.
The GRASS plugin provides access to GRASS GIS databases and functionalities (see GRASS-PROJECT in Literature and Web References). This includes visualizing GRASS raster and vector layers, digitizing vector layers, editing vector attributes, creating new vector layers and analysing GRASS 2-D and 3-D data with more than 400 GRASS modules.
In this section, we’ll introduce the plugin functionalities and give some examples of managing and working with GRASS data. The following main features are provided with the toolbar menu when you start the GRASS plugin, as described in section sec_starting_grass:
To use GRASS functionalities and/or visualize GRASS vector and raster layers in
QGIS, you must select and load the GRASS plugin with the Plugin Manager.
Therefore, go to the menu Plugins ‣
Manage Plugins, select
GRASS and click
[OK].
You can now start loading raster and vector layers from an existing GRASS
LOCATION
(see section sec_load_grassdata). Or, you can create a new
GRASS LOCATION
with QGIS (see section Creating a new GRASS LOCATION) and import
some raster and vector data (see section Importing data into a GRASS LOCATION) for further
analysis with the GRASS Toolbox (see section The GRASS Toolbox).
With the GRASS plugin, you can load vector or raster layers using the appropriate
button on the toolbar menu. As an example, we will use the QGIS Alaska dataset (see
section Sample Data). It includes a small sample GRASS LOCATION
with three vector layers and one raster elevation map.
grassdata
, download the QGIS ‘Alaska’ dataset
qgis_sample_data.zip
from http://download.osgeo.org/qgis/data/
and unzip the file into grassdata
.Gisdbase
, browse and select or enter the path to the newly created
folder grassdata
.alaska
and the MAPSET demo
.gtopo30
and click [OK]. The elevation layer will be visualized.alaska
and click [OK]. The Alaska boundary vector layer will be
overlayed on top of the gtopo30
map. You can now adapt the layer
properties as described in chapter The Vector Properties Dialog (e.g.,
change opacity, fill and outline color).rivers
and airports
, and
adapt their properties.As you see, it is very simple to load GRASS raster and vector layers in QGIS.
See the following sections for editing GRASS data and creating a new LOCATION
.
More sample GRASS LOCATIONs
are available at the GRASS website at
http://grass.osgeo.org/download/sample-data/.
Порада
GRASS Data Loading
If you have problems loading data or QGIS terminates abnormally, check to make sure you have loaded the GRASS plugin properly as described in section Starting the GRASS plugin.
GRASS data are stored in a directory referred to as GISDBASE. This directory, often
called grassdata
, must be created before you start working with the GRASS
plugin in QGIS. Within this directory, the GRASS GIS data are organized by projects
stored in subdirectories called LOCATIONs
. Each LOCATION
is defined
by its coordinate system, map projection and geographical boundaries. Each
LOCATION
can have several MAPSETs
(subdirectories of the
LOCATION
) that are used to subdivide the project into different topics or
subregions, or as workspaces for individual team members (see Neteler & Mitasova
2008 in Literature and Web References). In order to analyze vector and raster layers
with GRASS modules, you must import them into a GRASS LOCATION
. (This is
not strictly true – with the GRASS modules r.external
and v.external
you can create read-only links to external GDAL/OGR-supported datasets without
importing them. But because this is not the usual way for beginners to work with
GRASS, this functionality will not be described here.)
Figure GRASS location 1:
GRASS data in the alaska LOCATION
As an example, here is how the sample GRASS LOCATION alaska
, which is
projected in Albers Equal Area projection with unit feet was created for the
QGIS sample dataset. This sample GRASS LOCATION alaska
will be used for
all examples and exercises in the following GRASS-related sections. It is
useful to download and install the dataset on your computer (see Sample Data).
alaska.shp
shapefile (see section
Loading a Shapefile) from the QGIS Alaska dataset (see Sample Data).grassdata
, or create
one for the new LOCATION
using a file manager on your computer. Then
click [Next].MAPSET
within an existing
LOCATION
(see section Adding a new MAPSET) or to create a new
LOCATION
altogether. Select LOCATION
– we used ‘alaska’ – and click [Next].LOCATION
and
projection and haven’t memorized the EPSG ID, click on the LOCATION
bounds in the
north, south, east, and west directions. Here, we simply click on the button
[Set current |qg| extent], to apply the extent of the loaded layer
alaska.shp
as the GRASS default region extent.MAPSET
within our new LOCATION
(this
is necessary when creating a new LOCATION
). You
can name it whatever you like - we used ‘demo’. GRASS automatically creates a special MAPSET
called
PERMANENT
, designed to store the core data for the project, its default
spatial extent and coordinate system definitions (see Neteler & Mitasova 2008
in Literature and Web References).LOCATION
, ‘alaska’, and two MAPSETs
, ‘demo’ and ‘PERMANENT’,
are created. The currently opened working set is ‘demo’, as you defined.Figure GRASS location 2:
Creating a new GRASS LOCATION or a new MAPSET in QGIS
If that seemed like a lot of steps, it’s really not all that bad and a very quick
way to create a LOCATION
. The LOCATION
‘alaska’ is now ready for
data import (see section Importing data into a GRASS LOCATION). You can also use the already-existing
vector and raster data in the sample GRASS LOCATION
‘alaska’,
included in the QGIS ‘Alaska’ dataset Sample Data, and move on to
section The GRASS vector data model.
