Creating a monthly average from a matrix

2013 8 20
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My data, which has 30 years of monthly data, over a 2 degree grid of the globe:
>> whos KoppenA_prate
Name Size Bytes Class Attributes
KoppenA_prate 90x180x360 23328000 single
Not every pixel has data, just the pixels that fit into Koppen classification A
>> sum(A(:))
ans =
559
My question: Any ideas on how I could write a loop to create an average monthly value over the 30 years?
Thanks a bunch!

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Azzi Abdelmalek
Azzi Abdelmalek 2013-8-20
编辑：Azzi Abdelmalek 2013-8-20
This is not clear for me. What are your data? and what do you want?
Image Analyst
Image Analyst 2013-8-21
What does each of the 3 dimensions represent? Is any one of the dimensions the time point? If so, are the time points all one month apart? Or, are they days of the year, so that one month might have 30 data points sometimes, and 31 points at other times?
Kate
Kate 2013-8-21
90x180 is a 2 degree grid of the globe, 360 is 12 months x 30 years
Kate
Kate 2013-8-21
I would like to create a vector of precip data, for each month averaged over 30 years.
Kate
Kate 2013-8-21
This is as far as I've gotten, but my for loop isn't working:
%Create monthly averages over 30 years
Jan=[1:12:349];
Feb=[2:12:350];
Mar=[3:12:351];
Apr=[4:12:352];
May=[5:12:353];
Jun=[6:12:354];
Jul=[7:12:355];
Aug=[8:12:356];
Sep=[9:12:357];
Oct=[10:12:358];
Nov=[11:12:359];
Dec=[12:12:360];
for ii=length(num_mons)
if num_mons==Jan;
Jan_dat=[Jan,KoppenA_prate(3)];
elseif num_mons==Feb;
Feb_dat=[Feb,KoppenA_prate(3)];
end
end

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Cedric
Cedric 2013-8-21
编辑：Cedric 2013-8-21

1 个投票

Just
avgAllYears = mean(KoppenA_prate, 3) ;
where 3 is the dimension along which the mean must be computed. With that you get a 90x180 array of means over all times.
EDIT:
siz = size(KoppenA_prate) ;
avg = zeros(siz(1), siz(2), 12) ;
for mId = 1 : 12
avg(:,:,mId) = mean(KoppenA_prate(:,:,mId:12:end), 3) ;
end
EDIT2: or, more efficient but more complicated
siz = size(KoppenA_prate) ;
buf = mean(reshape(KoppenA_prate, siz(1), 12*siz(2), []), 3) ;
avg = reshape(buf, siz(1), siz(2), []) ;
Note: both produce a 90x180x12 array of monthly averages, were e.g. avg(:,:,5) is the average of 5th months over all years.

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Kate
Kate 2013-8-21
Thanks Cedric, but that would create a 30 year average, not a monthly average
Cedric
Cedric 2013-8-21
编辑：Cedric 2013-8-21
Ok, I misunderstood "monthly average" for "average over monthly values". See my edits.
Kate
Kate 2013-8-21
Thanks Cedric, that's great. How could I get a single, say Jan, value over the whole grid? I need to plot average yearly precip.
Thanks for all of your help!
Cedric
Cedric 2013-8-21
You're welcome; see my last note.

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