Re: [ferret_users] ferret failure to plot Netcdf file (precision problem ?)

[Jacques Stum <jacques.stum@xxxxxx>]

Ansley Manke a écrit :
> Hi Jacques,
> Thanks for including details on the contents of the file. That makes 
> it possible to see what's happening.
>
> The trouble with the coordinates in this data set is not the value 
> 9.20000000022014e-05 but the fact that the numbers jump from 359.xxx 
> to 0.xxx.  We humans know that this is the modulo branch cut but that 
> information isn't deduced from the coordinate values. The coordinate 
> values themselves must be strictly increasing or decreasing.  If for 
> instance the coordinates for the grid run from 180 up to 359.96, 0.0, 
> 0.04, 0.08, and on up to 179.96 then you'd get correct results if you 
> changed them to run from -180, ..., -0.04, 0.0, 0.4, ... to 179.96.
>
> (Here is the data standard that Ferret uses for netCDF files, just for 
> reference:
> http://ferret.pmel.noaa.gov/Ferret/documentation/coards-netcdf-conventions)
>
> Ferret can probably fix this file without having to remake the netCDF 
> file. To fix this, you can redefine the axis.  This is just an example 
> assuming the coordinates run as I said above, you can adjust this to 
> fit what the axis actually does, making the definition of the variable 
> new_longitude into a monotonic variable that is the equivalent of the 
> coordinates in the file.
>
>     yes? use TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.nc
>
>     yes? ! Define a variable with the same longitudes, but monotonic
>     yes? let xx = longitude
>     yes? let new_longitude = if xx GT 180 then (xx-180) else xx
>
>     yes? ! check what we have done
>     yes? list/i=1116:1129 new_longitude
>     yes? plot new_longitude
>
>     yes? ! Redefine the LONGITUDE axis
>     yes? define axis/modulo/units=degrees_east longitude = new_longitude
>
>     yes? !now you should see the plot on the longitude-latitude grid
>     yes? shade TEMPERATURE_DE_SURFACE_DE_LA_MER
>
>
> Jacques Stum wrote:
> > Hello Ferret users,
> >
> > Ferret failed to plot my NetCdf file as follows :
> >
> > $ferret
> >        NOAA/PMEL TMAP
> >        FERRET v6
> >        Linux(g77) 2.4.21-32 - 08/23/06
> >        29-Nov-07 14:14
> >
> > yes? set memory/size=128
> > Cached data cleared from memory
> > yes? use TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.nc
> > *** NOTE: Coordinates out of order or missing on axis Longitude at 
> > subscript 1126
> > *** NOTE: A dummy axis of subscripts will be used
> > yes? show data
> >     currently SET data sets:
> >    1> ./TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.nc  
> > (default)
> > name     title                             I         J         
> > K         L
> > LONGITUDE
> >          Longitudes                       1:4500    ...       
> > ...       ...
> >       (invalid coordinate axis)
> > TEMPERATURE_DE_SURFACE_DE_LA_MER
> >          sea surface temperature          ...       1:3500    
> > 1:4500    ...
> >
> > yes? shade TEMPERATURE_DE_SURFACE_DE_LA_MER
> > yes? quit
> >
> > The result is : Ferret outputs a 2-D plot, but with x = Latitude, and 
> > y = a number between 1 and 4500 (expected : a 2-D plot with x = 
> > Longitude, and y = Latitude !)
> >
> > Hereafter is the ncdump of my file : I just show you a subset of the 
> > LONGITUDE variable, between subscripts 1116 and 1129. It seems that 
> > Ferret does'nt interpret correctly subscript 1126 (Longitudes(1126) = 
> > 9.20000000022014e-05)
> >
> > netcdf TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.cfs {
> > dimensions:
> >    Latitude = 3500 ;
> >    Longitude = 4500 ;
> >    double Latitude(Latitude) ;
> >        Latitude:_FillValue = 1.84467440737096e+19 ;
> >        Latitude:long_name = "Latitudes" ;
> >        Latitude:units = "degrees_north" ;
> >        Latitude:standard_name = "latitude" ;
> >    double Longitude(Longitude) ;
> >        Longitude:_FillValue = 1.84467440737096e+19 ;
> >        Longitude:long_name = "Longitudes" ;
> >        Longitude:units = "degrees_east" ;
> >        Longitude:standard_name = "longitude" ;
> >    short Temperature_de_surface_de_la_mer(Longitude, Latitude) ;
> >        Temperature_de_surface_de_la_mer:_FillValue = 32767s ;
> >        Temperature_de_surface_de_la_mer:long_name = "sea surface 
> > temperature" ;
> >        Temperature_de_surface_de_la_mer:units = "degC" ;
> >        Temperature_de_surface_de_la_mer:scale_factor = 0.01 ;
> >        Temperature_de_surface_de_la_mer:add_offset = 0. ;
> >        Temperature_de_surface_de_la_mer:standard_name = 
> > "sea_surface_temperature" ;
> >        Temperature_de_surface_de_la_mer:valid_min = -162s ;
> >        Temperature_de_surface_de_la_mer:valid_max = 2793s ;
> >
> > Longitude(1116:1129) =   359.600092, 359.640092, 359.680092, 
> > 359.720092, 359.760092, 359.800092,
> > 359.840092, 359.880092, 359.920092, 359.960092, 9.20000000022014e-05,
> > 0.0400920000000022, 0.0800920000000022, 0.120092000000002
> >
> >
> > Is it a known unavoidable Ferret bug, or is there a means to avoid 
> > it, maybe by telling to Ferret something more on the input data 
> > precision (double ?)
> >
> > Thank you for your help,
> >
> > Jacques
>
>
> Cliquez ici 
> <https://www.mailcontrol.com/sr/3ZuPWjxzrFcBMC5TJeZ2dUfK8eMmGgcuf%2166NrBUsCcQG64UImLu5Ga3C10sUHexRuShiRIC6jRCCzZIChncDek80fUprGx%21E1f8ihh8CqRwR+z8XT0XFkR7EyZQW79JTFD7zlQ6sVB3gA8I8mUAYv7oe5OBIW4Elz63Uk5fl84nirg9zl+Fiyo5ffRBzBGOScxUbMTLFPbhZgB%21tlhEhRqRVZ%21%21rMKK> 
> si ce message est indésirable (pourriel).
Ansley, it does'nt work properly yet, but it is in good progress :

