Matlab in Chemical Engineering at CMU

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Interpolation of data

when we have data at two points but we need data in between them we use interpolation. Suppose we have the points (4,3) and (6,2) and we want to know the value of y at x=4.65, assuming y varies linearly between these points. we use the interp1 command to achieve this.

x = [4 6]
y = [3 2]

interp1(x,y,4.65)

x =

     4     6


y =

     3     2


ans =

    2.6750

multiple interpolation values

you can interpolate several values

interp1(x,y,[4.65 5.01 4.2 9])
% note that you cannot interpolate outside the region where data is
% defined. That is why the value at x=9 is NaN.

ans =

    2.6750    2.4950    2.9000       NaN

more sophisticated interpolation.

the default interpolation method is simple linear interpolation between points. Other methods exist too, such as fitting a spline to the data and using the spline representation to interpolate from.

x = [1 2 3 4];
y = [1 4 9 16]; % y = x^2

xi = [ 1.5 2.5 3.5]; % we want to interpolate on these values
y1 = interp1(x,y,xi);
y2 = interp1(x,y,xi,'spline')

y2 =

    2.2500    6.2500   12.2500

Compare these methods by plotting each set of interpolated values

the data we interpolated was constructed from y = x^2

figure
hold on
ezplot('x^2',[1 4])
plot(xi,y1,'rs',xi,y2,'bd')
legend('data','linear interpolation','spline interpolation')
hold off

summary: in this case the spline interpolation is a little more accurate than the linear interpolation. That is because the underlying data was polynomial in nature, and a spline is like a polynomial. That may not always be the case, and you need some engineering judgement to know which method is best.

% categories: Basic Matlab
% tags: math

Published with MATLAB® 7.10