Re: [ferret_users] reading BIG ascii files

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Leticia,
Ferret can read HDF files directly, if they are on an OPeNDAP server. If 
that's available for the data you're using, you can bypass all of this 
and just point to the data using its OPeNDAP address. You would need to 
use the /ORDER= qualifier to reverse the Y axis as I show at the end of 
this but otherwise you'd be all set.

To get the data from the axcii, file, here is an outline of what to do. 
When you know that the grid is regularly spaced, then it is far easier 
to just define the axes in Ferret and then read only the data field when 
reading the file. Ferret requires that latitudes run from south to 
north, but we can deal with that in a second step. First define the X 
axis and a Y axis which is the reverse of the Y axis you will want.

  yes? define axis/x=-179.917:179.917/npoints=2160/units=degrees_east 
lon_ax
  yes? define axis/y=-89.917:89.917/npoints=1080/units=degrees_north 
rev_lat_ax

  yes? define grid/x=lon_ax/y=rev_lat_x  xygrid
  yes? file/var="dum1, dum2, npp"/skip=1/grid=xygrid  my_file.dat

  yes? shade npp   ! the variable will be reversed in y with the south 
pole at the top.

If there is a missing-data  flag for the variable, set that after the 
FILE command, with the SET VARIABLE/BAD= command.

Now, Ferret can reverse coordinate axes if the data is coming in from a 
netCDF file (or an HDF file on an OPeNDAP server). If we save the data 
as netCDF, then we can visualize it correctly:

  yes? save/file=nppdata.nc npp
  yes? exit

Then to read and access the data with the Y axis reversed

> ferret
  yes? set data/ORDER=x-yt  nppdata.nc
  yes? show grid npp
  yes? shade npp


Leticia Cotrim da Cunha wrote:
> Hello,
>
> I tried to open big ASCII files organised in columns like this:
>
> lon               lat           NPP
> -179.917   89.917   -9999.
> -179.750   89.917   -9999.
> -179.583   89.917   -9999.
> etc
>
> The file has 1542035 lines.
> The format of the ASCII data is F10.3
> I wanted to use columns 1 and 2 (lon and lat) to define a grid for NPP.
> Longitude (col1) goes from -179.917 to 179.917 at an interval of 0.167 
> (2160 points).
> Latitude (col2) goes from 89.917 to -89.917 at an interval of 0.167 
> (1080 points).
> The files were extracted from 2160x1080 *.hdf files using SEADAS.
>
> I tried to follow the instructions at Ferret documentation, but it did 
> not work (are the files too big?) or I did not understand it (most 
> probable).
> Any hints? Thanks a lot for your help!
> Best regards
> Leticia

--

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Ansley Manke, NOAA/PMEL   ansley.b.manke@xxxxxxxx
7600 Sand Point Way NE, Seattle WA
Phone 206-526-6246,  FAX 206-526-6744