Hi All:
This has come up in the R list. it is referring to NCEP/NCAR Reanalysis 2
data [1] that the user obtained. here is what he wrote:
> Thank you very much Jeff. I think I’m too far to be able to explain myself.
> Perhaps, this is the wrong list for this question but I sent it in hope there
> is someone has deep understanding of netcdf data and use R. Let me tell the
> story simpler. Assume that you read a numeric vector of data from a netcdf
> file:
>
> data <- c(9.1999979, 8.7999979, 7.9999979, 3.0999980, 6.1000018, 10.1000017,
> 10.4000017, 9.2000017)
>
> you know that the values above are a model output and also you know that,
> physically, first and last values must be equal but somehow they are not.
>
> And now, you want to use “periodic” spline for the values above.
>
> spline(1:8, data, method = “periodic”)
>
> Voila! spline method throws a warning message: “spline: first and last y
> values differ - using y[1] for both”. Then I go on digging and discover 2
> attributes in netcdf file: “precision = 2” and “least_significant_digit = 1”.
> And I also found their definitions at [1].
>
> precision -- number of places to right of decimal point that are significant,
> based on packing used. Type is short.
> least_significant_digit -- power of ten of the smallest decimal place in
> unpacked data that is a reliable value. Type is short.
>
> Please, do not condemn me, english is not my main language :). At this point,
> as a scientist, what would you do according to explanations above? I think I
> didn’t exactly understand the difference between precision and
> least_significant_digit. One says “significant” and latter says “reliable”.
> Should I round the numbers to 2 decimal places or 1 decimal place after
> decimal point?
The data comes out that way because of the way R encodes floating points, and
for this case in R the correct thing would be to do:
> round(data, digits = 2)
But as the user later wrote:
> For instance, If you check the header information of omega.2015.nc file it
> says;
>
> $ ncdump -h omega.2015.nc
>
> ...
> omega:precision = 3s;
> omega:least_significant_digit = 3s;
> …
>
> So, I need to round values to 3 decimal places after point?
>
> and if you check the output of rhum.2015.nc;
>
> $ ncdump -h rhum.2015.nc
> ...
> rhum:precision = 2s ;
> rhum:least_significant_digit = 0s ;
> …
>
> Then I need to round values to 2 decimal places after point?
>
> Should I accomplish the rounding operation according to precision or
> least_significant_digit attributes?
If you have a good answer, please reply all so that the original poster can see
the response.
Thanks,
-Roy
1- http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html
**********************
"The contents of this message do not reflect any position of the U.S.
Government or NOAA."
**********************
Roy Mendelssohn
Supervisory Operations Research Analyst
NOAA/NMFS
Environmental Research Division
Southwest Fisheries Science Center
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