[ncl-talk] local_max and missing values

[Dennis Shea]

The very simple algorithm used does not handle boundary cases.

My suggestion:

Use the function on the global grid, then set all location N/N of 30 to
_FillValue.

HTH

On Fri, Jun 5, 2015 at 2:33 AM, Teresa Parker <tess.parker at monash.edu>
wrote:

> Hi - I'm using the local_max function to find local maxima in a variable
> field. The variable covers all longitudes, but is calculated for a 30 deg
> latitude band only. All values outside this latitude band are set to the
> @_FillValue for the variable.
>
> Where the local maximum for the variable is located well within the
> latitude band, there is no problem However, when the local maximum is
> located on the edge of the latitude band (i.e. at either the minimum or
> maximum latitude of the band), the function fails. This seems to be because
> the immediately neighbouring points are all "missing" values. However, the
> function documentation states that missing values are ignored.
>
> This can be seen from a simple example:
> *************************************************
> begin
>
> myvar = new((/3,3/),float)
> myvar at _FillValue = -999.
> myvar = (/ (/-999.0,-999.0,-999.0/),(/60,70,55/),(/43,45,23/)/)
> print(myvar)
> mymax = local_max(myvar, False, 0.0)
> print(mymax)
>
> mymaxval = mymax at maxval
> print(mymaxval)
>
> end
>
> However, if the missing values are set to e.g. zero, the function works:
> *********************************************************************
> begin
>
> myvar = new((/3,3/),float)
> myvar at _FillValue = -999.
> myvar = (/ (/0.0,0.0,0.0/),(/60,70,55/),(/43,45,23/)/)
> print(myvar)
> mymax = local_max(myvar, False, 0.0)
> print(mymax)
>
> mymaxval = mymax at maxvalmymaxval = mymax at maxval
> print(mymaxval)
>
> end
>
> While it might seem to be an easy workaround to set the next highest and
> lowest latitude band values to some miniscule value, this shouldn't really
> be necessary for the function to work. Can anyone help?
>
> --
> Tess Parker
> PhD Candidate
> School of Earth, Atmosphere and Environment
> Room 225, Building 28
> 9 Rainforest Walk
> Monash University, Clayton VIC 3800
> Phone: +61 3 9905 4458
> http://users.monash.edu.au/~tess/index.html
>
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