This archive contains answers to questions sent to Unidata support through mid-2025. Note that the archive is no longer being updated. We provide the archive for reference; many of the answers presented here remain technically correct, even if somewhat outdated. For the most up-to-date information on the use of NSF Unidata software and data services, please consult the Software Documentation first.
>From: Owen Cooper <address@hidden> >Organization: Aeronomy Laboratory/NOAA >Keywords: 200306251740.h5PHeMLd017765 McIDAS-X GRDCOPY Owen, >My RTGRIDS are coming from ADDE.UCAR.EDU OK, thought so. I found the problem. What is happening is that the selection code is comparing the grid number (McIDAS refers to this as the geographic ID) between the two grids, and sees that they are different. Given this, it is exiting since it doesn't recognize the grids to be the same. The following demonstrates what I mean: 1) I GRDCOPYed the U and V AVN grids for TIME=0 FHOUR=6 DAY=177 GPRO=MERC LEV=300 to a GRID file that was empty otherwise. The ADDE server used was ADDE.UCAR.EDU.Here is a long linsting of those grids: GRDLIST MYDATA/GRIDS.1 NUM=ALL FORM=ALL Dataset position 1 Directory Title= PAR LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ---- --------- ------------ -------- ---- ----- ------------ -------- ----- ---- U 300 MB 26 JUN 03177 12:00:00 AVN 6 26 JUN 03177 18:00:00 1 MERC Total pts= 41905 Num rows= 145 Num columns= 289 received: 2003177 155813Z u-component of wind GRIB ID numbers: Geographic = 43; PAR = 33; Model ID = 96; Level type =100 Units of gridded variable are MPS Scale of variable is: 2 Mercator Projection Min Lat= -90.00 Max Lat= 90.00 Min Lon=-180.00 Max Lon= 180.00 Latitude Increment= 1.2500 Longitude Increment= 1.2500 PAR LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ---- --------- ------------ -------- ---- ----- ------------ -------- ----- ---- V 300 MB 26 JUN 03177 12:00:00 AVN 6 26 JUN 03177 18:00:00 2 MERC Total pts= 41905 Num rows= 145 Num columns= 289 received: 2003177 155825Z v-component of wind GRIB ID numbers: Geographic = 44; PAR = 34; Model ID = 96; Level type =100 Units of gridded variable are MPS Scale of variable is: 2 Mercator Projection Min Lat= -90.00 Max Lat= 90.00 Min Lon=-180.00 Max Lon= 180.00 Latitude Increment= 1.2500 Longitude Increment= 1.2500 Number of grids listed = 2 GRDLIST - done 2) next, I pointed at the SSEC RTGRIDS server and did the exact same GRDCOPY into a different GRID file. Here is what those grids look like: GRDLIST MYDATA/GRIDS.2 NUM=2 FORM=ALL Dataset position 2 Directory Title= PAR LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ---- --------- ------------ -------- ---- ----- ------------ -------- ----- ---- U 300 MB 26 JUN 03177 12:00:00 AVN 6 26 JUN 03177 18:00:00 1 MERC Total pts= 41905 Num rows= 145 Num columns= 289 received: 2003177 155830Z u-component of wind GRIB ID numbers: Geographic =3744; PAR = 33; Model ID = 96; Level type =100 Units of gridded variable are MPS Scale of variable is: 2 Mercator Projection Min Lat= -90.00 Max Lat= 90.00 Min Lon=-180.00 Max Lon= 180.00 Latitude Increment= 1.2500 Longitude Increment= 1.2500 PAR LEVEL DAY TIME SRC FHOUR FDAY FTIME GRID PRO ---- --------- ------------ -------- ---- ----- ------------ -------- ----- ---- V 300 MB 26 JUN 03177 12:00:00 AVN 6 26 JUN 03177 18:00:00 2 MERC Total pts= 41905 Num rows= 145 Num columns= 289 received: 2003177 155852Z v-component of wind GRIB ID numbers: Geographic =3744; PAR = 34; Model ID = 96; Level type =100 Units of gridded variable are MPS Scale of variable is: 2 Mercator Projection Min Lat= -90.00 Max Lat= 90.00 Min Lon=-180.00 Max Lon= 180.00 Latitude Increment= 1.2500 Longitude Increment= 1.2500 Number of grids listed = 2 GRDLIST - done ***** Notice ***** - the 'Geographic =' for the grids copied from adde.ucar.edu are '43' for the first grid, and '44' for the second grid - the 'Geographic =' for the grids copied from the SSEC server are both '3744' Here is what is going on. The AVN grids are pieced together from 8 octet grids each of which as a separate geographic ID: 37, 38, ..., 44. The geographic ID that gets assigned to the stitched together grid that the v2002 XCD grib decoder creates is either the first or last (I don't remember which) octet grid that was stitched. Since the order that the octet grids are received in NOAAPORT is not fixed, one can not be sure of which geographic ID will be assigned to the stitched together composite grid. This accounts for the DERIVE=ABV working sometimes and not others! SSEC made a change in the v2003 version of XCD to set the geographic ID to a fixed number '3744' (representing any/all of 37, ..., 44). The selection code in McIDAS goes to great pains to make sure that the grids being operated on are exactly the same. It is exiting out the ABV calculation as soon as it sees that the geographic ID for the two grids does not match. I verified that if I change the geographic ID in the first grid copied from adde.ucar.edu (U, which was '43') to match that in the second grid copied from adde.ucar.edu (V, which is '44') then the DERIVE=ABV function works as it designed to. I see no _easy_ way to get around this problem from the user side. You could copy over the grids you want to use from the server one at a time and change the value in the grid so that hte selection will match, but that is kinda ugly. Even though I was not quite ready to load the v2003 version of XCD on adde.ucar.edu, I may well do so to help you get around this problem. More later... Tom