「Grass integration」の版間の差分

2013年9月18日 (水) 12:53時点における最新版

GRASS GISの機能を利用する

The GRASS plugin provides access to GRASS GIS [4] databases and functionalities. This includes visualization of GRASS raster and vector layers, digitizing vector layers, editing vector attributes, creating new vector layers and analysing GRASS 2D and 3D data with more than 300 GRASS modules.

In this Section we'll introduce the plugin functionalities and give some examples on managing and working with GRASS data. Following main features are provided with the toolbar menu, when you start the GRASS plugin, as described in Section 9.1:

Open mapset
New mapset
Close mapset
Add GRASS vector layer
Add GRASS raster layer
Create new GRASS vector
Edit GRASS vector layer
Open GRASS tools
Display current GRASS region
Edit current GRASS region

GRASSプラグインを使ってみよう

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 click 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 9.2). Or you create a new GRASS LOCATION with QGIS (see Section 9.3.1) and import some raster and vector data (see Section 9.4) for further analysis with the GRASS Toolbox (see Section 9.9).

GRASSラスター、ベクターレイヤをロードする

With the GRASS plugin, you can load vector or raster layers using the appropriate button on the toolbar menu. As an example we use the QGIS alaska dataset (see Section label_sampledata). It includes a small sample GRASS LOCATION with 3 vector layers and 1 raster elevation map.

Create a new folder grassdata, download the QGIS alaska dataset qgis_sample_data.zip from http://download.osgeo.org/qgis/data/ and unzip the file into grassdata.
Start QGIS.
If not already done in a previous QGIS session, load the GRASS plugin clicking on Plugins > Manage Plugins and selecting GRASS. The GRASS toolbar appears on the toolbar menu.
In the GRASS toolbar, click the Open mapset icon to bring up the MAPSET wizard.
For Gisdbase browse and select or enter the path to the newly created folder grassdata.
You should now be able to select the LOCATION alaska and the MAPSET demo.
Click OK. Notice that some previously disabled tools in the GRASS toolbar are now enabled.
Click on Add GRASS raster layer, choose the map name gtopo30 and click OK. The elevation layer will be visualized.
Click on Add GRASS vector layer, choose the map name 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 sec:vectorprops, e.g. change opacity, fill and outline color.
Also load the other two vector layers rivers and airports and adapt their properties.

As you see, it is very simple to load GRASS raster and vector layers in QGIS. See 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/data.php .

Tip 34 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 9.1.

GRASSのLOCATIONとMAPSETについて

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 LOCATION. 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, subregions, or as workspaces for individual team members (Neteler & Mitasova 2008 [2]). In order to analyze vector and raster layers with GRASS modules, you must import them into a GRASS LOCATION. <ref>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 data sets without importing them. But because this is not the usual way for beginners to work with GRASS, this functionality will not be described here.</ref>

**図 25:** GRASS data in the alaska LOCATION (adapted from Neteler & Mitasova 2008 [2]

Creating a new GRASS LOCATION

As an an example you find the instructions 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 GIS related chapters. It is useful to download and install the dataset on your computer label_sampledata).

**図 26:** Creating a new GRASS LOCATION or a new MAPSET in QGIS

Start QGIS and make sure the GRASS plugin is loaded
Visualize the alaska.shp Shapefile (see Section sec:load_shapefile) from the QGIS alaska dataset~label_sampledata.
In the GRASS toolbar, click on the Open mapset icon to bring up the MAPSET wizard.
Select an existing GRASS database (GISDBASE) folder grassdata or create one for the new LOCATION using a file manager on your computer. Then click Next.
We can use this wizard to create a new MAPSET within an existing LOCATION (see Section~sec:add_mapset) or to create a new LOCATION altogether. Click on the radio button Create new location (see Figure 26).
Enter a name for the LOCATION - we used alaska and click Next
Define the projection by clicking on the radio button Projection to enable the projection list
We are using Albers Equal Area Alaska (feet) projection. Since we happen to know that it is represented by the EPSG ID 2964, we enter it in the search box. (Note: If you want to repeat this process for another LOCATION and projection and haven't memorized the EPSG ID, click on the projector icon in the lower right-hand corner of the status bar (see Section label_projstart)).
Click Find to select the projection
Click Next
To define the default region, we have to enter the LOCATION bounds in north, south, east, and west direction. Here we simply click on the button Set current QGIS extent, to apply the extend of the loaded layer alaska.shp as the GRASS default region extend.
Click Next
We also need to define a MAPSET within our new LOCATION. You can name it whatever you like - we used demo. <ref>{When creating a new LOCATION, GRASS automatically creates a special MAPSET called PERMANENT designed to store the core data for the project, its default spatial extend and coordinate system definitions (Neteler & Mitasova 2008 [2]).</ref>
Check out the summary to make sure it's correct and click Finish
The new LOCATION alaska and two MAPSETs demo and PERMANENT are created. The currently opened working set is MAPSET demo, as you defined.
Notice that some of the tools in the GRASS toolbar that were disabled are now enabled.

