Interpolation of temperature with pressure

Hi all,

as a newbie in Matlab, I've got a simple problem :

1) I have two dataset of pressure and temperature which both have a length of 1000

2) There are some missing data in my temperature profile, I'd like to interpolate around these missing values

3) I use interp1(temperature,pressure) -> it doesn't interpolate (no error message)

Why ?

Thank you for your help ! Florian

Answer 1

[Star Strider]

You have to tell it at what value of what variable you want to interpolate your data. (Your call to interp1 needs a third argument.)

[Florian]

I agree but missing values are represented by NaN and when I try to interpolate like :

interp1(temperature,entire_file_that_contain_temperature&pressure,pressure)

I get :

Error using griddedInterpolant
The coordinates of the input points must be finite values; Inf and NaN are not permitted.

[Star Strider]

I don’t know what your data look like, so I generated my own. You may have to adapt this to your situation. You can trace it through to see how it functions.

This seems to be a solution:

% Create data —
x = 0:0.1:2*pi;
y = sin(x);

% Create NaNs in data — 
yn = y;
xn = x;
xn(5:5:end) = NaN;
yn(5:5:end) = NaN;

% Convert NaNs to missing values —
xm = xn;
ym = yn;
ym(isnan(ym)) = [];
xm(isnan(xm)) = [];

% Find indices of NaNs and interpolate for missing ‘x’ values —
ixn = find(isnan(xn));
for k1 = 1:size(ixn,2)
    ixr = [xn(ixn(k1)-1)  xn(ixn(k1)+1)];
    xi(k1) = mean(ixr);
end

% Use interpolated ‘x’ values to interpolate missing ‘y’ values —
yi = interp1(xm, ym, xi);

figure(1)
plot(xn, yn, 'ob')
hold on
plot(xi, yi, '+r')
hold off
grid

(Apologise for the delay. Out for a bit on errands.)

Answer 2

[Kelly Kearney]

Assuming temp and pressure are your vectors, you need to use the non-missing values to interpolate the missing ones:

ismissing = isnan(temp);
temp(ismissing) = interp1(pressure(~ismissing), temp(~ismissing), pressure(ismissing));