Re: Alternative to r.median for GRASS 7.x?

Nikos Alexandris <[email protected]> Fri, 26 Jan 2018 21:52:23 +0100

Newsgroups	gmane.comp.gis.grass.user
Message-ID	<[email protected]>

* Rich Shepard <[email protected]> [2018-01-26 11:12:14 -0800]:

>On Fri, 26 Jan 2018, Nikos Alexandris wrote:
>
>>how can we compute the median of a floating point cover raster map, over a
>>base map (using the conventional terminology as in r.stats.zonal)?
>
>Nikos,
>
>  By definition the median value has as many observations/measurements less
>than this value as greater than this value. Because floating point numbers
>are not represented exactly in computers (unless you extend them out to
>several dozen digits to the right of the decimal point), I suggest you
>create a map with integer values that will work for your application.

Something like multiply by * 1000, find the median, then divide by 1000
(as suggested in the past for other modules that don't treat fp
numbers).

>  You can find cells with values greater than some number, and less than
>some number but you'll have a difficult time finding that number and getting
>grass (or any application) to recognize it.

>HTH,

Yes it helps.

As for the last comment: do you mean it's going to be computationally
"difficult", as in running time?  There are algorithms that handle
this as I read on the net.  If floating point numbers of the cover map
that fall inside an area (as per the base map), are treated as a vector,
then sorting the figures will give an answer.  Even in case of even
number of numbers, the median can be, practically, the average of the
two middle-placed figures, or select the highest number of the lower
part.

Thank you, Nikos

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