[QGIS-Developer] [Qgis-user] Wrong result with Saga Raster calculator

[Eugenio Trumpy]

Hi Pedro,


I did the same for 5 pixels in the test raster I shared with you yesterday.

The different seems to be a little bit greater.

See the attached sheet.


I don't know if the version of saga, 2.3.1 the mine vs 2.3.2 the yours could generate this error,

or if the problem is in qgis process model.


E.





________________________________
Da: Pedro Venâncio <pedrongvenancio at gmail.com>
Inviato: mercoledì 31 maggio 2017 22:16
A: Eugenio Trumpy
Cc: qgis-user; qgis-developer
Oggetto: Re: [Qgis-user] Wrong result with Saga Raster calculator

Hi Eugenio,

I've created a sample processing model (attached), that works ok, except a small error in the result (decimals).

For instance:

Raster  ID      Pixel value

Z_top_res       a       1202,03510276

t_top_res       b       53,6123428345

topo    c1      -182,584899902

SAGA Result 1   c       15,1783676147

SAGA Result 2   d       31,9437332153   Diff
Expected Result 1
        15,1783679504   0,000000335741
Expected Result 2
        31,9740872222   0,0303540069

Formula
1       ifelse(c1>0,-(0.0065*c1-16),(0.0045*c1+16))
2       ifelse(a<500,0,((b-c)/a)*1000)

Using SAGA GUI there is no error.

Could this be because of some roundings in the input / output? (I'm sending for the developer's list also).

Thanks!

Best regards,
Pedro Venâncio



2017-05-31 10:56 GMT+01:00 Eugenio Trumpy <frippe12573 at hotmail.com<mailto:frippe12573 at hotmail.com>>:


Hi Pedro,

thank you very much to have tested the computation with my test data!
You are right, I checked again the computation and actually it works.

However, I was using Saga raster calculation in a processing modeler, and one of the layer I share with you
is the resulting from a previous computation.

In other word the raster layer c=temp_recal.tif comes from a previous computation (raster calculator):
ifelse(a>0,-(0.0065*a-16),(0.0045*a+16))
where a is the layer attached, and hence c of the previous  formula is the resulting of this first computation.

E.

________________________________
Da: Pedro Venâncio <pedrongvenancio at gmail.com<mailto:pedrongvenancio at gmail.com>>
Inviato: mercoledì 31 maggio 2017 11:37
A: frippe12573; qgis-user
Oggetto: Re: [Qgis-user] Wrong result with Saga Raster calculator

Hi Eugenio,

The result with SAGA Raster Calculator seems ok here, on QGIS 2.18.9, SAGA 2.3.2, both from OSGeo4W64 (result attached).

Z_top_res.tif have values from 724.403 to 1423.553, and so, the first condition is never true. Then, it always make ((b-c)/a)*1000, and the result seems ok to me.


Pedro



2017-05-31 10:19 GMT+01:00 frippe12573 <frippe12573 at hotmail.com<mailto:frippe12573 at hotmail.com>>:
Hi Paolo,

the raster have the same size and origin.
The pixels are precisely overlapped.

I tried also r.mapcalc, but I got a worst result.
Here the output of the log:
https://pastebin.com/jNyyzHgf

E.



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