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 9.4). You can also use the already existing vector and raster data in the sample GRASS LOCATION alaska included in the QGIS alaska dataset label_sampledata and move on to Section 9.5.

Adding a new MAPSET

A user has only write access to a GRASS MAPSET he created. This means, besides access to his own MAPSET, each user can also read maps in other user's MAPSETs, but he can modify or remove only the maps in his own MAPSET. All MAPSETs include a WIND file that stores the current boundary coordinate values and the currently selected raster resolution (Neteler & Mitasova 2008 [2], see Section 9.8).

Start QGIS and make sure the GRASS plugin is loaded
In the GRASS toolbar, click on the New mapset icon to bring up the MAPSET wizard.
Select the GRASS database (GISDBASE) folder grassdata with the LOCATION alaska, where we want to add a further MAPSET, called test.
Click Next.
We can use this wizard to create a new MAPSET within an existing LOCATION or to create a new LOCATION altogether. Click on the radio button Select location (see Figure 26) and click Next.
Enter the name text for the new MAPSET. Below in the wizard you see a list of existing MAPSETs and its owners.
Click Next, check out the summary to make sure it's all correct and click Finish

GRASS LOCATIONのインポート

This Section gives an example how to import raster and vector data into the alaska GRASS LOCATION provided by the QGIS alaska dataset. Therefore we use a landcover raster map landcover.img and a vector GML File lakes.gml from the QGIS alaska dataset label_sampledata.

Start QGIS and make sure the GRASS plugin is loaded.
In the GRASS toolbar, click the Open MAPSET icon to bring up the MAPSET wizard.
Select as GRASS database the folder grassdata in the QGIS alaska dataset, as LOCATION alaska, as MAPSET demo and click OK.
Now click the Open GRASS tools icon. The GRASS Toolbox (see Section 9.9) dialog appears.
To import the raster map landcover.img, click the module r.in.gdal in the Modules Tree tab. This GRASS module allows to import GDAL supported raster files into a GRASS LOCATION. The module dialog for r.in.gdal appears.
Browse to the folder raster in the QGIS alaska dataset and select the file landcover.img.
As raster output name define 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.
When it says Succesfully finished click View output. The landcover_grass raster layer is now imported into GRASS and will be visualized in the QGIS canvas.
To import the vector GML file lakes.gml, click the module v.in.ogr in the Modules Tree tab. This GRASS module allows to import OGR supported vector files into a GRASS LOCATION. The module dialog for v.in.ogr appears.
Browse to the folder gml in the QGIS alaska dataset and select the file lakes.gml as OGR file.
As vector output name define 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.
When it says Succesfully finished click View output. The lakes_grass vector layer is now imported into GRASS and will be visualized in the QGIS canvas.

GRASSベクターデータモデル

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 without gaps. An area is identified (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. <ref>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 prefere to store different geometry elements in different layers.</ref> It is possible to store more 'layers' in one vector dataset. For example, fields, forests and lakes can be stored in one vector. Adjacent forest and lake can share the same boundary, but they have separate attribute tables. It is also possible to attach attributes to boundaries. For example, the boundary between lake and forest is a road, so it can have a different attribute table.

The 'layer' of the feature is defined by 'layer' inside GRASS. 'Layer' is the number which defines if there are 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.

Tip 35 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/gdp/manuals.php for more information, books and tutorials in several languages.

新規GRASSベクターレイヤを作る

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 polygone geometries, following the procedure described in Section 9.7.

In GRASS it is possible to organize all sort 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 sec:create shape).

Tip 36 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 31).

