Re: [GRASS-user] compare a DCELL and FCELL question

Hi,
Is it important to cross-post this discussion on both dev and user lists?

Regards,
Zoltan


On 2021-01-24 17:56, Markus Metz wrote:
> Trying to answer the original question: with a DCELL map 
> "cat1_acc_riv" and a FCELL map "cat1_minacc", why is 
> "float(cat1_acc_riv) == float(cat1_minacc)" not equal to 
> "int(cat1_acc_riv) == int(cat1_minacc)" ?
>
> int truncates to integer while float converts to single precision 
> floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, 
> "float(cat1_acc_riv) == float(cat1_minacc)" becomes "1.1 == 1.9" 
> whereas "int(cat1_acc_riv) == int(cat1_minacc)" becomes "1 == 1", thus 
> the results are different.
>
> Another reason for possible differences is that float can only 
> represent max 7 decimal digits. E.g. float(194320567) becomes 
> 194320560 but int(194320567) preserves the value 194320567.
>
> Thus the safest is to cast everything to the type with the highest 
> precision. In this case with FCELL and DCELL, use 
> "double(cat1_acc_riv) == double(cat1_minacc)" or even better the 
> suggestion of Markus N.
>
> Markus M
>
>
> On Sun, Jan 24, 2021 at 3:51 PM ming han <[email protected] 
> <mailto:[email protected]>> wrote:
> >
> > Hi Markus and Micha
> >
> >      I am just trying to find grids have the same values in these 
> two rasters, I will try the threshold approach.
> >
> > Thanks
> > Ming
> >
> > Markus Neteler <[email protected] <mailto:[email protected]>> 
> 于2021年1月24日周日 上午6:58写道：
> >>
> >> Hi Ming,
> >>
> >> On Sun, Jan 24, 2021 at 10:49 AM ming han <[email protected] 
> <mailto:[email protected]>> wrote:
> >> >
> >> > Hi Micha
> >> >
> >> >      Many thanks for your reply.
> >> >      Here is the command I am using:
> >> >
> >> >      if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())
> >> >
> >> >       The str_r is a CELL raster. the result is different when I 
> change it to:
> >> >        if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())
> >>
> >> Note that numerical "equality" is better tested with a threshold test
> >> against the map pixel difference.
> >> As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.
> >>
> >> Hence the test needs to be implemented in a different way, i.e. by
> >> using an epsilon.
> >> Essentially something like this:
> >>
> >> if(fabs(map_A - map_B) <= 1.0e-15, ... )
> >>
> >> In your case (untested):
> >> r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , 
> null())
> >>
> >> See related discussions here: [1], [2] and elsewhere.
> >>
> >> [1] Comment by Glynn: 
> https://trac.osgeo.org/grass/ticket/2854#comment:9 
> <https://trac.osgeo.org/grass/ticket/2854#comment:9>
> >> [2] Comment by Glynn:
> >> 
> https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html 
> <https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html>
> >>
> >> Best,
> >> Markus
> >
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>
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-- 

=============================================
Zoltan Szecsei GPrGISc 0031
Geograph (Pty) Ltd.
GIS and Photogrammetric Services

Cape Town, South Africa.

Mobile: +27-83-6004028
www.geograph.co.za
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