yes? let xx=longitude
yes? list/i=1116:1129 xx
            VARIABLE : LONGITUDE
            DATA SET : SST collated file: avhrr17_g 20070917
            FILENAME : 
TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.cfs.nc
            SUBSET   : 14 points (X)
1116   / 1116:  359.6
1117   / 1117:  359.6
1118   / 1118:  359.7
1119   / 1119:  359.7
1120   / 1120:  359.8
1121   / 1121:  359.8
1122   / 1122:  359.8
1123   / 1123:  359.9
1124   / 1124:  359.9
1125   / 1125:  360.0
1126   / 1126:    0.0
1127   / 1127:    0.0
1128   / 1128:    0.1
1129   / 1129:    0.1
yes? let new_longitude = if xx GT 180 then (xx-360) else xx
yes? list/i=1116:1129 new_longitude
            VARIABLE : IF XX GT 180 THEN (XX-360) ELSE XX
            DATA SET : SST collated file: avhrr17_g 20070917
            FILENAME : 
TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.cfs.nc
            SUBSET   : 14 points (X)
1116   / 1116: -0.3999
1117   / 1117: -0.3599
1118   / 1118: -0.3199
1119   / 1119: -0.2799
1120   / 1120: -0.2399
1121   / 1121: -0.1999
1122   / 1122: -0.1599
1123   / 1123: -0.1199
1124   / 1124: -0.0799
1125   / 1125: -0.0399
1126   / 1126:  0.0001
1127   / 1127:  0.0401
1128   / 1128:  0.0801
1129   / 1129:  0.1201
yes? define axis/modulo/units=degrees_east longitude=new_longitude
*** NOTE: Orientation not specified via /X,/Y,/Z, or /T. X assumed.
Replacing definition of axis LONGITUDE
*** NOTE: grid  used by data set 
TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.cfs
*** NOTE: Redefinition may alter apparent contents of data set
*** NOTE: grid  used by data set 
TsurfAv17jL2pnv_20070917_103456_0079_-060_-044_0135.cfs
*** NOTE: Redefinition may alter apparent contents of data set
yes? shade TEMPERATURE_DE_SURFACE_DE_LA_MER : plots in x axis the 
latitude and in y axis a variable named "z (degrees_east)" which values 
are longitude values

How may I tell to ferret to put the longitude in x and latitude in y ?


--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
JACQUES STUM                                  	     ~~	  E-mail	 :  jstum@xxxxxx	
Collecte Localisation Satellite,
8-10 rue Hermès,                                 	     ~~	  Phone   :  33 5 61 39 48 12
Parc Technologique du Canal,
31526 Ramonville Saint-Agne, France	     ~~    	  Fax	 :  33 5 61 39 37 82
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