[GRASSLIST:5838] Re: create new map of TS with min value month

[Glynn Clements]

Martin Wegmann wrote:

> I have a time series and I want to create a map containing the information 
> which decade has the annual minimum value. 
> 
> e.g.:
> 
> values for one pixel
> 
> jan 1 = 10
> jan 2 = 11
> jan 3 = 18
> feb 1 = 9
> feb 2 = 3
> feb 3 = 5
> .....
> 
> output =   22 or 5 (either 2= feb ; 2=2. decade OR 5th decade)
> 
> The ouput map would hold the information for each pixel which decade has the 
> annual minimum value.

> however before I start writing a script with a if condition for each decade 
> map I would like to know if somebody knows a more straight forward method. 

Add it to r.series, i.e.:

1. Copy c_min.c to e.g. c_minx.c.
2. Change the copy to return the x value instead of the y (see below).
3. Search for all occurrences of c_min in the files in the r.series
   directory and add equivalents for c_minx.
4. Repeat for c_max while you're at it.
5. Re-compile.

It's likely to be simpler than writing a script.

Sample (untested) c_minx implementation:

	void c_minx(DCELL *result, DCELL *values, int n)
	{
		DCELL min, minx;
		int i;
	
		G_set_d_null_value(&min, 1);
		G_set_d_null_value(&minx, 1);
	
		for (i = 0; i < n; i++)
		{
			if (G_is_d_null_value(&values[i]))
				continue;
	
			if (G_is_d_null_value(&min) || min > values[i])
			{
				min = values[i];
				minx = i;
			}
		}
	
		if (G_is_d_null_value(&minx))
			G_set_d_null_value(result, 1);
		else
			*result = minx;
	}

Note: you're stuck with DCELL; r.series only generates DCELL maps. Use
r.mapcalc afterwards if you really need CELL.

-- 
Glynn Clements <glynn at gclements.plus.com>