Thank you Roy. I use NCEP/NCAR Reanalysis 2 data [1]. More precisely, u-wind data of the year 2015 [2]. I am also pretty sure that the variables like scale_factor or add_offset should be precise like 0.01 or 187.65 but somehow (I hope this is not an issue originated by me) they are not, including data. Also let me note that I already contacted to author of ncdf4 package and also sent an email to ESRL, too, but no luck yet.
For a vectoral data, absolute and mutual u components of wind speed at the poles must be equal. For instance, at “2015-01-01 00 GMT”, u-wind at longitude=0 and latitude=90 is 9.1999979 m/s and u-wind at longitude=180 and latitude=90 is -9.2000017 m/s. Minus sign comes from positive north direction. Physically, their absolute values must be equal. 1- http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html 2- ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis2.dailyavgs/pressure/uwnd.2015.nc > On 08 Jul 2016, at 02:27, Roy Mendelssohn - NOAA Federal > <roy.mendelss...@noaa.gov> wrote: > > Hi Ismail: > > Can you point me to a particular netcdf file you are working with. I would > like to play with it for awhile. I am pretty certain the scale factor is > 0.01 and what you are seeing in rounding error (or mor precisely I should say > problems with representations of floating point numbers), but i would like > to see if there is away around this. > > Thank, > > -Roy > >> On Jul 7, 2016, at 4:16 PM, Ismail SEZEN <sezenism...@gmail.com> 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? >> >> Thanks, >> >> 1- >> http://www.esrl.noaa.gov/psd/data/gridded/conventions/cdc_netcdf_standard.shtml >> >> >>> On 08 Jul 2016, at 01:29, Jeff Newmiller <jdnew...@dcn.davis.ca.us> wrote: >>> >>> Correction: >>> >>> ?options (not par) >>> -- >>> Sent from my phone. Please excuse my brevity. >>> >>> On July 7, 2016 3:26:06 PM PDT, Jeff Newmiller <jdnew...@dcn.davis.ca.us> >>> wrote: >>>> Same as with any floating point numeric computation environment... you >>>> don't. There is always uncertainty in any floating point number... it >>>> is just larger in this data than you might be used to. >>>> >>>> Once you get to the stage where you want to output values, read up on >>>> >>>> ?round >>>> ?par (digits) >>>> >>>> and don't worry about the incidental display of extra digits prior to >>>> presentation (output). >>>> -- >>>> Sent from my phone. Please excuse my brevity. >>>> >>>> On July 7, 2016 12:50:54 AM PDT, Ismail SEZEN <sezenism...@gmail.com> >>>> wrote: >>>>> Hello, >>>>> >>>>> I use ncdf4 and ncdf4.helpers packages to get wind data from ncep/ncar >>>>> reanalysis ncetcdf files. But data is in the form of (9.199998, >>>>> 8.799998, 7.999998, 3.099998, -6.8000018, …). I’m aware of precision >>>>> and least_significant_digit attributes of ncdf4 object [1]. For uwnd >>>>> data, precision = 2 and least_significant_digits = 1. My doubt is that >>>>> should I round data to 2 decimal places or 1 decimal place after >>>>> decimal point? >>>>> >>>>> Same issue is valid for some header info. >>>>> >>>>> Output of ncdf4 object: >>>>> >>>>> >>>>> Output of ncdump on terminal: >>>>> >>>>> >>>>> for instance, ncdump's scale factor is 0.01f but ncdf4 object’s >>>>> scale_factor is 0.00999999977648258. You can notice same issue for >>>>> actual_range and add_offset. Also a similar issue exist for the data. >>>>> How can I truncate those extra unsignificant decimal places or round >>>>> the numbers to significant decimal places? >>>>> >>>>> 1 - >>>>> http://www.esrl.noaa.gov/psd/data/gridded/conventions/cdc_netcdf_standard.shtml >>>>> <http://www.esrl.noaa.gov/psd/data/gridded/conventions/cdc_netcdf_standard.shtml> >>>>> ______________________________________________ >>>>> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >>>>> https://stat.ethz.ch/mailman/listinfo/r-help >>>>> PLEASE do read the posting guide >>>>> http://www.R-project.org/posting-guide.html >>>>> and provide commented, minimal, self-contained, reproducible code. >>>> >>>> ______________________________________________ >>>> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >>>> https://stat.ethz.ch/mailman/listinfo/r-help >>>> PLEASE do read the posting guide >>>> http://www.R-project.org/posting-guide.html >>>> and provide commented, minimal, self-contained, reproducible code. >>> >> >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ********************** > "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 > ***Note new address and phone*** > 110 Shaffer Road > Santa Cruz, CA 95060 > Phone: (831)-420-3666 > Fax: (831) 420-3980 > e-mail: roy.mendelss...@noaa.gov www: http://www.pfeg.noaa.gov/ > > "Old age and treachery will overcome youth and skill." > "From those who have been given much, much will be expected" > "the arc of the moral universe is long, but it bends toward justice" -MLK Jr. > ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
