Interpolation between two curves (matlab)

Question

Ok guys. I have the following problem: I have the data of the following plot. So the data file of this plot contains three columns. The 2nd and 3rd ones the x,y points. And the 1st one is to which system those points belong. In this case the red ones are for the system of 20 years. The blue ones for the 30 years. What I want to find is the curve at 25 years. So if I plot it it should be between the red and blue curves. I have no idea how interpolate the data in order to obtain what I want. Actually I want to have for 21,22,...29 years, I guess if we can find it for a time in between these two, then the method should work for any time between 20 and 30. PS: I guess the interpolation for each curve (in this case red or blue one) is quite easy. Just using interp1(x,y,xx) will work. But what happened with the other "dimension" (M) The data. ``` 20.0000 3.4076 0 20.0000 3.4226 99.5405 20.0000 3.4701 196.3360 20.0000 3.5592 287.0781 20.0000 3.6248 328.8516 20.0000 3.6643 348.3373 20.0000 3.…

Accepted Answer

Actually, by looking at the Matlab documentation I found a simpler way. You can use the function griddata. (The doc in matlab help shows visual example). The resampling on a common grid and the interpolation is embedded in the function.

```
%// First separate (and name your column to identify them better)
t = d(:,1) ;
x = d(:,2) ;
y = d(:,3) ;

%// use the function 'griddata'
[TI,YI] = meshgrid( 20:30 , 0:20:500 ) ; %// change these values to change the grid limits
XI = griddata(t,y,x,TI,YI) ;

%// show result in 3D ... but could be projected in X-Y plane if necessary
plot3(TI,YI,XI , 'Marker','o' )
xlabel('Time') ; ylabel('Y') ; zlabel('X')
```

The last line of the code shows this plot:

All your interpolated data are in the XI matrix. The way to retrieve them depends on how you want to organize them ultimately.

EDIT: To place all the interpolated data in a single table `InterpData` organized the same way of your original table, use the following:

```
nLine = numel(XI) ;
InterpData = [ reshape(TI,nLine,[]) reshape(XI,nLine,[]) reshape(YI,nLine,[]) ] ;
```

Regarding the `NaN`s. They will come to bother you every time you ask to do an interpolation outside of the initially known values. For example, if your time in the original data is in the [20 to 30] interval, matlab will gladly interpolate anything within that interval, but will return `NaN` if you ask to return a value for time = 19 for example. Same goes for `Y`, the grid on which to interpolate has to be within the initial range. (as in this implementation we use a base grid formed by `Time` (column 1) and `Y`(column 3), to interpolate the `X` column).

Interpolation between two curves (matlab)

Solution

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