[GRASS-user] Script for idw cross validation

[francesco sapienza]

Hello! i'm trying to validate my IDW model for field values of pH (and
other parameters) using the following:

# Partition sample points in 4 groups
Module("v.kcv", map="pHsr", npartitions=4, column="part")

Module("v.extract", flags="r", input="pHsr", output="pHcalibration",
       where="part=1")
Module("v.extract", input="pHsr", output="pHvalidation",
       where="part=1")

Module("v.surf.idw", input="pHcalibration", column="pH_values",
       npoints=12, power=2, output="pH_CAL_idw")

Module("v.what.rast", map="pHvalidation", raster="pHsr",
       column="actual")
Module("v.what.rast", map="pHvalidation", raster="pH_CAL_idw",
column="Estimated")

Things work pretty good so far, i obtained a vector with the actual values
and the estimated ones, the next step would be to evaluate the root mean
square error for this routine. (Eventually i also want to loop the idw
estimation for different set of Validation and Calibration groups).
*So far i haven't used the script but instead i used the GUI interface for
every single comand (v.kcv, v.extract, v.surf.idw and v.what.rast).*
For the following section i wanted to use a script but i have trouble
working it, as stats is 'not defined'.

sqlstat = "SELECT actual,estimated FROM pHvalidation"
stats = Module("db.select", flags="c", sql=sqlstat,
               stdout_=PIPE).outputs.stdout
stats = stats.replace("\n", "|")[:-1].split("|")
stats = (np.asarray([float(x) for x in stats], dtype="float").
         reshape(len(stats)/2, 2))

rsme = np.sqrt(np.mean(np.diff(stats, axis=1)**2))

I tried to run the GUI for db.select specifing in the sql SELECT= SELECT
actual,estimated FROM pHvalidation".
I obtained in the output of the GUI the right selected values, but i don't
know how to procede calculating the RMSE value, when i try to run the
script above, stats stay as 'not defined'. Also i was expecting a vector
with two columns* with my selected values as an output of the GUI, but i
dont know if and where such file is been saved.
I guess that the line ''stats= Module(''db.select'', flags=''c'',
sql=sqlstat, stdout_=PIPE).outputs.stdout" is required for define ''stats''
but must be  missing the reference to ''stats'' somewhere before that.
Is it possible that i just have to create a vector with the column im
trying to select and named that vector ''stats''?


*I get the idea that the output of the GUI is providing me the right values
but i dont know how to create a vector from that output to call it
''stats'' and procede with the script.

I hope that i was able to explain the issue properly, i may be missing
something simple because i'm not an expert at all in using Gis GRASS,
especially when it comes to writing scripts.
Thanks in advance,
FS.
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