【John Van Hoesen】Mastering QGIS

Mastering QGIS

By: Dr. John Van Hoesen, GISP; Dr. Luigi Pirelli; Dr. Richard Smith Jr., GISP; Kurt Menke, GISP

Publisher: Packt Publishing

Pub. Date: March 30, 2015

Web ISBN-13: 978-1-78439-006-8

Print ISBN-13: 978-1-78439-868-2

Pages in Print Edition: 420

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1. Importing KML into SpatiaLite
   
2. To import a KML file into a SpatiaLite database, complete the following steps:
   
3. 
   
4. Open DB Manager by clicking on DB Manager under Database. Expand SpatiaLite and select GiffordPinochet.sqlite on the Tree panel.
   
5. Navigate to Table | Import layer/file to open the Import vector layer dialog.
   
6. Click on the ellipsis button at the right-hand side of the Input drop-down box and select and open streams.kml from the sample dataset that is available for download on the Packt Publishing website.
   
7. Click on the Update options button to load the remainder of the dialog box. The output table name will populate as streams, and it will match the base name of the input file.
   
8. Set the following options as shown in the next screenshot:
   
9. Select Source SRID and enter 4326. This is the EPSG code for all KML datasets.
   
10. Select Target SRID and enter 26910. This is the EPSG code for NAD 83/UTM Zone 10 North.
    
11. Select Create spatial index.
    
12. Refer the following screenshot to make sure your settings match. If so, click on the OK button to import the file.
    
13. Note

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1. Importing a shapefile into SpatiaLite
   
2. Open DB Manager by clicking on DB Manager under Database. Expand SpatiaLite and select GiffordPinochet.sqlite on the Tree panel.
   
3. Navigate to Table | Import layer/file to open the Import vector layer dialog, as shown in the following screenshot.
   
4. Click on the ellipsis button at the right-hand side of the Input drop-down box and select and open NF_roads.shp from the sample dataset that is available for download on the Packt Publishing website.
   
5. Click on the Update options button to load the remainder of the dialog box. The output table name will populate as NF_roads, and it will match the base name of the input file.
   
6. Set the following options:
   
7. Select Source SRID and enter 26910. This is the EPSG code for NAD 83/UTM Zone 10 North. Since we don't want to change the coordinate system during import, we do not need to set Target SRID.
   
8. Select Create spatial index.
   
9. Click on the OK button to import the file.

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1. Importing tables into SpatiaLite
   
2. To import a table file into a SpatiaLite database, complete the following steps:
   
3. 
   
4. Open DB Manager by clicking on DB Manager under Database. Expand SpatiaLite and select GiffordPinochet.sqlite on the Tree panel.
   
5. Navigate to Table | Import layer/file to open the Import vector layer dialog.
   
6. Click on the ellipsis button to the right-hand side of the Input drop-down box and select and open Waterfalls.xls from the sample dataset that is available for download on the Packt Publishing website.
   
7. Click on the Update options button to load the remainder of the dialog box. The output table name will populate as Waterfalls, and it match the base name of the input file. Note that all options related to spatial datasets are not modifiable and are grayed out (as shown in in the following screenshot). This is because SpatiaLite treats the input as a nonspatial table, even though it has coordinates stored in the table. We will add the spatial component to the table in a later step.
   
8. Click on the OK button to import the file.

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1. Exporting tables out of SpatiaLite as a shapefile
   
2. 
   
3. To export a table as a shapefile, perform the following steps:
   
4. 
   
5. Open DB Manager by clicking on DB Manager under Database. Expand SpatiaLite and select the database from which you wish to export a table in the Tree panel.
   
6. In the Tree panel, select the table that you wish to export.
   
7. Navigate to Table | Export to file to open the Export vector file dialog.
   
8. Click on the ellipsis button at the right-hand side of the Output file text box and name the output file. Note that you can only export to the shapefile format using this tool.
   
9. Set the Source SRID, Target SRID, and Encoding options or leave them unselected to use the default values. Select Drop existing one if you wish to overwrite an existing shapefile.

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1. Creating a SQL query
   
2. To create a SQL query, perform the following steps:
   
3. 
   
4. Open DB Manager by clicking on DB Manager under Database.
   
5. In the Tree panel, navigate to and select the database on which you wish to perform a SQL query.
   
6. Navigate to Database | SQL window, or press F2 on your keyboard, to open the SQL window.
   
7. Enter a SQL query in the textbox at the top. Click on the Execute button or F5 on your keyboard to execute the SQL query against the database. The results of the query will be displayed in the results box at the bottom, and the number of affected rows and execution time will appear next to the Execute button. An example of a successfully run query is shown in the following screenshot:

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1. Creating a spatial view
   
2. Creating a spatial view on a SpatiaLite database using the SQL window in DB Manager is a two-step process. The first step is to create a view that includes a field with unique identifiers and the geometry column. The second step is to insert a new record in the views_geometry_columns table to register the view as a spatial view. In this section, we will create a spatial view on the Waterfalls table to show all the waterfalls in the Mowich Lake quad; to do this, perform the following steps:
   
3. 
   
4. Open DB Manager by clicking on DB Manager under Database.
   
5. In the Tree panel, navigate to and select the GiffordPinochet.sqlite database.
   
6. Navigate to Database | SQL window, or press F2 on your keyboard, to open the SQL window.
   
7. Enter the following query:
   
8. CREATE VIEW mowich_lake_waterfalls AS
   
9. SELECT w.pk as ROWID, w.NAME, w.TYPE, w.geom from Waterfalls as w
   
10. WHERE w.quadname = 'Mowich Lake';
    
11. In the CREATE VIEW query, two fields are required to be included in the SELECT statement: the unique identifier field should be renamed to ROWID and the geometry field. You must rename the unique identifier to ROWID or the view cannot be registered as a spatial view.
    
12. 
    
13. Click on the Execute button to create the view. The following screenshot displays a successfully written and executed view of the Waterfalls table:

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1. Paletted raster band rendering
   
2. The paletted raster band renderer applies a single color to a single raster value. QGIS supports the loading of rasters with paletted colors stored within and the changing the color assigned to the raster value. QGIS does not currently support the creation of color palettes for single band rendering. However, existing QGIS layer style files (.qml) that contain palettes can be applied by clicking on the Load Style button in the layer properties.
   
3. 
   
4. As an example of a raster with a color palette stored within it, add NA LC 1km.tif from the sample data to the QGIS canvas and open the Style tab under Layer Properties. The following figure shows the Paletted band renderer being applied to

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1. gray raster band rendering
   
2. Singleband gray raster band rendering
   
3. The singleband gray band renderer stretches a gradient between black and white to a single raster band. Additionally, contrast enhancements are available to adjust the way the gradient is stretched across the raster band's values. Let's apply a singleband gray renderer and a contrast enhancement to the sample GRAY_50M_SR_W.tif raster file that represents shaded relief, hypsography, and flat water for Earth.

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1. Singleband pseudocolor raster band rendering
   
2. The singleband pseudocolor band renderer stretches a color ramp to a single raster band. Additionally, three Color interpolation methods are available to adjust the way the color ramp is stretched across the raster band's values with respect to the min and max cell values (for a discussion on determining min and max values, see the preceding section).

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