GRASSベクターレイヤのデジタイズと編集

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 28 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.

Tip 37 Digitizing polygones in GRASS

If you want to create a polygone in GRASS, you first digitize the boundary of the polygone, 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 is, that a topological vector model links attribute information of a polygon always to the centroid and not to the boundary.

Toolbar

In Figure 27 you see the GRASS digitizing toolbar icons provided by the GRASS plugin. Table tab:grass_tools explains the available functionalities.

**図 27:** GRASS Digitizing Toolbar

**表 3:** GRASS Digitizing Tools
Icon	Tool	Purpose
	New Point	Digitize new point
	New Line	Digitize new line (finish by selecting new tool)
	New Boundary	Digitize new boundary (finish by selecting new tool)
	New Centroid	Digitize new centroid (label existing area)
	Move vertex	Move one vertex of existing line or boundary and identify new position
	Add vertex	Add a new vertex to existing line
	Delete vertex	Delete vertex from existing line (confirm selected vertex by another click)
	Move element	Move selected boundary, line, point or centroid and click on new position
	Split line	Split an existing line to 2 parts
	Delete element	Delete existing boundary, line, point or centroid (confirm selected element by another click)
	Edit attributes	Edit attributes of selected element (note that one element can represent more features, see above)
	Close	Close session and save current status (rebuilds topology afterwards)

Category Tab

The Category tab allows you to define the way in which the category values will be assigned to a new geometry element.

**図 28:** GRASS Digitizing Category Tab

Mode: what category value shall be applied to new geometry elements.
- Next not used - apply next not yet used category value to geometry element.
- Manual entry - manually define the category value for the geometry element in the 'Category'-entry field.
- No category - Do not apply a category value to the geometry element. This is e.g. used for area boundaries, because the category values are connected via the centroid.
Category - A number (ID) is attached to each digitized geometry element. It is used to connect each geometry element with its attributes.
Field (layer) - Each geometry element can be connected with several attribute tables using different GRASS geometry layers. Default layer number is 1.

Tip 38 Creating an additional GRASS 'layer' with QGIS

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.

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GRASS region ツール

The region definition (setting a spatial working window) in GRASS is important for working with raster layers. Vector analysis is per default not limited to any defined region definitions. 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/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. It also allows 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 provide a lot more parameters to define an appropriate region extend and resolution for your raster analysis. You can use these parameters with the GRASS Toolbox, described in Section 9.9.

GRASSツールボックス

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 where you have write-permission (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.

Working with GRASS modules

図 32: GRASS Toolbox and searchable Modules List

(a) Modules Tree

(b) Searchable Modules List

The GRASS Shell inside the GRASS Toolbox provides access to almost all (more than 300) GRASS modules in command line modus. To offer a more user friendly working environment, about 200 of the available GRASS modules and functionalities are also provided by graphical dialogs. These dialogs are grouped in thematic blocks, but are searchable as well. You find a complete list of GRASS modules available in QGIS version \CURRENT in appendix appdx_grass_toolbox_modules. It is also possible to customize the GRASS Toolbox content. It is described in Section sec:toolbox-customizing.

As shown in Figure 32, you can look for the appropriate GRASS module using the thematically grouped Modules Tree or the searchable Modules List tab.

Clicking on a grapical module icon a new tab will be added to the toolbox dialog providing three new sub-tabs Options, Output and Manual. In Figure 33 you see an example for the GRASS module v.buffer.

図 33: GRASS Toolbox Module Dialogs

(a) Module Options

(b) Modules Output

(c) Module 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. 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.

Output

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

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 if you use the module g.manual

Tip 40 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.

Working with the GRASS LOCATION browser

Another useful feature inside the GRASS Toolbox is the GRASS LOCATION browser. In Figure 34 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.

**図 34:** GRASS LOCATION browser

The toolbar inside the Browser tab offers following tools to manage the selected LOCATION:

Add selected map to canvas
Copy selected map
Rename selected map
Delete selected map
Set current region to selected map
Refresh browser window

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.

Customizing the GRASS Toolbox

Nearly all GRASS modules can be added to the GRASS toolbox. A XML interface is provided to parse the pretty simple XML files which configures 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 the modules group, etc. can be found on the QGIS wiki at
http://wiki.qgis.org/qgiswiki/Adding_New_Tools_to_the_GRASS_Toolbox .