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> 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> 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> 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> 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@lists.osgeo.org > > https://lists.osgeo.org/mailman/listinfo/grass-dev > > _______________________________________________ > grass-dev mailing > listgrass-dev@lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/grass-dev > > > -- > > ============================================= > Zoltan Szecsei GPrGISc 0031 > Geograph (Pty) Ltd. > GIS and Photogrammetric Services > > Cape Town, South Africa. > > Mobile: +27-83-6004028www.geograph.co.za > ============================================= > > _______________________________________________ > grass-dev mailing list > grass-dev@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/grass-dev >
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