A user has write access only to a GRASS MAPSET
he or she created. This means that
besides access to your own MAPSET
, you can read maps in other users’
MAPSETs
(and they can read yours), but you can modify or remove only the maps in your own MAPSET
.
All MAPSETs
include a WIND
file that stores the current boundary
coordinate values and the currently selected raster resolution (see Neteler & Mitasova
2008 in Literature and Web References, and section The GRASS region tool).
grassdata
with the
LOCATION
‘alaska’, where we want to add a further MAPSET
called ‘test’.MAPSET
within an existing
LOCATION
or to create a new LOCATION
altogether. Click on the
radio button text
for the new MAPSET
. Below in the wizard, you
see a list of existing MAPSETs
and corresponding owners.This section gives an example of how to import raster and vector data into the
‘alaska’ GRASS LOCATION
provided by the QGIS ‘Alaska’ dataset.
Therefore, we use the landcover raster map landcover.img
and the vector GML
file lakes.gml
from the QGIS ‘Alaska’ dataset (see Sample Data).
grassdata
in the QGIS
Alaska dataset, as LOCATION
‘alaska’, as MAPSET
‘demo’ and
click [OK].landcover.img
, click the module
r.in.gdal
in the Modules Tree tab. This GRASS module
allows you to import GDAL-supported raster files into a GRASS
LOCATION
. The module dialog for r.in.gdal
appears.raster
in the QGIS ‘Alaska’ dataset
and select the file landcover.img
.landcover_grass
and click
[Run]. In the Output tab, you see the currently running GRASS
command r.in.gdal -o input=/path/to/landcover.img
output=landcover_grass
.landcover_grass
raster layer is now imported into GRASS and
will be visualized in the QGIS canvas.lakes.gml
, click the module
v.in.ogr
in the Modules Tree tab. This GRASS module allows
you to import OGR-supported vector files into a GRASS LOCATION
. The
module dialog for v.in.ogr
appears.gml
in the QGIS ‘Alaska’ dataset and select the
file lakes.gml
as OGR file.lakes_grass
and click [Run]. You
don’t have to care about the other options in this example. In the
Output tab you see the currently running GRASS command
v.in.ogr -o dsn=/path/to/lakes.gml output=lakes\_grass
.lakes_grass
vector layer is now imported into GRASS and will be
visualized in the QGIS canvas.It is important to understand the GRASS vector data model prior to digitizing.
In general, GRASS uses a topological vector model.
This means that areas are not represented as closed polygons, but by one or more boundaries. A boundary between two adjacent areas is digitized only once, and it is shared by both areas. Boundaries must be connected and closed without gaps. An area is identified (and labeled) by the centroid of the area.
Besides boundaries and centroids, a vector map can also contain points and lines. All these geometry elements can be mixed in one vector and will be represented in different so-called ‘layers’ inside one GRASS vector map. So in GRASS, a layer is not a vector or raster map but a level inside a vector layer. This is important to distinguish carefully. (Although it is possible to mix geometry elements, it is unusual and, even in GRASS, only used in special cases such as vector network analysis. Normally, you should prefer to store different geometry elements in different layers.)
It is possible to store several ‘layers’ in one vector dataset. For example, fields, forests and lakes can be stored in one vector. An adjacent forest and lake can share the same boundary, but they have separate attribute tables. It is also possible to attach attributes to boundaries. An example might be the case where the boundary between a lake and a forest is a road, so it can have a different attribute table.
The ‘layer’ of the feature is defined by the ‘layer’ inside GRASS. ‘Layer’ is the number which defines if there is more than one layer inside the dataset (e.g., if the geometry is forest or lake). For now, it can be only a number. In the future, GRASS will also support names as fields in the user interface.
