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local_max

Determines the relative maxima for a 2-dimensional array.

Prototype

	function local_max (
		x [*][*] : float or double,  
		cyclic   : logical,          
		delta    : numeric           
	)

	return_val [1] :  integer

Arguments

x

A 2-dimensional float or double array. Missing data are allowed and must be indicated by x@_FillValue.

cyclic

Set to False if the data array is not cyclic in x.

Set to True if data are cyclic in the x-axis direction. In this case, x should not include the cyclic point.

delta

Tolerance level. If values are within delta of surrounding values it will not be counted as a local max value. Generally, delta=0.0

Return value

Returns the number of relative max values found (nmax). If nmax is greater than zero, then nmax will have three 1-dimensional attribute arrays associated with it:

  1. nmax@xi - this will contain the x-axis indices (e.g. longitudes)
  2. nmax@yi - this will contain the y-axis indices (e.g. latitudes)
  3. nmax@maxval - this will contain the maximum values
Each of these 1-dimensional attribute vectors will be of length nmax.

If x is of type float, the return value is float. If x is of type double, the return value is of type double.

Description

This function uses a simple algorithm. It looks at the points surrounding x and checks to see if they are greater than the central value. The central value is x + delta.

       1-----------8-----------7
       |           |           |
       |           |           |
       |           |           |
       2-----------0-----------6
       |           |           |
       |           |           |
       |           |           |
       3-----------4-----------5

See Also

local_min, local_min_1d, local_max_1d

Examples

Example 1

Application example.

Example 2

Let x be a 2-dimensional array with dimension sizes nlat = 64 and mlon = 128 with coordinate variables "lat" and "lon," respectively. The data are cyclic. Do not use any tolerance [delta = 0.0]

    nmax = local_max(x, True, 0.0)
If nmax = 3, then the attribute vectors might look like:
    nmax@xi     = (/  6,    114,     180 /)
    nmax@yi     = (/ 42,     76,     104 /)
    nmax@minval = (/ 70.0, -28.6,   -47.8 /)
If it is desired to write this to, say, an ascii file, then:

   ncol   = 3
   data   = new ( (/nmin,ncol/) , typeof(x) )
   data(:,0) = nmax@xi
   data(:,1) = nmax@yi
   data(:,2) = nmax@minval

   opt = True
   opt@fout = "sample.max.txt"
   write_matrix( data, ncol + "f10.2", opt)
The output file ["sample.max.txt"] would look like:
      6.00     42.00     70.00
    114.00     76.00    -28.60
    180.00    104.00    -47.80