[GRASS-user] Uploading NumPy array as a new column to a vector map

[César Augusto Ramírez Franco]

On Fri, Feb 15, 2019, 06:01 Micha Silver <tsvibar at gmail.com wrote:

>
> On 2/14/19 4:49 PM, César Augusto Ramírez Franco wrote:
>
> Greetings,
>
> I have a municipality polygon layer with a poverty index (percent) and a
> population count, I'm multiplying them to visualize the vulnerability for
> each municipality, due to the distribution of that new variable I want to
> compute the logarithm, which is not possible in sqlite, so I tried reading
> the column with numpy and issuing a np.log10(). I'm happy with the result,
> but how can I get this new numpy array to a new column in the vector map in
> a simple way?
>
> This is what I already have:
>
> #!/usr/bin/env python
>
> import grass.script as gs
> import numpy as np
> import matplotlib.pyplot as plt
>
> nbi_pob = np.array(gs.vector_db_select("muniant", columns =
> "nbi_pob")["values"].values()).astype(float).squeeze()
>
> plt.hist(nbi_pob)
> plt.show()
>
> log_nbi_pob = np.log10(nbi_pob)
>
> plt.hist(log_nbi_pob)
> plt.show()
>
> I'm trying to introduce this kind of analysis to students who are not
> familiar nor proficient with programming (this is an GIS introductory
> course) so the numpy bit is already complex enough to them, and I'd like to
> avoid using for loops with cursors to achieve this.
>
>
> I also can't think of any simple way to do this. However PostgreSQL does
> have a log() function. So if you migrate your backend DB to PostgreSQL you
> should be able to do (not tested):
>
>
> gs.run_command('v.db.addcolumn', map_="muniant", column="log_nbi_pob
> DOUBLE")
>
> gs.run_command('v.to.db', map_="muniant", column="log_nbi_pob",
> query="log(nbi_pob)")
>
>
> But I must say that it seems strange that you are introducing numpy,
> matplotlib and the GRASS python interface to students who don't yet have
> the basic tools of programming, such as 'for' loops. Isn't this is a case
> of "putting the wagon in front of the horse"?? :-)
>

That script hasn't been introduced to students, I was just fiddling with
the data after a class when the question arose, trying to figure out how to
do that. I do most of the Lab research analysis with R, and lately I've
been studying the scipy ecosystem to try and do them within R, but maybe
Markus' suggestion would do to get students going with their course's
projects.

Thanks!

Is there a simple solution to this?
>
> --
> *César Augusto Ramírez Franco*
>
> _______________________________________________
> grass-user mailing listgrass-user at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/grass-user
>
> --
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>
>
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