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

[Markus Metz]

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
>
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