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[GEMPAK #ESY-440130]: Problem with rotated lat/lon



Nilsson,

GDS octed 28 contains the scanning mode flag, which defines whether the grid 
points
are north to south or south to north in the "j" direction. The GRIB convention 
is that 
bits are numbered from most significant being position "1", and least 
significant being 
position "8". The bit "2" should be the second bit from the left, eg the "64" 
value. Note that
bits 4-8 are reserved, and should likely be zero.

In your nagrib output below, I see:
 GDS byte 28 (mode) = 2

That value of 2 seems to be reversed in bit position, and I would expect it to 
be "64" for 
south to north grid points. Since the position "7" bit is reserved, it should 
be "0", so you should
not have a value of 2 (unless your conversion program is using reserved 
bits...and
the decoder wouldn't know about them anyway).

See if you can correct this bit ordering.

Steve Chiswell
Unidata User Support





> Hi,
> 
> I'm having problem with gdplot after 'unrotating' a rotated lat/lon
> grib.
> 
> Since Gempak doesn't handle rotated lat/lon gribs I followed these
> steps:
> 
> (Original rotated grib file: swan.grb)
> 
> swan.grb -> unrotate with help of XCONV  output file is (in ntecdf form)
> : swan-wvhgt.nc
> 
> swan-wvhgt.nc -> transformed back to grib with help of lats4d. Result is
> swan-wvhgt.grb (If I plot this, using xconv, it looks OK)
> 
> swan-wvhgt.grb -> transformed to gempak format, swan-wvhgt.grd, with
> nagrib. (Se listing below)
> 
> When I do a gdplot2 on swan-wvhgt.grd I get a 'correct' plot. except for
> one thing: It's upside down, ie mirrowed in north/south . I don't
> understand why. Can you pls give me a hint.
> 
> I attach the the grib file <<swan-wvhgt.grb>>  that I used for input to
> nagrib.
> 
> /Regards Bengt
> 
> The gdplot parameters are as follows
> 
> GDFILE    Grid file
> /home/bengt/proj/swan-wvhgt.grd
> GDATTIM   Grid date/time                    061221/0600F000
> GLEVEL    Grid level                        0
> GVCORD    Grid vertical coordinate          none
> PANEL     Panel loc/color/dash/width/regn   0
> SKIP      Skip_cntr/skip_plt_x;skip_plt_y   0
> SCALE     Scalar scale / vector scale       0
> GFUNC     Scalar grid                       HGHTSW
> CTYPE     Contour type:  C/F                C
> CONTUR    Subbox/smooth                     2/0
> CINT      Contour interval/min/max          0
> LINE      Color/type/width/label/smth/fltr  8/1/2
> FINT      Fill interval/min/max             0
> FLINE     Fill colors/fill types            31-18--1
> HILO      Color/symbol/rng/rad/cnt/intp
> HLSYM     HILO txt size/posn/font/wdth/hw
> CLRBAR    Color/ornt/anch/x;y/ln;wd/freq|t  1
> GVECT     Vector grid                       WND
> WIND      Wind symbol/siz/wdth/typ/hdsz     bm6
> REFVEC    Mag;x;y;txtsiz/font/wdth/HW;labl
> TITLE     Title color/line/title            0
> TEXT      Size/fnt/wdth/brdr/N-rot/just/hw  1.3/23//hw
> CLEAR     Clear screen flag                 y
> GAREA     Graphics area                     51.8;-10.0;64.5;37.3
> IJSKIP    Iskp;Istrt;Istp/Jskp;Jstrt;Jstp
> PROJ      Map projection/angles/margins|dr  MER/0;0;0/nm
> MAP       Map color/dash/width/filter flag  0
> MSCALE    fgc;bgc;mask/units/lat;hide/valu  0
> LATLON    Line color/dash/width/freq/inc/l  0
> DEVICE    Device|name|x size;y size|color   gif|/tmp/wave.gif|1400;719
> STNPLT    Txtc/txt attr|marker attr|stnfil
