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From: Michael on 8 Jul 2010 09:45
MATLAB Version 7.10.0.499 (R2010a)

I need help understanding how the interp1 function works. I have looked at the documentation (in particular example 1see below), and website (still working through all of these) and it still have not found an answer to my questions. I have varied the step of xi to values larger and smaller than x see example. When I plot the results for xi step of 2 it skips over every other point. However, with a value smaller than 1 (x step) the interval of xi is the default of x (in this case one).

So here is the question: How is this a finer abscissa?? What do I not understand?

--------
interp1
example 1
Generate a coarse sine curve and interpolate over a finer abscissa.

x = 0:1:10;
y = sin(x);
xi = 0:0.5:10;
%xi = 0:2:10; %larger step size than x
yi = interp1(x,y,xi);
plot(x,y,'o',xi,yi)
From: John D'Errico on 8 Jul 2010 09:58
"Michael" <michael.lisowski(a)gentex.com.extrachar> wrote in message <i14kp4$lh1$1(a)fred.mathworks.com>...
> MATLAB Version 7.10.0.499 (R2010a)
>
> I need help understanding how the interp1 function works. I have looked at the documentation (in particular example 1see below), and website (still working through all of these) and it still have not found an answer to my questions. I have varied the step of xi to values larger and smaller than x see example. When I plot the results for xi step of 2 it skips over every other point.


> However, with a value smaller than 1 (x step) the interval of xi is the default of x (in this case one).
>

I have no idea what you mean by this last statement.
Please show an example, because I can only guess that
you have made a mistake in your code.


> So here is the question: How is this a finer abscissa?? What do I not understand?

When you show what is confusing you, then we can help.


> --------
> interp1
> example 1
> Generate a coarse sine curve and interpolate over a finer abscissa.
>
> x = 0:1:10;
> y = sin(x);
> xi = 0:0.5:10;
> %xi = 0:2:10; %larger step size than x
> yi = interp1(x,y,xi);
> plot(x,y,'o',xi,yi)

When diff(xi) is smaller than diff(x), the abscissa might
be said to be finer.

John
From: Michael on 8 Jul 2010 10:14
Sorry,

I figured out my issue in the example
x = 0:10;
y = sin(x);
xi = 0:.25:10;
yi = interp1(x,y,xi);
plot(x,y,'o',xi,yi)

The plot generated a perficly straight line between the (x(1),y(1)) and (x(2),y(2)). With the value interpelated value of yi egual to exactly half of y(2). Hence, a linear interpolation. This example may have been better off if they had used
yi = interp1(x,y,xi,'spline');
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