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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
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Hi Chris: netcdf-java has several levels of API. The lowest level (NetcdfFile) doesnt touch the data, so you just get whatever is in there. this is the level that users of the C library are used to. the next highest level of the netcdf-java API (NetcdfDataset) process missing data, scale/offset, CF conventions, and other things. At this layer the various attribute conventions are used to identify missing data. John On Wed, Apr 15, 2015 at 10:27 AM, Chris Barker <chris.barker@xxxxxxxx> wrote: > As I read those docs, and as I've always interpreted netcdf, those "rules" > are there to inform the user of the data -- I don't expect client software > reading the data to replace values in the the arrays for me -- at least not > low-level generic data reading libraries. > > What were you expecting, and why? > > Higher level libs that represent a data model _may_ do some QA/QC on the > data, I suppose, but even then, I'd want to clearly specify what I wanted > it to do -- certainly not silently toss out values because the were outside > the limits specified in the meta-data. > > -Chris > > > > On Wed, Apr 15, 2015 at 9:18 AM, John Caron <caron@xxxxxxxx> wrote: > >> The netcdf-java library does indeed ignore that rule. We should revisit >> the manual and clarify things i guess. >> >> On Wed, Apr 15, 2015 at 9:33 AM, Russ Rew <russ@xxxxxxxxxxxxxxxx> wrote: >> >>> Hi Phil, >>> >>> I think that's a misinterpretation of the Users Guide attribute >>> conventions. Under the description for _FillValue, the Guide states: >>> >>> The fill value ... is normally outside the valid range and therefore >>> treated as missing when read by generic applications. It is legal (but not >>> recommended) for the fill value to be within the valid range. >>> >>> >>> The last sentence implies that the valid range is not determined by the >>> _Fill_Value. >>> >>> I think it's intended that the rule about _Fill_Value under the >>> description for valid_range only applies in case none of valid_min, >>> valid_max, or valid_range are specified, so it wouldn't apply to your >>> example: >>> >>> If neither valid_min, valid_max nor valid_range is defined then generic >>> applications should define a valid range as follows. ... >>> >>> >>> However, I suspect that the rule is confusing enough that writers of >>> generic clients might just ignore it, even in that case. >>> >>> --Russ >>> >>> On Wed, Apr 15, 2015 at 8:55 AM, Bentley, Philip < >>> philip.bentley@xxxxxxxxxxxxxxxx> wrote: >>> >>>> Hi folks, >>>> >>>> I was hoping someone can clarify for me the correct use of the >>>> valid_min, valid_max and valid_range attributes by *well-behaved* netCDF >>>> clients. >>>> >>>> Given the netCDF file shown below (in CDL form), and considering the >>>> rules for handling the aforementioned attributes, as defined in Appendix A >>>> of the NetCDF user guide (see >>>> http://www.unidata.ucar.edu/software/netcdf/docs/attribute_conventions.html), >>>> I *think* I'd expect conforming netCDF clients to represent the data for >>>> each of the variables var1..var3 as an array with values (_, -272, 0, 100, >>>> 9999, _), where _ signifies missing data values. >>>> >>>> --- >>>> netcdf mditest { >>>> >>>> dimensions: >>>> dim1 = 6; >>>> >>>> variables: >>>> float var1(dim1); >>>> var1:long_name = "var1"; >>>> var1:units = "1"; >>>> var1:valid_min = -273.0f; >>>> var1:valid_max = 1.0e5f; >>>> >>>> float var2(dim1); >>>> var2:long_name = "var2"; >>>> var2:units = "1"; >>>> var2:valid_min = -273.0f; >>>> var2:_FillValue = 1.0e5f; // constrains valid_max >>>> >>>> float var3(dim1); >>>> var3:long_name = "var3"; >>>> var3:units = "1"; >>>> var3:_FillValue = -273.0f; // constrains valid_min >>>> var3:valid_max = 1.0e5f; >>>> >>>> // global attributes >>>> :Conventions = "CF-1.0"; >>>> >>>> data: >>>> var1 = -300, -272, 0, 100, 9999, 1e6 ; >>>> var2 = -300, -272, 0, 100, 9999, 1e6 ; >>>> var3 = -300, -272, 0, 100, 9999, 1e6 ; >>>> } >>>> --- >>>> >>>> However, having tried several different netCDF clients - some C, some >>>> Java, some Python - none of them appear to adhere consistently to the >>>> aforementioned rules for handling the valid min/max/range attributes. The >>>> python-based clients, in particular, only seem to honour the _FillValue >>>> attribute, reflecting, I believe, the current behaviour of the >>>> netcdf4-python module. >>>> >>>> Am I perhaps misinterpreting the nc attribute-handling conventions? >>>> >>>> Regards, >>>> >>>> -- >>>> Phil Bentley PhD, Climate Science IT Apps Group >>>> Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, United Kingdom >>>> Tel: +44 (0)1392 886881 >>>> Email: philip.bentley@xxxxxxxxxxxxxxxx Web: >>>> http://www.metoffice.gov.uk/ >>>> >>>> >>>> _______________________________________________ >>>> netcdfgroup mailing list >>>> netcdfgroup@xxxxxxxxxxxxxxxx >>>> For list information or to unsubscribe, visit: >>>> http://www.unidata.ucar.edu/mailing_lists/ >>>> >>>> >>> >>> _______________________________________________ >>> netcdfgroup mailing list >>> netcdfgroup@xxxxxxxxxxxxxxxx >>> For list information or to unsubscribe, visit: >>> http://www.unidata.ucar.edu/mailing_lists/ >>> >> >> >> _______________________________________________ >> netcdfgroup mailing list >> netcdfgroup@xxxxxxxxxxxxxxxx >> For list information or to unsubscribe, visit: >> http://www.unidata.ucar.edu/mailing_lists/ >> > > > > -- > > Christopher Barker, Ph.D. > Oceanographer > > Emergency Response Division > NOAA/NOS/OR&R (206) 526-6959 voice > 7600 Sand Point Way NE (206) 526-6329 fax > Seattle, WA 98115 (206) 526-6317 main reception > > Chris.Barker@xxxxxxxx > > _______________________________________________ > netcdfgroup mailing list > netcdfgroup@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/ >
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