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Re: radius and cutoff values in scat2gridgauss_xy command



Hello Ansley and Billy,

Many thanks for the new function SCATGRID_NOBS_XY.

I tried this function in ferret v5.8 and it works fine.

Regards,
Yogesh

Manke wrote:
Hi all,
Billy suggested a set of functions which return the number of scattered
points that will fall in each grid cell of the output grid, if they are interpolated
onto that grid with one of the SCAT2GRID functions. I've gone ahead
and put together this set of functions. They'll be relased with future Ferret
releases, but they're available as external functions which you can download now.

A tar file containing these functions is linked to the "objective analysis" demo
web page, at
http://www.ferret.noaa.gov/Ferret/Demos/objective_analysis_demo/objective_analysis_demo.html#gridding_nobs_functions

The functions are SCATGRID_NOBS_XY, SCATGRID_NOBS_YZ,
etc. Briefly, they are called as follows:

yes? LET a = SCATGRID_NOBS_XY(xpts, ypts, xout, yout)

where xpts and ypts are the coordinates of the scattered points to be gridded, and
xout and yout are variables which have the X and Y axes of the output grid.
The result is on the xout, yout grid and at each x,y contains a count of the
scattered points in that output grid cell. This count is independent of the gridding
function that may be called in a separate step (Gaussian or Laplace method); it
relates the set of scattered locations to the grid we will interpolate them onto.

Ansley

Hello Yogesh -

First, I'm not exactly sure what you mean by "radius" in your
example, since there is no such argument to the SCAT2GRIDGAUSS*
functions.

But I assume that you mean the arguments XSCALE and YSCALE,
which control the width of the Gaussian function used to do
the mapping.

However, there is no direct way to implement your rule:
SCAT2GRIDGAUSS will fill any gridbox that has the weight
function

W_n(x_n,y_n) = exp{-[(x_n-x_0)/XSCALE]^2 - [(y_n-y_0)/YSCALE]^2}

greater than exp{-CUTOFF^2}. (See note re CUTOFF at bottom).

In general, this will "spread" the observations over gridpoints
that had no obs to begin with.

We are implementing an additional external function that will
count the number of obs/gridbox. If you need to get your answer
today, then you could think about how to make a mask for your
grid based on the number of obs in each gridbox, and use that
to blank out gridpoints you don't want.

Note re CUTOFF: CUTOFF should be a single number, not separate
numbers for the x and y axes. This was a mistake when the fortran
code was implemented as an external function. It will be fixed in
a future release. IN the meantime, please use the same value for
XCUTOFF and YCUTOFF. A good value is 2, which means that data points
far enough away from the gridpoints so that the value of the weight function is less than exp{-4} = 0.018 will not be used in the calculation
for that gridpoint. This will exclude data values more than twice the
scale distances from each gridpoint.
Billy K



Date: Mon, 07 Feb 2005 12:44:10 +0100
From: "Yogesh K. Tiwari" <ktiwari@bgc-jena.mpg.de>
To: ferret_users <oar.pmel.ferret_users@noaa.gov>
Subject: radius and cutoff values in scat2gridgauss_xy command

Hello Ferret Users,

I could not understand exactly the meaning of cutoff
and radius values in the scat2gridgauss* ferret command.

I want to make a rule while griding my scattered data as,

I required that there are at least 10 observations
in a grid box 1x1 while using the scat2gridgauss_xy command.

Whether my following radius and cutoff values are right for my
above rule (10 obs in a grid).


define symbol radius = 0.25
define symbol cutoff = 2.0

Many many thanks in advance for your help.

Regards,
Yogesh

--

===========================================
Yogesh K. Tiwari,
Max-Planck Institute for Biogeochemistry,
Beutenberg Campus, Hans-Knoell-Strasse 10,
D-07745 Jena,
Germany

Office : 0049 3641 576376
Home : 0049 3641 223163
Fax : 0049 3641 577300
Handy : 0049 173 698 8789
e-mail : yogesh.tiwari@bgc-jena.mpg.de
===========================================





--

===========================================
Yogesh K. Tiwari,
Max-Planck Institute for Biogeochemistry,
Beutenberg Campus, Hans-Knoell-Strasse 10,
D-07745 Jena,
Germany

Office   : 0049 3641 576 376
Home     : 0049 3641 223 163
Fax      : 0049 3641 577 300
Cell     : 0049 1520 4591 008
e-mail   : yogesh.tiwari@bgc-jena.mpg.de
===========================================



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