Re: GRADS binary -> Ferret

[hankin@xxxxxxxxxxxxxxxxxxx (Steve Hankin)]

On Feb 12,  7:37pm, Ilana Wainer wrote:
> Subject: GRADS binary -> Ferret
>
>
> Question: how to use a GRADS binary file with Ferret. In other words
> what would the " FILE/FORMAT " comand look like ?
>
> thanks
>
> -Ilana
===================================================

Hi Illana,

The GrADS binary files are typically (always?) grids of single precision
floating point values. Ferret can read 'em.

It is generally best to use Ferret to read the data and then SAVE the results
into a NetCDF file (with titles and units etc.). The reason for this is that
Ferret reads NetCDF much faster than unformatted binary and it cannot do direct
access or memory management on the binary data file. In Ferret we opted for
flexibility in ASCII and binary files at some cost of performance. Anyway, the
NetCDF files are standardized, machine independent, and self-describing ... all
that good stuff.

To read a variable from the binary file follow these steps:

1) Use DEFINE AXIS and DEFINE GRID to describe the coordinate system of the
variable

2) Use the FILE command with
	/GRID=grid_from_step_1
	/FORMAT=stream
	/SKIP=#_of_words_to_skip_at_file_start
	/VAR=varname

3) (optional) Use
	SET VARIABLE/TITLE="My variable"/UNITS="those units" varname
   to describe the variable

4) Use
	SAVE/FILE=myfile.nc/TITLE="data set title" varname

5) for multiple variables repeat 2-4 and add "/APPEND" to the SAVE command in 4

Note: it is actually possible to read multiple variables simultaneously in step
2 by specifying /VAR="var1,var2,..." and
/COLUMNS=#_of_words_between_variables_in_file ... for simplicity we'll stick
with one variable at a time in this message)


Below is a more concrete example.

	Cheers - steve

=================================================================

Suppose your file, GrADS.dat, was organized on a 2x2 degree global grid like
this:

          Time 1, Level    ?, Variable slp  (sea level pressure)
          Time 1, Level 1000, Variable td
          Time 1, Level  850, Variable td
          Time 1, Level  700, Variable td
          Time 1, Level  500, Variable td
          Time 1, Level  400, Variable td
          Time 1, Level  300, Variable td
          Time 1, Level 1000, Variable u
          Time 1, Level  850, Variable u
                 then levels 700, 500, 400, 300, 250, 200, then
          Time 1, Level  150, Variable u
          Time 1, Level  100, Variable u
          Time 1, Level 1000, Variable v
          Time 1, Level  850, Variable v
                 then levels 700, 500, 400, 300, 250, 200, then
          Time 1, Level  150, Variable v
          Time 1, Level  100, Variable v

	 ditto all that for Time 2, ...

For this example we will read the 2D field of U at the first level at Time 1.

Step 1: define the grid for U

	DEFINE AXIS/X=21E:19E:2/units=degrees xax	! 180 points
	DEFINE AXIS/Y=89S:89N:2/unit=degrees  yax	! 90 points
	DEFINE AXIS/Z=1000:1000:1/units=mbars zax
	DEFINE AXIS/t=1-JAN-1986:1-JAN-1986:1/unit=days tax  ! or whatever ...

	DEFINE GRID/X=xax/Y=yax/Z=zax/T=tax ugrid2d

Step 2: describe the variable in the file

	FILE/VAR=u/SKIP=`180*90*7`/GRID=ugrid2d/FORMAT=stream   GrADS.dat

Step 3: more information about variable "u"

	SET VARIABLE/TITLE="Zonal Velocity"/UNITS="cm/sec" u

Step 4: Save the data to a NetCDF file

	SAVE/FILE=GrADS.cdf/TITLE="My NetCDF data set" u

To keep this simple I've avoided the steps to handle the decreasing coordinates
on the Z axis of U. Write me if you want that.


-- 

		|  NOAA/PMEL               |  ph. (206) 526-6080  
Steve Hankin	|  7600 Sand Point Way NE  |  FAX (206) 526-6744
		|  Seattle, WA 98115-0070  |  hankin@pmel.noaa.gov