No inconvenience - general list etiquette usually says start with 'user'
and then move to 'dev' if it looks like a SW issue.
But I am NOT lecturing you, so please do not take offense.
Keep well,
Zoltan
On 2021-01-24 18:28, ming han wrote:
sorry for the inconvenience. I was just not sure if it is problem that
needs developers to answer.
Best regards
Ming
Zoltan <zolt...@geograph.co.za <mailto:zolt...@geograph.co.za>>
于2021年1月24日周日 上午11:13写道：
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 <dustm...@gmail.com
<mailto:dustm...@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 <nete...@osgeo.org <mailto:nete...@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 <dustm...@gmail.com
<mailto:dustm...@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
<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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--
=============================================
Zoltan Szecsei GPrGISc 0031
Geograph (Pty) Ltd.
GIS and Photogrammetric Services
Cape Town, South Africa.
Mobile: +27-83-6004028
www.geograph.co.za <http://www.geograph.co.za>
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=============================================
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Geograph (Pty) Ltd.
GIS and Photogrammetric Services
Cape Town, South Africa.
Mobile: +27-83-6004028
www.geograph.co.za
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