Resampling dataset from monthly to annual
Guizmo
Mon, 02/14/2011 - 09:22 am
I am new to Igor and I have been trying to use the resampling option to change a monthly dataset to an annual dataset, with a value every year centered on the mid-year (i.e 1991.5: 1992.5; 1992.5 ...). My new dataset should get 12 times shorter than my original one.
I have been trying to use the resample tool window, with intepolated:12, and decimate:0.5, I left the filter lenght onthe default value (21).
and...I just can't click on the button "do it" ...
do I do something wrong? should I try to type the command in?
I am attaching my file with a column with the decimal year (monthly), 1991.04; 1991.12; 1991.21...and a column with the monthly dataset which I need to resample from years 1850 to 2002)
Thanks a lot!
Guizmo
My guess is that after selecting your waves in the Waves list, you did not press the yellow arrow to the right of that list. This will add the selected waves to the box below the row of headings (Wave, Points, etc.). Then the "Do It" button becomes active.
To get the sampling rate that you want try using Interpolate 1, Decimate 12 and, I think, Filter Length 1 if you want the mid-point of your data rather than an average. The ratio of Inerpolate to Decimate controls the final sampling rate. So Interpolate 1 Decimate 12 sets the output point density at 1/12 of the starting wave point density. Also, I had to delete the first 6 data points from both "cal years" and "concs" to get an output wave containing the point at mid year.
Others might have better suggestions for these settings.
I attached a modified version of your experiment with the outputs of my tests.
Jeff
February 15, 2011 at 07:59 am - Permalink
I executed:
Print conc_samp[0] // Print resampled output for first 12 points
I got 0.0862091 and 0.0922917.
I'm not sure how to solve this using Resample.
Once you get the right output, you can use wave scaling to set the X values of the output wave, like this:
Display conc_samp
Now you no longer need the X wave ('cal years_samp').
To learn about X scaling, execute this:
and then:
February 15, 2011 at 09:28 am - Permalink
... Interpolate 1, Decimate 12 and, ... Filter Length 1 .... Also, I had to delete the first 6 data points from both "cal years" and "concs" ...
was to get the value of the data point occuring at the middle of each year rather than the average for the year. That was my interpretation of Guizmo's message. I could be wrong in that assumption.
Like Howard, I don't know how to get the average for each year using Resample. However, here is a short function that will return the average for every n points in a wave, using the usual wave assignment syntax:
Wave w
Variable CurrentPoint
Variable PointsInAvg
Variable first = PointsInAvg * CurrentPoint
Variable last = first + PointsInAvg -1
wavestats/Q/R=[first, last] w
print CurrentPoint
return V_avg
End
To return the annual average from a wave with 12 points per year, one would write:
concs_samp = avgpts(concs, p, 12)
It would be necessary to appropriately size the output wave.
For Guizmo's data the first three annual points are: 0.0862091, 0.32879 and 0.358203.
Jeff
February 15, 2011 at 11:40 am - Permalink
wave calendar, data
make/O/N=(numpnts(data)/12) red_cal, red_concs
red_cal = calendar[12*p+5]
red_concs = mean(data , 12*p, 12*p+11 )
end
February 16, 2011 at 07:24 am - Permalink