How can I interpolate for a particular range of data?

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Wed, 12/12/2012 - 11:30 pm

I need to interpolate a data set for a specific range of data. How can I specify the range?

The actual problem is like this.
I have several set of x_data and y_data, where x_data is the control parameter, y_data is corresponding values. In different data set the x_data has different range with different Delta_X. Now I need to map this data to a two dimensional wave. Where I need evenly spaced x_data. That is why I need to interpolate all the data set for a fixed range of x_data with a fixed number of points.
I do something similar when comparing experimental and reference data. Pick the x_data wave with the highest density of datapoints as your reference. Next we are going to loop over the commonly shared x-range of all the x-waves. You can do this by making two waves (one contains the minimum values while the other contains the maximum values). We will start the loop at the Max(minimum_wave) and go to min(maximum_wave). This ensures that each Y-wave has data within the range we will loop over. Use these start/stop values and the delta_x of your reference wave to determine the number of steps in the loop.

Then, simply iterate a for loop where you interpret each of the other XY-pair waves based on your reference x-wave. One thing to note is that interpolate estimates a value between the two nearest points via a linear line. So if there is a large discrepancy between the delta_x values of your waves, you could end up with some rather odd looking data. If this is the case, you could use the lowest density x_wave as your reference, and basically discard the extra data from your other waves (the process would be the same).

December 13, 2012 at 07:42 am - Permalink

Quote:
I need to interpolate a data set for a specific range of data. How can I specify the range?


If you mean that you want to create an interpolated version of a subset of an XY pair, you must first create a 1D wave containing the subset. You can do this using the Duplicate /R operation.

After doing that you then can do the interpolation. Execute this for a discussion of interpolating XY pairs to waveform data:
DisplayHelpTopic "Converting XY Data to a Waveform"


Once you have done that you can store the resulting interpolated data in a column of a 2D wave using waveform assignment:
dest[][0] = source[p]   // Store interpolated data in matrix column 0

December 13, 2012 at 09:26 am - Permalink

hrodstein][quote wrote:

If you mean that you want to create an interpolated version of a subset of an XY pair, you must first create a 1D wave containing the subset. You can do this using the Duplicate /R operation.


Yes, I need to interpolate in a subset of XY pair. I want specific x_data values, say, 0, 0.1, 0.2, ... ,1 for all the data sets. Unfortunately, the recorded data are not in such nice form, they looks something like -0.106, 0.012, 0.097, 0.205...,1.011 this. So if I create subset of data, I will not get specific data points which I want, rather I will get 0.012,...., 1.011 and in-between equally spaced points.

December 14, 2012 at 12:44 am - Permalink

You can interpolate at a set of specific X values. For details execute:
DisplayHelpTopic "Destination X Coordinates from Destination Wave"


I think using this technique you don't need to create a subset of your input waves.

December 14, 2012 at 09:57 am - Permalink