[ncl-talk] lat,lon not accurately generated by NCL

[Guoqing Ge - NOAA Affiliate]

Hi Craig,  yes, it's this kind of difference.

I did not find any option to specify an alternate earth radius in NCL when
reading GRIB files. If someone knows, please kindly let me know.

Thanks,

Guoqing


--
Guoqing Ge, PhD
CIRES Research Scientist
at NOAA/ESRL/GSD (Rm 2B502)
325 Broadway, Boulder, CO 80305
(303)497-4163
&
Thursdays at
Developmental Testbed Center (FL3-1039)
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On Thu, Aug 2, 2018 at 12:32 PM, Craig Mattocks <mattocks at mac.com> wrote:

> Hi Guoqing,
>
> Are you talking about this sort of difference (using a Lambert CONUS 2.5
> km grid)?
>
> Location: Cape Hatteras, NC
> lon = -75.528824
> lat =  35.250538
>
> alias mm5="proj +proj=lcc +lat_1=25 +lat_2=25 +lat_0=25 +lon_0=-95 +a=6370
> +b=6370 +towgs84=0,0,0 +no_defs -f %.6f"
>
> mm5 << EOF
> -75.528824 35.250538
> EOF
>
> alias ndfd="proj +proj=lcc +lat_1=25 +lat_2=25 +lat_0=25 +lon_0=-95 +x_0=0
> +y_0=0 +a=6371.229 +b=6371.229 +no_defs -f %.6f"
>
> ndfd << EOF
> -75.528824 35.250538
> EOF
>
> Results (km):
> x = 1791.188308   MM5/WRF spheroid, r = 6370000 m
> y = 1274.741690
>
> x = 1791.533892   NDFD grib octet 3.15 = 6 spheroid, r = 6371229 m
> y = 1274.987633
>
> Difference = 0.424165 km or 424.165 m
>
> Craig
>
> P.S. I recently learned that you have to use cs2cs, instead of proj, when
> converting between different datums (not relevant in this case).
>
> > On Wed, 1 Aug 2018 13:57:55 -0600 Guoqing Ge - NOAA Affiliate <
> guoqing.ge at noaa.gov> wrote:
> >
> > We have a similar issue when using wgrib2 to read HRRR grib files. In
> this
> > situation, we have identified these inaccurate lat-lon values are due to
> > the use of different earth radius by WRF/geogrid.exe and wgrib2
> > respectively. Explicitly specify the geogrid-used earth radius (instead
> of
> > the default one) in the command line option, wgrib2 can reproduce the
> > original lat-lon values.  Based on similarity, we guess NCL should have
> the
> > same problem.
> >
> > Is there an option to specify earth radius in NCL when reading GRIB
> files?
> >
> > Thanks,
> >
> > Guoqing
>
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