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Good afternoon:
I am operating on a NetCDF file that contains data for 20 variables over a period of 8 months. This is too much data, so I am trying to extract data based on the time of day. That is, to extract data for all variables from 11pm to 4am for each day in the data file. I have been able to pull out the date / time in the format "dd-mmm-yyyy hh:mm:ss". I can extract and work on ranges of time, but not a range of time per day, for many days.
I can see in my head how to do this, but I am unsure of an efficient way to code it. Experimenting with different time functions, (datenum,hours, datetime, datevec) with other structure and NetCDF tools have been unsucessful. I could use a shove in the right direction. Thank you.

Accepted Answer

Walter Roberson
Walter Roberson on 5 Jul 2017
See https://www.mathworks.com/matlabcentral/answers/312198-how-to-extract-data-from-nc-file-based-on-latitude-longitude-time-and-wind#comment_464820 and note that in my sample "expanding the selection" code that you could code hours and minutes into the from date and to date strings.
NATHAN MURRY on 1 Aug 2017
Hi Walter: So I see I had the correct two statements, but I didn't try to join them together in a larger logical statement as you showed. With some further experimentation and additions, the function works great.
Thank you again for all of your help. I learned quite a bit in wrestling through this problem. Take care.

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More Answers (2)

Tanziha Mahjabin
Tanziha Mahjabin on 29 Jan 2020
Edited: Walter Roberson on 29 Jan 2020
I want to cut some time from a bid data, using ncread(source,varname,start,count).
for your information,
Size: 69x69x45588
Dimensions: J,I,TIME
Datatype: single
long_name = 'Standard deviation of sea water velocity U component values in 1 hour.'
units = 'm s-1'
valid_min = -10
valid_max = 10
cell_methods = 'TIME: standard_deviation'
_FillValue = 999999
ancillary_variables = 'NOBS1 NOBS2 UCUR_quality_control'
Now if i write,
u=ncread(ncfile,'UCUR',[1 1 1],[Inf Inf 44931]);
it takes the command as the start time is from the start.
But what should i write if i want cut the time from somewhere middle?
I tried to define index,
u=ncread(ncfile,'UCUR',[1 1 ind],[Inf Inf 44931]);
But it is not working. Any helpful suggestion please.
  1 Comment
Walter Roberson
Walter Roberson on 29 Jan 2020
netcdf times are never in MATLAB serial datenum . Instead they are in some time units relative to a particular epoch that is defined in the attributes, such as "seconds since Jul 1, 1983 00:00:00 UTC" . You need to examine the attributes for the TIME coordinate and do the conversion.

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Tanziha Mahjabin
Tanziha Mahjabin on 30 Jan 2020
Hi Walter,
Thanks for the comment. I did the conversion.
When i write something like this, ru=ncread(ncfile,'UCUR',[1 1 1],[Inf Inf 931]); it works as the time starts from the beginning.
But i want to start the time from somewhere else as i mentioned in my question. So i defined index and try to start according to that.
u=ncread(ncfile,'UCUR',[1 1 ind],[Inf Inf 44931]);
It didn't work.
Walter Roberson
Walter Roberson on 30 Jan 2020
I don't think you want that read inside a for loop??

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