[GRASS-user] v.out.ascii output is different than the original ascii raster

[Maciek Sieczka]

On Sun, 21 May 2006 23:09:14 +0200 (CEST)
Roger Bivand <Roger.Bivand at nhh.no> wrote:

> On Sun, 21 May 2006, Maciek Sieczka wrote:
> 
> > Hi,
> > 
> > I've got double floating point Grass ASCII raster (dem.asc,
> > attached).
> > 
> > I do:
> > 
> > 	r.in.ascii -d input=dem.asc output=dem
> > 
> > 	r.out.ascii input=dem output=dem2.asc
> > 
> > and compare dem.asc vs dem2.asc, to notice that all values in
> > dem2.asc (attached) are slightly different (sometimes lower,
> > sometimes higher).

<snip>

> > A bug?
> 
> No, the result of representing the input ASCII as a double (which
> will include fuzz), and writing it out with more digits of fuzz. The
> R FAQ sums it up nicely:
> 
> http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
> 
> or a link refered to there:
> 
> http://docs.sun.com/source/806-3568/ncg_goldberg.html
> 
> To a computer using 64 bits for a double, both will look the same
> within machine precision, so the tiny difference should have no
> practical impact.
> 
> Hope this helps,

Yes, absolutely. Thank you very much!

Best,
Maciek

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