[netcdfgroup] Interpretation of valid_min/max/range attributes

• To: "netcdfgroup@xxxxxxxxxxxxxxxx" <netcdfgroup@xxxxxxxxxxxxxxxx>
• Subject: [netcdfgroup] Interpretation of valid_min/max/range attributes
• From: "Bentley, Philip" <philip.bentley@xxxxxxxxxxxxxxxx>
• Date: Wed, 15 Apr 2015 14:55:40 +0000

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/

• Follow-Ups:
  • Re: [netcdfgroup] Interpretation of valid_min/max/range attributes
    • From: Russ Rew