> 
> 
> 
> 
> GEMPAK-NAGRIB>d
> GBFILE    GRIB data file name               swan-wvhgt.grb
> INDXFL    GRIB index file name
> GDOUTF    Output grid file                  list
> PROJ      Map projection/angles/margins|dr
> GRDAREA   Area covered by grid
> KXKY      Number of grid points in x;y      10;10
> MAXGRD    Maximum number of grids           200
> CPYFIL    Grid file whose navigation is to  gds
> GAREA     Graphics area                     51;-10;66;38
> OUTPUT    Output device/filename            T
> GBTBLS    Input GRIB decoding tables
> GBDIAG    GRIB diagnostic elements          all
> PDSEXT    Y or N, add PDS extension if fou  NO
> OVERWR    Overwr flag                       yes
> GEMPAK-NAGRIB>r
> IDS bytes 0 -  4 (GRIB msg id)    = GRIB
> IDS bytes 5 -  7 (ids.msg_length) = 29096
> IDS byte       8 (ids.edition)    = 1
> PDS bytes  1- 3 (pds.length)      = 28
> PDS byte      4 (pds.version)     = 128
> PDS byte      5 (pds.center)      = 82
> PDS byte      6 (pds.process)     = 130
> PDS byte      7 (pds.grid_id)     = 255
> PDS byte      8 (pds.flag)        = 192
> PDS byte      9 (pds.parameter)   = 105
> PDS byte     10 (pds.vcoord)      = 102
> PDS bytes    11 (pds.level_1)     = 0
> PDS bytes    12 (pds.level_2)     = 0
> PDS bytes 11-12 (pds.level)       = 0
> PDS byte     13 (pds.year)        = 6
> PDS byte     14 (pds.month)       = 12
> PDS byte     15 (pds.day)         = 21
> PDS byte     16 (pds.hour)        = 6
> PDS byte     17 (pds.minute)      = 0
> PDS byte     18 (pds.time_unit)   = 1
> PDS byte     19 (pds.time_p1)     = 0
> PDS byte     20 (pds.time_p2)     = 0
> PDS byte     21 (pds.time_range)  = 0
> PDS bytes 22-23 (pds.avg_num)     = 0
> PDS byte     24 (pds.avg_miss)    = 0
> PDS byte     25 (pds.century)     = 21
> PDS byte     26 (pds.izero)       = 0
> PDS bytes 27-28 (pds.dec_scale)   = 0
> PDS EXT FLAG (1-app,0-nc,-1-rep)  = 0
> PDS EXT STRING                    =
> Changing center table to cntrgrib1.tbl
> Changing vertical coord table to vcrdgrib1.tbl
> Changing WMO parameter table to wmogrib128.tbl
> Changing center parameter table to smhigrib128.tbl
> GDS bytes  1 -  3 (gds.length)    = 32
> GDS byte        4 (gds.NV)        = 0
> GDS byte        5 (gds.PV)        = 255
> GDS byte        6 (gds.grid_proj) = 0
> GDS bytes  7 -  8 (Nx)            = 189
> GDS bytes  9 - 10 (Ny)            = 199
> GDS bytes 11 - 13 (La1)           = 51800
> GDS bytes 14 - 16 (Lo1)           = -10000
> GDS byte       17 (flag)          = 128
> GDS bytes 18 - 20 (La2)           = 64575
> GDS bytes 21 - 23 (Lo2)           = 37733
> GDS bytes 24 - 25 (skipped)
> GDS bytes 26 - 27 (skipped)
> GDS byte       28 (mode)          = 2
> GDS bytes 29 - 32 (skipped)
> GDS bytes 33 - 35 (skipped)
> GDS bytes 36 - 38 (skipped)
> GDS byte       39 (skipped)
> GDS bytes 40 - 42 (skipped)
> GDS bytes 43 - 45 (skipped)
> GDS bytes 46 - 48 (skipped)
> GDS byte       49 (skipped)
> GDS bytes 50 - 52 (skipped)
> 
> MESG# NMCGRD# PRM#   VCD#    GEMPAK_TIME       LEVL1 LEVL2  VCRD PARM
> 1   255    105    102    061221/0600F000        0        NONE HGHTSW
> 
> 
> 
> 1 GRIB messages were read or scanned from the GRIB file:
> swan-wvhgt.grb
> 
> 
> Parameters requested:
> GBFILE,INDXFL,GDOUTF,PROJ,GRDAREA,KXKY,MAXGRD,CPYFIL,
> GAREA,OUTPUT,GBTBLS,GBDIAG,PDSEXT,OVERWR.
> 
> 


Ticket Details
===================
Ticket ID: ESY-440130
Department: Support GEMPAK
Priority: Normal
Status: Closed