Re: UTM projected data

Hi Robert:


On 2/3/2014 8:26 PM, Schmunk, Robert B. (GISS-611.0)[TRINNOVIM, LLC] wrote:
> 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.

Are you seeing an error message? presumably the UTM projection is
failing, but not sure why. can you send me an example file?

look at the code in

  ucar.nc2.dataset.transform.UTM

to see how we handle the attributes. Maybe my docs need updating?


> 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.

i think you are right

> 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?

yes, thats my understanding

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?

probably predated my realizing zone number wass sufficient, not sure.

> Thanks for any help/guidance,
>
> rbs
>
>
>
>
>
> -- Robert B. Schmunk Webmaster / Senior Systems Programmer NASA
> Goddard Institute for Space Studies 2880 Broadway, New York, NY
> 10025