.. 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 ` |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