.. only:: html
|LS| GRASS Tools
===============================================================================
In this lesson we will present a selection of tools to give you an idea of the
capabilities of GRASS.
.. _grass_aspect:
|basic| |FA| Create an aspect map
-------------------------------------------------------------------------------
#. Open the :guilabel:`GRASS Tools` tab
#. Load the :file:`g_dem` raster layer from the :guilabel:`grass_mapset` Mapset
#. Look for the :guilabel:`r.aspect` module by searching for it in the
:guilabel:`Filter` field of the :guilabel:`Modules List` tab
#. Open the tool and set it up like this and click on the :guilabel:`Run` button:
.. image:: img/grass_aspect.png
:align: center
|
#. When the process is finished click on :guilabel:`View Output` to load the
resulting layer in the canvas:
.. image:: img/grass_aspect_result.png
:align: center
The :file:`g_aspect` layer is stored within the :guilabel:`grass_mapset` Mapset
so you can remove the layer from the canvas and reload it whenever you want.
|basic| |FA| Get basic statistic of raster layer
-------------------------------------------------------------------------------
We want to know some basic statistics of the :file:`g_dem` raster layer.
#. Open the :guilabel:`GRASS Tools` tab
#. Load the :file:`g_dem` raster layer from the :guilabel:`grass_mapset` Mapset
#. Look for the :guilabel:`r.info` module by searching for it in the
:guilabel:`Filter` field of the :guilabel:`Modules List` tab
#. Set up the tool like this and click on :guilabel:`Run`:
.. image:: img/grass_raster_info.png
:align: center
|
#. Within the Output tab you will see some raster information printed, like the
path of the file, the number of rows and columns and other useful information:
.. image:: img/grass_raster_info_result.png
:align: center
|moderate| |FA| The Reclass Tool
-------------------------------------------------------------------------------
Reclassifying a raster layer is a very useful task. We just created the
:file:`g_aspect` layer from the :file:`g_dem` one. The value range gets from 0
(North) passing through 90 (East), 180 (South), 270 (West) and finally to 360
(North again). We can reclassify the :file:`g_aspect` layer to have just 4
**categories** following specific *rules* (North = 1, East = 2, South = 3 and
West = 4).
Grass reclassify tool accepts a ``txt`` file with the defined rules. Writing the
rules is very simple and the GRASS Manual contains very good description.
.. tip:: Each GRASS tool has its own Manual tab. Take the time to read the
description of the tool you are using to don't miss some useful parameters
#. Load the :file:`g_aspect` layer or, if you don't have create it, go back to the
:ref:`grass_aspect` section.
#. Look for the :guilabel:`r.reclass` module by searching for it in the
:guilabel:`Filter` field of the :guilabel:`Modules List` tab
#. Open the tool and set it up like the following picture. The file containing the
rules is in the :file:`exercise_data/grass/` folder, named :file:`reclass_aspect.txt`.
#. Click on :guilabel:`Run` and wait until the process is finished:
.. image:: img/grass_reclass.png
:align: center
|
#. Click on :guilabel:`View Output` to load the reclassified raster in the canvas
The new layer is made up by just 4 values (1, 2, 3, and 4) and it is easier to
manage and to process.
.. image:: img/grass_reclass_result.png
:align: center
.. tip:: Open the :file:`reclass_aspect.txt` with a text editor to see the rules
and to start becoming used to them. Moreover, take a deep look at the GRASS
manual: a lot of different examples are pointed out.
.. _backlink-grass_reclass:
|moderate| |TY| Reclassify with your rules
-------------------------------------------------------------------------------
Try to reclassify the :file:`g_dem` layer into 3 new categories:
* from 0 to 1000, new value = 1
* from 1000 to 1400, new value = 2
* from 1400 to the maximum raster value, new value = 3
:ref:`Check your results <grass_reclass>`
|moderate| |FA| The Mapcalc Tool
------------------------------------------------------------------------------
The Mapcalc tools is similar to the Raster Calculator of QGIS. You can perform
mathematical operation on one or more raster layers and the final result will
be a new layer with the calculated values.
The aim of the next exercise is to extract the values greater than 1000 from the
:file:`g_dem` raster layer.
#. Look for the :guilabel:`r.mapcalc` module by searching for it in the
:guilabel:`Filter` field of the :guilabel:`Modules List` tab.
#. Start the tool.
The :guilabel:`Mapcalc` dialog allows you to construct a sequence of analyses
to be performed on a raster, or collection of rasters. You will use these tools
to do so:
.. image:: img/map_calc_tools.png
:align: center
|
In order, they are:
* :guilabel:`Add map`: Add a raster file from your current GRASS mapset.
* :guilabel:`Add constant value`: Add a constant value to be used in functions,
1000 in this case
* :guilabel:`Add operator or function`: Add an operator or function to be connected
to inputs and outputs, we will use the operator ``greater equals than``
* :guilabel:`Add connection`: Connect elements. Using this tool, click and drag
from the red dot on one item to the red dot on another item. Dots that are
correctly connected to a connector line will turn gray. If the line or dot is
red, it is not properly connected!
* :guilabel:`Select item`: Select an item and move selected items.
* :guilabel:`Delete selected item`: Removes the selected item from the current
mapcalc sheet, but not from the mapset (if it is an existing raster)
* :guilabel:`Open`: Open an existing file with the operation defined
* :guilabel:`Save`: Save all the operation in a file
* :guilabel:`Save as`: Save all the operations as a new file on the disk.
#. Using these tools, construct the following algorithm:
.. image:: img/grass_mapcalc.png
:align: center
|
#. Click on :guilabel:`Run` and then on :guilabel:`View output` to see the output
displayed in your map:
.. image:: img/grass_mapcalc_result.png
:align: center
|
This shows all the areas where the terrain is higher than 1000 meters.
.. tip:: You can also save the formula you have created and load it in another
QGIS project by clicking on the last button on the GRASS Mapcalc toolbar.
|IC|
-------------------------------------------------------------------------------
In this lesson, we have covered only a few of the many tools GRASS offers. To
explore the capabilities of GRASS for yourself, open the :guilabel:`GRASS
Tools` dialog and scroll down the :guilabel:`Modules List`. Or for a more
structured approach, look under the :guilabel:`Modules Tree` tab, which
organizes tools by type.
.. Substitutions definitions - AVOID EDITING PAST THIS LINE
This will be automatically updated by the find_set_subst.py script.
If you need to create a new substitution manually,
please add it also to the substitutions.txt file in the
source folder.
.. |FA| replace:: Follow Along:
.. |IC| replace:: In Conclusion
.. |LS| replace:: Lesson:
.. |TY| replace:: Try Yourself
.. |basic| image:: /static/global/basic.png
.. |moderate| image:: /static/global/moderate.png