Attributes can be stored inside the GRASS LOCATION
as dBase or SQLite3 or
in external database tables, for example, PostgreSQL, MySQL, Oracle, etc.
Attributes in database tables are linked to geometry elements using a ‘category’ value.
‘Category’ (key, ID) is an integer attached to geometry primitives, and it is used as the link to one key column in the database table.
Порада
Learning the GRASS Vector Model
The best way to learn the GRASS vector model and its capabilities is to download one of the many GRASS tutorials where the vector model is described more deeply. See http://grass.osgeo.org/documentation/manuals/ for more information, books and tutorials in several languages.
To create a new GRASS vector layer with the GRASS plugin, click the
Create new GRASS vector toolbar icon.
Enter a name in the text box, and you can start digitizing point, line or polygon
geometries following the procedure described in section Digitizing and editing a GRASS vector layer.
In GRASS, it is possible to organize all sorts of geometry types (point, line and area) in one layer, because GRASS uses a topological vector model, so you don’t need to select the geometry type when creating a new GRASS vector. This is different from shapefile creation with QGIS, because shapefiles use the Simple Feature vector model (see section Creating new Vector layers).
Порада
Creating an attribute table for a new GRASS vector layer
If you want to assign attributes to your digitized geometry features, make sure to create an attribute table with columns before you start digitizing (see figure_grass_digitizing_5).
The digitizing tools for GRASS vector layers are accessed using the
Edit GRASS vector layer icon on the toolbar. Make sure you
have loaded a GRASS vector and it is the selected layer in the legend before
clicking on the edit tool. Figure figure_grass_digitizing_2 shows the GRASS
edit dialog that is displayed when you click on the edit tool. The tools and
settings are discussed in the following sections.
Порада
Digitizing polygons in GRASS
If you want to create a polygon in GRASS, you first digitize the boundary of the polygon, setting the mode to ‘No category’. Then you add a centroid (label point) into the closed boundary, setting the mode to ‘Next not used’. The reason for this is that a topological vector model links the attribute information of a polygon always to the centroid and not to the boundary.
Toolbar
In figure_grass_digitizing_1, you see the GRASS digitizing toolbar icons provided by the GRASS plugin. Table table_grass_digitizing_1 explains the available functionalities.
Figure GRASS digitizing 1:
GRASS Digitizing Toolbar
Table GRASS Digitizing 1: GRASS Digitizing Tools
Category Tab
The Category tab allows you to define the way in which the category values will be assigned to a new geometry element.
Figure GRASS digitizing 2:
GRASS Digitizing Category Tab
Порада
Creating an additional GRASS ‘layer’ with |qg|
If you would like to add more layers to your dataset, just add a new number in the ‘Field (layer)’ entry box and press return. In the Table tab, you can create your new table connected to your new layer.
Settings Tab
The Settings tab allows you to set the snapping in screen pixels. The threshold defines at what distance new points or line ends are snapped to existing nodes. This helps to prevent gaps or dangles between boundaries. The default is set to 10 pixels.
Figure GRASS digitizing 3:
GRASS Digitizing Settings Tab
Symbology Tab
The Symbology tab allows you to view and set symbology and color settings for various geometry types and their topological status (e.g., closed / opened boundary).
Figure GRASS digitizing 4:
GRASS Digitizing Symbology Tab
Table Tab
The Table tab provides information about the database table for a given ‘layer’. Here, you can add new columns to an existing attribute table, or create a new database table for a new GRASS vector layer (see section Creating a new GRASS vector layer).
Figure GRASS digitizing 5:
GRASS Digitizing Table Tab
Порада
GRASS Edit Permissions
You must be the owner of the GRASS MAPSET
you want to edit. It is
impossible to edit data layers in a MAPSET
that is not yours, even
if you have write permission.
The region definition (setting a spatial working window) in GRASS is important
for working with raster layers. Vector analysis is by default not limited to any
defined region definitions. But all newly created rasters will have the spatial
extension and resolution of the currently defined GRASS region, regardless of
their original extension and resolution. The current GRASS region is stored in
the $LOCATION/$MAPSET/WIND
file, and it defines north, south, east and
west bounds, number of columns and rows, horizontal and vertical spatial resolution.
It is possible to switch on and off the visualization of the GRASS region in the QGIS
canvas using the Display current GRASS region button.
With the Edit current GRASS region icon, you can open
a dialog to change the current region and the symbology of the GRASS region
rectangle in the QGIS canvas. Type in the new region bounds and resolution, and
click [OK]. The dialog also allows you to select a new region interactively with your
mouse on the QGIS canvas. Therefore, click with the left mouse button in the QGIS
canvas, open a rectangle, close it using the left mouse button again and click
[OK].
