[GRASS-dev] GSoC Proposal: Python package for topology tools

Tue Apr 23 13:19:23 PDT 2019

On Mon, Apr 22, 2019 at 4:45 AM Facundo Ferrín <facundo.ferrin at gmail.com>
wrote:
>
> Hello!
>
> I don't know if I'm expressing myself well. The use of GRASS is
command-line, in the sense:
>
> rass run --mapset=/some/directory/grassdata/ncspm/practice1 r.lake
elevation=some/file.tiff
>
>
> What I propose is the use of a package to import from any Python script.
For example, read a TIFF with GDAL or GeoPandas, perform some operations
like removing some data and finally simplify the polygon.

GRASS is essentially a toolset consisting of several hundred modules which
are called on the command line or in scripts. If you want to use this
functionality, you need to call GRASS modules as done in e.g.
https://trac.osgeo.org/grass/browser/grass/trunk/scripts
>
> GRASS is something like GDAL's option to create polygons from images:
>
> https://gdal.org/gdal_polygonize.html

The GRASS equivalent is 'r.to.vect type=area'
>
> My idea, or my need, is to use GRASS functions programmatically, as in
this case.
>
>
https://pcjericks.github.io/py-gdalogr-cookbook/raster_layers.html#polygonize-a-raster-band

Convert an OGR File to a Raster
more generic: convert a vector to a raster
The GRASS equivalent is 'v.to.rast'

Clip a GeoTiff with Shapefile
more generic: clip a raster to vector polygons
In GRASS: convert to vector polygons to raster with 'v.to.rast', then
create a MASK with 'r.mask'

Calculate zonal statistics
more precisely here: get raster statistics for each vector polygon
The GRASS equivalent is 'v.rast.stats'

Raster to vector line
In GRASS: use 'r.reclass' or 'r.mapcalc' to select pixel values that need
to be converted to vector lines, then use r.to.vect type=line. BTW, the
green lines in the example are wrong because they don#t go through the
pixel centers.

Create raster from array
The GRASS equivalent is 'r.in.ascii'

Create least cost path
The GRASS equivalent is 'r.cost' + 'r.path'

Replace No Data Value of Raster with new value
The GRASS equivalent is 'r.mapcalc' using "if(isnull(rastermap),
<new_value>, rastermap)"

>
> Is that among your ideas for the future? If that's the case and you're
interested, let me know. If so, you could make some prototype use to start
migrating the functionalities.

All this can be easily implemented as a GRASS GIS addon in a Python script
with much less code than in your example. Please have a look at
https://trac.osgeo.org/grass/browser/grass/trunk/scripts
to get started.
Alternatively, a QGIS plugin using GRASS GIS functionality would be an
option (QGIS has a GRASS GIS interface that can be used for inspiration).

