GRASS Python scripting

Hands on lunch (30-4-2018, Deltares, Delft) by Willem Ottevanger and Marc Weeber

1. Preliminary steps

1.1 Install GRASS

GRASS is deliverd with QGIS. This is the easiest option to get started. Choose the QGIS Standalone Installer Version 2.18 (64 bit).

1.2 Setup GRASS mapset location

Before starting we need to setup the GRASS projection and mapset. In the scripts folder you will find the following command:

> setup_grass_location.bat

This will bring up the following window. Click 'New' to create a new GRASS location

Fill in Project Location as: RD_new and the GRASS Mapset to Amersfoort / RD New

Select a method to choose the EPSG code:

We will use Amersfoort / RD new

including the datum transformation

A summary is provided of the newly defined location

A new mapset can be created by using the 'New' button. Call this mapset 'mapset1'

After this you can press 'Quit'.

1.3 Running python packaged with QGIS

• Load a command window with the necessary paths loaded for GRASS > cmd_grass.bat

• Run the following command within the command window to check your setup > %GRASS_PYTHON% setup_grass_environment.py

After running you should see the following output:

Current GRASS GIS 7 environment:
{u'MAPSET': u'mapset1', u'GISDBASE': u'D:\\user\\Documents/grassdata', u'LOCATION_NAME': u'RD_new'}
Available raster maps:
Available vector maps:

1.4 Interactive python enviroment

Instead of using the python environment, it is also possible to connect to the grass python libraries from a locally setup python environment using Anaconda. This has advantages of interactive code development, as well as interfacing with other python libraries which can be installed using conda (or otherwise pip).

• Start an Anaconda prompt (from Start menu)
• Install a clean anaconda environment with jupyter

> conda create -n gispy python=2 jupyter

• Activate the conda enviroment

> activate gispy

• In a later phase you can run start the environment using from the scripts folder

> start_conda_cmd.cmd

• Nexy set all relevant paths

> set_conda_grass_env.bat

• In the exercises folder run

> jupyter notebook ex1.ipynb

2. GRASS basics

GRASS commands are preceded by the following prefixes depending on the functionality:

prefix	section	description
g.	general	generic commands
v.	vector	2D/3D vector data processing
r.	raster	2D raster data processing
db.	database	attribute data management
d.	display	display commands
i.	imagery	imagery data processing
ps.	postscript	map outputs
r3.	raster3D	3D raster data processing

Further reading

1. https://github.com/GISMentors/grass-gis-workshop-jena-2018
2. https://grass.osgeo.org/grass74/manuals/
3. https://github.com/wenzeslaus/python-grass-addon
4. https://grasswiki.osgeo.org/wiki/GRASS_and_Python#External_Python_editors_.28IDE.29