The GRASS module g.region
provides a lot more parameters to define an
appropriate region extent and resolution for your raster analysis. You can use
these parameters with the GRASS Toolbox, described in section The GRASS Toolbox.
The Open GRASS Tools box provides GRASS module functionalities
to work with data inside a selected GRASS
LOCATION
and MAPSET
.
To use the GRASS Toolbox you need to open a LOCATION
and MAPSET
that you have write permission for (usually granted, if you created the MAPSET
).
This is necessary, because new raster or vector layers created during analysis
need to be written to the currently selected LOCATION
and MAPSET
.
Figure GRASS Toolbox 1:
The GRASS shell inside the GRASS Toolbox provides access to almost all (more than 300) GRASS modules in a command line interface. To offer a more user-friendly working environment, about 200 of the available GRASS modules and functionalities are also provided by graphical dialogs within the GRASS plugin Toolbox.
A complete list of GRASS modules available in the graphical Toolbox in QGIS version 2.8 is available in the GRASS wiki at http://grass.osgeo.org/wiki/GRASS-QGIS_relevant_module_list.
It is also possible to customize the GRASS Toolbox content. This procedure is described in section Customizing the GRASS Toolbox.
As shown in figure_grass_toolbox_1, you can look for the appropriate GRASS module using the thematically grouped Modules Tree or the searchable Modules List tab.
By clicking on a graphical module icon, a new tab will be added to the Toolbox dialog, providing three new sub-tabs: Options, Output and Manual.
Options
The Options tab provides a simplified module dialog where you can usually select a raster or vector layer visualized in the QGIS canvas and enter further module-specific parameters to run the module.
Figure GRASS module 1:
The provided module parameters are often not complete to keep the dialog clear. If you want to use further module parameters and flags, you need to start the GRASS shell and run the module in the command line.
A new feature since QGIS 1.8 is the support for a Show Advanced Options
button below the simplified module dialog in the Options tab. At the
moment, it is only added to the module v.in.ascii
as an example of use, but it will
probably be part of more or all modules in the GRASS Toolbox in future versions
of QGIS. This allows you to use the complete GRASS module options without the need
to switch to the GRASS shell.
Output
Figure GRASS module 2:
The Output tab provides information about the output status of the
module. When you click the [Run] button, the module switches to the
Output tab and you see information about the analysis process. If
all works well, you will finally see a Successfully finished
message.
Manual
Figure GRASS module 3:
The Manual tab shows the HTML help page of the GRASS module. You can
use it to check further module parameters and flags or to get a deeper knowledge
about the purpose of the module. At the end of each module manual page, you see
further links to the Main Help index
, the Thematic index
and the
Full index
. These links provide the same information as the
module g.manual
.
Порада
Display results immediately
If you want to display your calculation results immediately in your map canvas, you can use the ‘View Output’ button at the bottom of the module tab.
The following examples will demonstrate the power of some of the GRASS modules.
The first example creates a vector contour map from an elevation raster (DEM).
Here, it is assumed that you have the Alaska LOCATION
set up as explained in section
Importing data into a GRASS LOCATION.
gtopo30
elevation raster by clicking
gtopo30
raster from the demo location.gtopo30
raster should appear as
the Name of input raster.ctour_100
.Successfully finished
appears in the output window. Then click [View Output]
and [Close].Since this is a large region, it will take a while to display. After it finishes rendering, you can open the layer properties window to change the line color so that the contours appear clearly over the elevation raster, as in The Vector Properties Dialog.
Next, zoom in to a small, mountainous area in the center of Alaska. Zooming in close, you will notice that the contours have sharp corners. GRASS offers the v.generalize tool to slightly alter vector maps while keeping their overall shape. The tool uses several different algorithms with different purposes. Some of the algorithms (i.e., Douglas Peuker and Vertex Reduction) simplify the line by removing some of the vertices. The resulting vector will load faster. This process is useful when you have a highly detailed vector, but you are creating a very small-scale map, so the detail is unnecessary.
Порада
The simplify tool
Note that the QGIS fTools plugin has a Simplify geometries ‣ tool that works just like the GRASS v.generalize Douglas-Peuker algorithm.
However, the purpose of this example is different. The contour lines created by
r.contour
have sharp angles that should be smoothed. Among the v.generalize
algorithms, there is Chaiken’s, which does just that (also Hermite splines). Be
aware that these algorithms can add additional vertices to the vector,
causing it to load even more slowly.
