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Hi,<br>
Is it important to cross-post this discussion on both dev and user
lists?<br>
<br>
Regards,<br>
Zoltan<br>
<br>
<br>
<div class="moz-cite-prefix">On 2021-01-24 17:56, Markus Metz wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAG+h=FEkeiib1uQqkbvTLP3o85dWMJfnb-HwH0nDoYe4MSpGUQ@mail.gmail.com">
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<div>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)" ?<br>
</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Markus M<br>
</div>
<div><br>
</div>
<div><br>
On Sun, Jan 24, 2021 at 3:51 PM ming han <<a
href="mailto:dustming@gmail.com" moz-do-not-send="true">dustming@gmail.com</a>>
wrote:<br>
><br>
> Hi Markus and Micha<br>
><br>
> I am just trying to find grids have the same values
in these two rasters, I will try the threshold approach.<br>
><br>
> Thanks<br>
> Ming <br>
><br>
> Markus Neteler <<a href="mailto:neteler@osgeo.org"
moz-do-not-send="true">neteler@osgeo.org</a>>
于2021年1月24日周日 上午6:58写道：<br>
>><br>
>> Hi Ming,<br>
>><br>
>> On Sun, Jan 24, 2021 at 10:49 AM ming han <<a
href="mailto:dustming@gmail.com" moz-do-not-send="true">dustming@gmail.com</a>>
wrote:<br>
>> ><br>
>> > Hi Micha<br>
>> ><br>
>> > Many thanks for your reply.<br>
>> > Here is the command I am using:<br>
>> ><br>
>> > if(float(cat1_acc_riv) ==
float(cat1_minacc), str_r, null())<br>
>> ><br>
>> > The str_r is a CELL raster. the result is
different when I change it to:<br>
>> > if(int(cat1_acc_riv) == int(cat1_minacc),
str_r, null())<br>
>><br>
>> Note that numerical "equality" is better tested with
a threshold test<br>
>> against the map pixel difference.<br>
>> As the threshold, we use GRASS_EPSILON which is
defined as 1.0e-15.<br>
>><br>
>> Hence the test needs to be implemented in a different
way, i.e. by<br>
>> using an epsilon.<br>
>> Essentially something like this:<br>
>><br>
>> if(fabs(map_A - map_B) <= 1.0e-15, ... )<br>
>><br>
>> In your case (untested):<br>
>> r.mapcalc diffepsilon = if( abs( map_A - map_B) <=
1.0e-15, str_r , null())<br>
>><br>
>> See related discussions here: [1], [2] and elsewhere.<br>
>><br>
>> [1] Comment by Glynn: <a
href="https://trac.osgeo.org/grass/ticket/2854#comment:9"
moz-do-not-send="true">https://trac.osgeo.org/grass/ticket/2854#comment:9</a><br>
>> [2] Comment by Glynn:<br>
>> <a
href="https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html"
moz-do-not-send="true">https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html</a><br>
>><br>
>> Best,<br>
>> Markus<br>
><br>
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<pre class="moz-signature" cols="72">--
=============================================
Zoltan Szecsei GPrGISc 0031
Geograph (Pty) Ltd.
GIS and Photogrammetric Services
Cape Town, South Africa.
Mobile: +27-83-6004028
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