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

[Rainer M Krug]

On Friday, May 24, 2013, Glynn Clements wrote:

>
> Rainer M. Krug wrote:
>
> > >> Hmm - sounds interesting, but also for other purposes. But as the
> result
> > >> will be a raster map, I don't think this approach is usable here
> (unless
> > >> I can feed the numbers back into a raster map, i.e. the inverse of
> > >> r.stats -c)
> > >
> > > r.cross + r.reclass ?
> >
> > There is one aspect in r.cross which makes this approach quite
> > difficult to implement: the value of the result map layer can not easily
> > be traced back to the values of the two input layers:
>
> If you only have a small number of classes, you can just create the
> cross-product manually, e.g.:
>
>         r.mapcalc 'result = ageClass * 10 + dc'


Yes - This is effectively what I am doing now.


>
> So a category of 15 would be ageClass=1, dc=5.
>
> r.cross is more useful when the total number of pairs which actually
> occur is much fewer than the product of the numbers of categories, as
> it only creates a category for each combination which actually occurs,
> not for each potential combination.


True. The problem I have with r.cross is that I have to manually decode
afterwords what combination the value refers to. I could use a she'll
script to translate the category values, but this sounds a little tedious,
especially as the format is not that easily parsable.

Cheers,

Rainer


>
> --
> Glynn Clements <glynn at gclements.plus.com <javascript:;>>
>


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