my2c,
Markus M

>
> Cheers!
>
>
> On 10/04/2019 16:47, Vaclav Petras wrote:
>
> Hello Facundo,
>
> Using the GRASS GIS algorithms in various settings is certainly something
we are aiming for. However, as you probably understood from the
conversation here, it is not clear what exactly you are trying to achieve
considering the already available ways (APIs if you will). Also, to
evaluate your proposal, we need to test your skills for this particular
project. Therefore, I suggest you 1) implement a prototype of what you have
in mind, 2) compare it with the existing approaches (see e.g. email from
Markus M.), and 3) identify what is missing in the existing approaches and
what would you need to add in order to make you prototype work.
>
> Alternatively, or ideally in addition to the above, you can implement
couple of the test and training tasks from the proposal already linked by
Luca:
>
>
https://trac.osgeo.org/grass/wiki/GSoC/2019#Neweasy-to-useCLIandAPIforGRASSGIS
>
> Best,
> Vaclav
>
> On Thu, Mar 28, 2019 at 4:13 AM Markus Metz <markus.metz.giswork at gmail.com>
wrote:
>>
>>
>>
>> On Thu, Mar 28, 2019 at 9:02 AM Maris Nartiss <maris.gis at gmail.com>
wrote:
>> >
>> > Hello Facundo,
>> > the easiest way would be moving functions of v.generalize into a
>> > library (e.g. grass_generalize) and thus make available for calling
>> > via ctypes.
>> > In the past I have had a good success manipulating GRASS vectors via
>> > ctypes. It takes more skill than a plain Python implementation but it
>> > is easier than a full blown C code and faster than pure Python one.
>> >
>> > Māris.
>> >
>> > ceturtd., 2019. g. 28. marts, plkst. 03:13 — lietotājs Facundo Ferrin
>> > (<facundo.ferrin at gmail.com>) rakstīja:
>> > >
>> > > Hi Luca!
>> > >
>> > > Thanks for replying! In my job, there were things we had to do
programmatically. For example, to manipulate geometries that reach the
backend from a GeoJSON we use tools like these:
>> > >
>> > >
https://pcjericks.github.io/py-gdalogr-cookbook/geometry.html#create-geometry-from-wkt
>> > >
>> > > However, polygon simplification does not work very well because it
does not take topology into account. My idea was to port part of the GRASS
algorithms to be able to use them without needing the graphical interface
or command line, but only importing a library in a Python script.
>>
>> In this particular case, the core of the corresponding python script
would be three lines (import, simplify, export):
>>
>> -->
>> import grass.script as grass
>>
>> grass.run_command('v.in.ogr', ...)
>> grass.run_command('v.generalize', ...)
>> grass.run_command('v.out.ogr', ...)
>> <--
>>
>> The import step with v.in.ogr is needed because the vector to be
simplified must be a native GRASS vector with topology.
>>
>> How does your proposal differ from the QGIS-GRASS interface?
>>
>> Markus M
>> > >
>> > > Is it something that you have in mind to do or that might be useful
to you?
>> > >
>> > >
>> > > El jue., 28 de mar. de 2019 a la(s) 00:32, Luca Delucchi (
lucadeluge at gmail.com) escribió:
>> > >>
>> > >> On Tue, 26 Mar 2019 at 03:11, Facundo Ferrin <
facundo.ferrin at gmail.com> wrote:
>> > >> >
>> > >> > Hi there!
>> > >>
>> > >> Hi Facundo,
>> > >> >
>> > >> >
>> > >> > My name is Facundo Ferrin. I am a nuclear engineer who is taking
a master in Computer Vision in Barcelona, and finally I found my
opportunity to contribute to OSGeo by applying two things that I really
like: Python and Backend development . I do not know exactly what I should
write in this first email, so I'll start by listing the projects I'm
interested in.
>> > >> >
>> > >> > I'm working in a company that is developing a platform for
precision agriculture called Auravant (https://www.auravant.com/). I work
as a backend developer and data analyst and I use daily almost every tool
that you post in the ideas: GeoServer, PostGIS, QGis. I'm also porting a
tool for polygon simplification called topoJSON (
https://github.com/fferrin/topojson).
>> > >> >
>> > >> > ---
>> > >> > MY MAIN IDEA is to start porting GRASS tools into a python
package that can be used in other projects (beyond the client to use by
command line). I don't know if it's something you have in mind but for
offline and automated analysis it would be very useful. I particularly had
problems when I tried to simplify geometries since the geometry of polygons
was not taken into account.
>> > >> > ---
>> > >>
>> > >> Your idea is not clear to me, there are already two Python library
to
>> > >> work with GRASS. you can find some ideas in the proposal page
>> > >> https://trac.osgeo.org/grass/wiki/GSoC/2019 (for example
>> > >> Neweasy-to-useCLIandAPIforGRASSGIS) and
>> > >> https://trac.osgeo.org/grass/wiki/GSoC/2018 (Improve GRASS
integration
>> > >> in QGIS 3)
>> > >>
>> > >> > Hope to hear from you soon!
>> > >> >
>> > >>
>> > >> --
>> > >> ciao
>> > >> Luca
>> > >>
>> > >> www.lucadelu.org
>> > >
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