[GRASSLIST:7595] Re: how does v.to.rast work...

[Zaboj Hrazsky, DAPHNE CR]

Zaboj Hrazsky, DAPHNE ČR napsal(a):
> Hi
> 
> One can say daily task...
> 
> I need to get rasters with cell 100m from large vector file with 
> variable area of polygons: from 20 m2 to hectars. I must not lost any 
> occurence of small polygons (which often are generalized out in such 
> cases). The best result for me is raster with values of "my vector" area 
> in each cell, but also the only presence/absence in 100m grid can be 
> used for further analysis.
> 
> There are more ways:
> 
> A) use v.mkgrid and overlay these vectors works well, but in case of 1 
> 400 000 cells in grid I've found some limits :-(.
> 
>    v.mkgrid  map=grid100 grid=2926,4728 position=region
>    Creating vector grid ...
>    Writing out vector rows ...
>    Writing out vector columns ...
>    Creating centroids ...
>    dbmi: Protocol error (invalid table/column name or unsupported column 
> type)
>    ERROR: Cannot insert row: insert into gridall values ( 822223, 174, 
> 4279 )
> 
> 
> 
> B) use v.to.rast
> 
> B1) plain v.to.rast - see file v.to.rast_centroids.png - leave many 
> pieces outside the result raster. What is the algorithm, how does GRASS 
> know which cell falls in resultant raster?
> 
> B2) to add category to vector boundaries and run  v.to.rast  ...  see  
> boundary.png ...  the  result raster eats to much ... empty cells were 
> involved, Question is why?
> 
> B3) the most complicated, but with best results now: v.to.rast in small 
> (5m or less) cell size -> r.to.vect feature=point -> v.to.rast in 100m 
> cell size works with points well. I tried to use v.to.points and patch 
> these "linepoints" and "rasterpoints" before second v.to.rast, but with 
> no better result.
> 
> B4) convert raster with small size cell direct to 100m raster, but I 
> didn't find any functionality which can simulate the Agregate() function 
> known from ESRI  GIS products, which sumarize values from finer raster 
> into cell of coarser one. Any mc
> 
> 
> There is no C for me (not programator :), but maybe anybody can navigate 
> me a bit ...
> 
> Thanks for your time
> 

I found the solution on:

http://grass.gdf-hannover.de/twiki/bin/view/GRASS/AggregateValues

shortly:

g.region res=REQUIRED_RESOLUTION
r.random.cells out=BASE dist=0
g.region res=ORIGINAL_RESOLUTION
r.statistics base=BASE cover=ORIG_RASTER method=sum out=AGGREG_RASTER

aggregated values are stored as labels (accesible as @AGGREG_RASTER in 
r.mapcalc modul), for final raster run:

g.region res=REQUIRED_RESOLUTION
r.mapcalc FINAL_AGGREG_RASTER=@AGGREG_RASTER

thats it, but could me somebody direct to any description of v.to.rast 
mechanism?

Thanks... Zaboj