Successfully finished
appears in
the output windows, click [View output] and then [Close].Figure GRASS module 4:
Порада
Other uses for r.contour
The procedure described above can be used in other equivalent situations. If you have a raster map of precipitation data, for example, then the same method will be used to create a vector map of isohyetal (constant rainfall) lines.
Several methods are used to display elevation layers and give a 3-D effect to maps. The use of contour lines, as shown above, is one popular method often chosen to produce topographic maps. Another way to display a 3-D effect is by hillshading. The hillshade effect is created from a DEM (elevation) raster by first calculating the slope and aspect of each cell, then simulating the sun’s position in the sky and giving a reflectance value to each cell. Thus, you get sun-facing slopes lighted; the slopes facing away from the sun (in shadow) are darkened.
gtopo30
elevation raster. Start the GRASS
Toolbox, and under the Raster category, double-click to open Spatial analysis
‣ Terrain analysis.gtopo30_shade
for the new hillshade raster, and click [Run].gtopo30
together, move
the hillshade map below the gtopo30
map in the table of contents, then open
the Properties window of gtopo30
, switch to the
Transparency tab and set its transparency level to about 25%.You should now have the gtopo30
elevation with its colormap and transparency
setting displayed above the grayscale hillshade map. In order to see the
visual effects of the hillshading, turn off the gtopo30_shade
map, then turn
it back on.
Using the GRASS shell
The GRASS plugin in QGIS is designed for users who are new to GRASS and not familiar with all the modules and options. As such, some modules in the Toolbox do not show all the options available, and some modules do not appear at all. The GRASS shell (or console) gives the user access to those additional GRASS modules that do not appear in the Toolbox tree, and also to some additional options to the modules that are in the Toolbox with the simplest default parameters. This example demonstrates the use of an additional option in the r.shaded.relief module that was shown above.
Figure GRASS module 5:
The module r.shaded.relief can take a parameter zmult
, which multiplies
the elevation values relative to the X-Y coordinate units so that the hillshade
effect is even more pronounced.
gtopo30
elevation raster as above, then start the GRASS Toolbox
and click on the GRASS shell. In the shell window, type the command
r.shaded.relief map=gtopo30 shade=gtopo30_shade2 azimuth=315 zmult=3
and
press [Enter].gtopo30_shade2
raster to display it in QGIS.gtopo30
raster in
the table of contents, then check the transparency of the colored gtopo30
layer.
You should see that the 3-D effect stands out more strongly compared with the
first shaded relief map.Figure GRASS module 6:
The next example shows how a GRASS module can aggregate raster data and add columns of statistics for each polygon in a vector map.
shapefiles
directory into GRASS.forest_areas
vector and display the types of forests - deciduous,
evergreen, mixed - in different colors: In the layer Properties
window, Symbology tab, choose from Legend type
gtopo30
and forest_areas
.elev
,
and click [Run]. This is a computationally heavy operation, which will run
for a long time (probably up to two hours).forest_areas
attribute table, and verify that several new
columns have been added, including elev_min
, elev_max
, elev_mean
,
etc., for each forest polygon.Another useful feature inside the GRASS Toolbox is the GRASS LOCATION
browser. In figure_grass_module_7, you can see the current working LOCATION
with its MAPSETs
.
In the left browser windows, you can browse through all MAPSETs
inside the
current LOCATION
. The right browser window shows some meta-information
for selected raster or vector layers (e.g., resolution, bounding box, data source,
connected attribute table for vector data, and a command history).
Figure GRASS module 7:
The toolbar inside the Browser tab offers the following tools to manage
the selected LOCATION
:
The Rename selected map and
Delete selected map only work with maps inside your currently selected
MAPSET
. All other tools also work with raster and vector layers in
another MAPSET
.
Nearly all GRASS modules can be added to the GRASS Toolbox. An XML interface is provided to parse the pretty simple XML files that configure the modules’ appearance and parameters inside the Toolbox.
A sample XML file for generating the module v.buffer
(v.buffer.qgm) looks
like this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE qgisgrassmodule SYSTEM "http://mrcc.com/qgisgrassmodule.dtd">
<qgisgrassmodule label="Vector buffer" module="v.buffer">
<option key="input" typeoption="type" layeroption="layer" />
<option key="buffer"/>
<option key="output" />
</qgisgrassmodule>
The parser reads this definition and creates a new tab inside the Toolbox when you select the module. A more detailed description for adding new modules, changing a module’s group, etc., can be found on the QGIS wiki at http://hub.qgis.org/projects/quantum-gis/wiki/Adding_New_Tools_to_the_GRASS_Toolbox.