[GRASS-dev] [GRASS-user] compare a DCELL and FCELL question

[Markus Metz]

On Sun, Jan 24, 2021 at 5:06 PM ming han <dustming at gmail.com> wrote:
>
> Hi Everyone
>
>    Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, ... )
approach may increase the runtime compare to '==' way?

The formulas are very simple, I don't think that differences in runtime can
be reliably measured. (De-)compression of the data and the operating
system's file cache have a much stronger influence on the runtime.

Markus M
>
> Thanks
> Ming
>
> Markus Metz <markus.metz.giswork at gmail.com> 于2021年1月24日周日 上午10:57写道：
>>
>> 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 <dustming at gmail.com> 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 <neteler at osgeo.org> 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 <dustming at gmail.com> 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
>> >> [2] Comment by Glynn:
>> >> https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html
>> >>
>> >> Best,
>> >> Markus
>> >
>> > _______________________________________________
>> > grass-dev mailing list
>> > grass-dev at lists.osgeo.org
>> > https://lists.osgeo.org/mailman/listinfo/grass-dev
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