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20021212: apparent navigation error when display nex2gini radars in McIDAS

>From: Robert Mullenax <address@hidden>
>Organization: Universal Weather
>Keywords: 200212122157.gBCLvr401500 McIDAS NEXRCOMP


>I am running the nex2gini script every 10 minutes to generate U.S. base
>composites, and have been using them in GARP/NMAP2.  Everything
>seems to work fine.  I have occasionally looked at them in McIDAS 
>and they looked okay as well.  However, I am thinking of making
>a sort of radar cgi type thing out of them using McIDAS and
>noted today that the navigation of the composite
>seems to be off by approximately 45nm.  The data is correctly displayed
>in GEMPAK, but all the echoes are shifted ~45nm to the north of their
>actual location in McIDAS.

I just looked at the RTNEXRAD/N0R N0R product for ID=TWB and the
NEXRCOMP/1KN0R-NAT product loaded centered on TWB and the
correspondence look pretty good:


NEXRCOMP/1KN0R-NAT loaded with TWB at the center

The feature whose location I compared was the upper right most 55 dBZ
echo to the ENE of Tampa.

I then IMGCOPYied the 00:22 NEXRCOMP/1KN0R-NAT product centered on TBW,
made a copy of it, and remapped the 00:20 N0R product for TBW onto the copy.
The results of this operation can be seen in the following two GIFs

IMGCOPYed NEXRCOMP/1KN0R-NAT with TWB at the center:

Remapped NEXRAD Level III N0R for TWB:

The location of the echos looks pretty consistent to me!?  I animated
the two displays in McIDAS and see what looks to be at most a 1 pixel
shift between the two displays.  A 1 pixel shift is more-or-less 1 km,
so my quickie test does not replicate your 45 nm offset (which would
be 83.340 km).

How does your base reflectivity composite compare to ours?  You can
access ours on papagayo:

DATALOC ADD NEXRCOMP papagayo.unl.edu

>I have attached two sample images from
>the same time, each app looking at the same file.  You will see that
>McIDAS shows precip covering Mobile Bay for example while
>GEMPAK does not.  GEMPAK is correct.   I create the grid on
>an RH Linux 7.2 box and have tried the McIDAS display on the same
>box (McIDAS 2002a) and on a McIDAS 7.704 Sun SPARC with
>the same result. <<gem.gif>>  <<MCIDAS.GIF>> 
>Here is the .nts file I use for the nex2gini:
>GRDAREA 21.8;-122;48.2;-62.5
>PROJ    lcc/40;-100;40
>KXKY    4750;3020
>GFUNC   n0r
>RADTIM  current
>STNFIL  rad.tbl
>COMPRESS no                                

I will see if Chiz has any comments about these GEMPAK settings.


>From address@hidden Thu Dec 12 22:12:04 2002

I don't why it didn't occur to me to look at your NEXRCOMP
stuff.  Anyway, I verified tonight (at home) that while
your NEXRCOMP imagery is correct, ours is  shifted 
to the north again..roughly 45nm in McIDAS, but it does display correctly
in GARP.  I looked at the stuff over Florida.

I did change the garea and kxky Steve had in his nex2gini example
as I wanted to get more of the eastern edge of the Maine radars
and a little further south and west.

Tommorrow I will set my settings back to his suggested ones
as a test.

Thanks for the quick reply.


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