
%for i = 1:length(xticks)
%   xticklabels{i} = num2str(xticks(i),3);
%end
%set(gca,'XTickLabels',xticklabels);
imagesc(k,f,abs(st));
colorbar; 
%title('FFT2');
%xlabel('k (1/m)')
%ylabel('f (Hz)')

%spec=st.*conj(st)./df./dk; 

%figure(2)
%imagesc(f,k,log10(spec)); axis xy
%colormap(jet)
%shg
%xlabel('k (1/m)')
%ylabel('f (Hz)')

[Image Analyst]

You forgot to attach any data or the ReadDasLogV4() function:

Unrecognized function or variable 'ReadDasLogV4'.

Error in test8 (line 16)

Dasdata = ReadDasLogV4(folder_path, file_start);

We'll check back later for it. In the meantime, check very carefully the three inputs your sending to imagesc() and validate that it can take those types of arguments.

Answer 1

[Walter Roberson]

T=Dasdata.Time(channels);

Your T is a column vector.

dt=T(2:end)-T(1:end-1);

That makes dt a column vector, because indexing a column vector with a vector index returns a column vector no matter whether the index itself is row or column.

Nt=length(T);

Scalar.

df=1./Nt./dt; 

Nt is scalar, 1./Nt is scalar. dt is column vector, scalar ./ column vector gives column vector.

f=[-fliplr(1:(Nt/2)) 0 (1:(Nt/2-1))].*df;

1:(Nt/2) is a row vector. fliplr() of a row vector is a row vector. 1:(Nt/2-1) is a row vector. [row vector, 0, row vector] gives a row vector.

Then you .* the row vector with a column vector. Implicit expansion is used, and you get a result which is length() of the column vector, by length() of the row vector -- a 2D array.

You then try to pass in f as the YData parameter to imagesc(), but the YData parameter to imagesc() must be one of:

1. Empty
2. A scalar
3. A vector with two or more elements. In this case, the first and last entries are used and the rest are ignored

Your 2D array is not empty or a scalar or a vector with two elements, so you are given an error message.

You have the same problem with construction of k: it will be a 2D array as well, for the same reason: you are combining row vectors and column vectors with implicit expansion.

Easiest fix:

T = Dasdata.Time(channels) .';

X = Dasdata.Position(channels) .';

[Iffat Arisa]

Thanks for your help. I fixed T and X and run, but it again shows the following error:

Arrays have incompatible sizes for this operation.

Error in a6 (line 40)

f=[-fliplr(1:(Nt/2)) 0 (1:(Nt/2-1))].*df;

[Walter Roberson]

Please attach a copy of as much of the 2.5 Lpm Step Test for 2 inch pipe with 0 insulation folder as we need to test your code. You can zip the files and attach the zip.

[Iffat Arisa]

I've fixed it, now it is running. Thanks a lot for your help.

I have a question about getting slope of the graph. I used:

b = polyval(f,k, abs(st));

slope = b(1)

Is this a correct Matlab command?

[Walter Roberson]

No, to get the slope you can use polyfit() (not polyval) with degree 1

[Iffat Arisa]

I have attached the figure which is frequency vs. wavenumber. Would you please tell me how can I get slope from this? Which Matlab function is required to get slope of this figure?

[Walter Roberson]

I suspect that what you need is gradient() rather than slope.

Answer 2

[Iffat Arisa]

I've attached ReadDasLogV4.m

[Image Analyst]

I think you forgot to attach a .das file, didn't you?

[Iffat Arisa]

I tried to attach das file, but it is not attached here.

After running the script, I found the attached section. Please see it.