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UTM projected data

John,

I have a Panoply user at USGS who is working with data on a UTM grid. Although 
this projected grid is not in CF, I noticed that you do indicate it's 
availablity in netCDF-Java with some example CDL on the page at
https://www.unidata.ucar.edu/software/thredds/current/netcdf-java/reference/StandardCoordinateTransforms.html

However, I am having trouble actually implementing the UTM grid handler in NJ.

If I have a variable which uses a grid_mapping and the mapping variable has 
attribute grid_mapping_name = "universal_transverse_mercator" per the following

float tmin(time=1, y=547, x=592);
  :long_name = "minimum temperature";
  :units = "degrees_Celsius";
  :_FillValue = 1.0E20f; // float
  :grid_mapping = "UTM_Projection";

char UTM_Projection;
  :grid_mapping_name = "universal_transverse_mercator";
  :utm_zone_number = 10;
  :semi_major_axis = 6378137.0f;
  :inverse_flattening = 298.257f;

then when I query the tmin VariableDS for its CoordinateSystem and call 
getProjection on the coordinate system, it returns a null rather than, as I 
would have hoped, a ucar.unidata.geoloc.projection.UtmProjection.

Okay, I put that aside and opted to directly construct a UtmProjection object. 
In this case, I ran into the problem that the example CDL on the 
StandardCoordinateTransforms.html page mentioned above uses a semi-major axis 
with units of km. However, I found that I had to feed UtmProjection a value 
with units of meters in order for it to work. This seems borne out by the 
default semi-major axis found in EarthEllipsoid being in meters, as well as 
Appendix F of the CF convention stating that a semi_major_axis attribute should 
have units of meters.

The user also asked, and I too am not sure of, how to handle the UTM zone 
number. The sample CDL on the StandardCoordinateTransforms.html page shows an 
attribute that is an integer value. Does this mean that a zone in the southern 
hemisphere should be indicated by a negative number? If not, then how should it 
be indicated? If a negative number is the way to indicate the southern 
hemisphere, then why does the UtmProjection constructor require a boolean 
isNorth?

Thanks for any help/guidance,

rbs


 


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