[GRASS-user] How to implement 2D lookup table?

[Moritz Lennert]

On 23/05/13 10:26, Rainer M Krug wrote:
>
> Hi
>
> I have two raster layers
>
> ,----
> | dc: CELL, 1, ..., 7, NULL
> | ageClass: CELL, 1,2,3,NULL
> `----
>
> And I want to create a third one, depending on the value in a lookup table. dc corresponds to the row, ageClass to the column,
>
> The lookup table looks as follow:
>
> ,----
> |     seedling young adult
> | dc1   425000 87500 37500
> | dc2   270000 55000 23500
> | dc3   127500 26000 11250
> | dc4    32500  7100  3200
> | dc5     5550  1220   500
> | dc6      605   132    55
> | dc7       56    12     6
> `----
>
> Now I want to have something like (using r.mapcalc as an example)
>
> ,----
> | r.mapcalc expression = "result = lookupTable[col=ageClass, row=dc]"
> `----
>
> and e.g. for one cell
>
> ,----
> | ageClass = 3
> | dc = 5
> | ==>
> | result = 500
> `----
>
> I haven't found anything like this in GRASS.
>
> I could use MASKS and then r.reclass(), or generate dynamically a
> complex if() construct, but both options sound a little bit
> awkward to me.
>
> I am scripting GRASS from R, so there is always the option of doing it
> in R, but there is the issue of reading the data into R and writing it
> back to GRASS, which takes time. As I am doing this in a simulation
> which runs several times, this is not an unimportant issue.
>
> Is there better way of achieving this?

I don't see a possibility to include the lookup table into r.mapcalc, 
but anyway, IIUC, the result you are looking for is not a raster map, 
but a numerical value, or ?

My best first guess is:

r.stats -c in=dc,ageClass

possibly combined with a little awk to do the lookup:

r.stats -c in=dc,ageClass | awk '{if($1==5 && $2==3) print $3}'

Or, if the maps are heavy, feed the data resulting from the r.stats call 
into a new db table and then use sql queries to do the lookup. Or feed 
the results of r.stats into an R dataframe.

Moritz