[GRASS-user] Re: count pixels by attribute in sampling units

[Patrick Giraudoux]

Complement: any radius > 100 gives inconsistent results in the 'units' 
file of r.le.par:

/Thèse Fritsch/PrépaFragStat/grass$ cat r.le.para/units
         1    # of scales
         5    # of units of scale 1.
-1078225628-1217465035   u_w, u_l of units in scale 1
     100.5             radius of circles in scale 1
 539113457 608735128   left, top of unit[1]
 539113466 608734109   left, top of unit[2]
 539114864 608734976   left, top of unit[3]
 539115149 608733823   left, top of unit[4]
 539113914 608733783   left, top of unit[5]

When I use a shorter radius, I get:

grass > cat r.le.para/units
         1    # of scales
         5    # of units of scale 1.
        51        51   u_w, u_l of units in scale 1
      25.5             radius of circles in scale 1
       618      2586   left, top of unit[1]
       627      1567   left, top of unit[2]
      2025      2434   left, top of unit[3]
      2310      1281   left, top of unit[4]
      1075      1241   left, top of unit[5]


Patrick Giraudoux wrote:
> Doubts confirmed... Circles in r.le.setup may not reach the expected 
> size.  For instance, using the option 'sampling units centered over 
> sites', if I take a 25.5 cells radius everything works fine (I get a 
> total of 2053 pixels in each sampling units, which looks reasonable 
> 25.5^2 * pi = 2042.8), but if I want a larger sampling unit of 125.5 
> pixel radius, I get a warning asking confirmation whether I want a 
> radius > 100 pixels, and then get a total area of 2601 pixels, which 
> is far below what is expected from a 125.5 radius (125.5^2 * pi = 
> 49480.87).
>
> The pixel size is 4 m in reality, and the two radius correspond to 
> appx 100m and 500m...
>
> Any hint welcome !
>
> Patrick
>
>
> Patrick Giraudoux wrote:
>> Sorry to reply to myself... I just though this could help other 
>> listers...
>>
>> It seems that r.le.patch provides a way to get a count of cells by 
>> attribute in each sampling unit with the option att=a5. One must 
>> previously define each attribute value as a group (e.g. with 
>> r.le.setup) and things work fine at least with rectangles. I will 
>> have some control trials with circles later on...
>>
>> Patrick
>>
>>
>> Patrick Giraudoux wrote:
>>> Dear listers,
>>>
>>> I am exploring r.le and r.li, but I am afraid those two sets of 
>>> programmes are providing what I need only partly. Before using them 
>>> for shape and diversity analysis, I just would like to get the 
>>> number of pixels (or the area) by attributes types (eg the attribute 
>>> is a class of CORINE landcover or any classified image) in each 
>>> sampling unit into a text file. Sampling units would be circles of a 
>>> given radius centered over sites (overlayed on the raste to 
>>> analyse). I can provide either a vector file with the center points 
>>> or imported polygon shapefiles. Surprisingly it seems that this 
>>> simple statistics is not provided in r.le (maybe wrong ?). I suppose 
>>> this may be due to the fact there is a much more simple way to do it 
>>> in GRASS, but cannot find it in Neteler and Mitasova book.
>>>
>>> Any hint welcome,
>>>
>>> Patrick
>>>
>>>
>>>
>>
>>
>
>