I need correct syntax of interpolation in this case

Hello, I have a 40x32x360 double array. (latitude x longitude x time) which presents precipitation for these latitudes and longitude for 360 months.

I want to interpolate this array to found how much the precipitation value for latitude = 30.3772 and longitude 38.2147 in all 360 months (the output is 1x1x360 array).

I read about interp2 and interp3 but I don't know what is suitable in this case and how to use them. Can you tell me what is the true syntax in order to do this?

Answer 1

[S. Walter]

Behzad,

Check out griddedInterpolant (https://www.mathworks.com/help/matlab/ref/griddedinterpolant.html).

You'll want to make sure your latitude, longitude, and time are all 40x32x360 arrays. For that you can use ndgrid (https://www.mathworks.com/help/matlab/ref/ndgrid.html) if they are not.

In your case, you'd want something along these lines:

% Create a interpolation surface that uses latitude, longitude, 
% and time to interpolate over precipitation
F = griddedInterpolant(lat,long,t,precipitation)
% Pick the time of interest
tOfInterest = [1:1:360]';
% Pick the latitude and longitude of interest but make sure
% it's the same size as your time vector
latOfInterest = 30.3772*ones(size(tOfInterest));
longOfInterest = 38.2147*ones(size(tOfInterest));
% Now interpolate to those values
AnswerYouAreLookingFor = F(latOfInterest,longOfInterest,tOfInterest)

Hope that helps!

[BN]

Dear S. Walter

I attached my data, unfortunately it gives me an error that says Grid arrays must have NDGRID structure.

I think my arrays are ndgrided, can you look at my data and help me with this issue?

Thank you so much for your help

[S. Walter]

Your data is in vectors, not arrays. You will have to use ndgrid to convert it from vectors to arrays.

You can check your variables by using the "whos" command:

whos

and the output you'll get is:

  Name                 Size                  Bytes  Class       Attributes

  dates              360x1                    2880  datetime              
  lati                 1x32                    128  single                
  loni                 1x40                    160  single                
  precipitation       40x32x360            1843200  single                

If you look at the "Size" column, it shows you "dates" is 360x1, "lati" is 1x32, and "loni" is 1x40. But what you're interested in interpolating over is 40x32x360. The interpolator doesn't know how to organize those vectors into a matrix, the programmer will have to do that. In order to create that matrix, you use the ndgrid function.

Looking at your data though, you can see precipitation is 40x32x360 - meaning it's organized as longitude by latitude by date. When you go to use ndgrid, you have to keep that same ordering

  [x,y,z] = ndgrid(loni,lati,dates);

When you do that, you should see that x, y, and z are now the same size as precipitation. When you then organize your griddedInterpolant, you will have to keep the same order

  F = griddedInterpolant(x,y,z,precipitation)

But Wait! The other part of your problem is that, looking at the "Class" column of your variables - dates is in datetime format. The interpolator is not set up to handle classes other than single or double. So you'll have to find a way to convert the "dates" variable into a numerical value. I can't help you how to do that, it depends on the rest of your code.

Lastly! Make sure when you go to use the interpolator you just created, you always keep the same order - longitude, latitude, dates - otherwise you will receive an erroneous answer.

[BN]

Dear S. Walter,

I really appreciate you, you are really helped me, Thanks.