interp1 vector of interpolated sample points using Euler method

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NOtway 2022-10-13

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评论： Torsten 2022-10-13

Hi, I have sompe tabular data that I have imported and have to interpolate between.

[date, t, Rainfall, Evap, I_t] = readvars("BOM_Data.xlsx");
% Define functions to allow interpolation of data
RAINFALL = @(time) interp1(t,Rainfall,time);
EVAP = @(time) interp1(t,Evap,time);

However when I use these values later on in the code it isnt working and those functions spit out NaN values for the first timestep.

For example, one of those equations is:

P_in(i) = RAINFALL(time(i)) *A_catch * C ;

Where A_catch and C are constant variables. The 2nd and further timesteps seem to work, however the 1st one is NaN, which flows on through to toher calcs and messes up others.

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Torsten 2022-10-13

The t array from your Excel data must start with t(1) = 0. Is this the case ?

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Answer 1

Jiri Hajek 2022-10-13

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Please have a look into the documentation of interp1, namely the section on extrapolation method. You'll see that NaNs are the default result when you try to extrapolate, which is quite sensible.You have to check your input data and see whether you really want to extrapolate, as this is not really a typical task for an iterpolation method.

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NOtway 2022-10-13

I dont need to extrapolate, only interpolate (eg. take my daily data and use it for hourly Euler timesteps). Do you have any suggestions on more suitable methods to go about this then?

Jiri Hajek 2022-10-13

Perhaps you dont need it or want it, but it is the most straightforward explanation for the NaNs you are geting. You have to carefully check the inputs into the interpolator, that is all. You see, we do not know what are the values in your variables,you have to check them yourself...

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