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QGIS 2 Cookbook

QGIS 2 Cookbook

By : Mandel, Olaya Ferrero, Anita Graser
5 (1)
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QGIS 2 Cookbook

QGIS 2 Cookbook

5 (1)
By: Mandel, Olaya Ferrero, Anita Graser

Overview of this book

QGIS is a user-friendly, cross-platform desktop geographic information system used to make maps and analyze spatial data. QGIS allows users to understand, question, interpret, and visualize spatial data in many ways that reveal relationships, patterns, and trends in the form of maps. This book is a collection of simple to advanced techniques that are needed in everyday geospatial work, and shows how to accomplish them with QGIS. You will begin by understanding the different types of data management techniques, as well as how data exploration works. You will then learn how to perform classic vector and raster analysis with QGIS, apart from creating time-based visualizations. Finally, you will learn how to create interactive and visually appealing maps with custom cartography. By the end of this book, you will have all the necessary knowledge to handle spatial data management, exploration, and visualization tasks in QGIS.
Table of Contents (14 chapters)
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Preface
Preface
What this book covers
What you need for this book
Who this book is for
Sections
Conventions
Reader feedback
Customer support
Free Chapter
1
1. Data Input and Output
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1. Data Input and Output
Introduction
Finding geospatial data on your computer
Describing data sources
Importing data from text files
Importing KML/KMZ files
Importing DXF/DWG files
Opening a NetCDF file
Saving a vector layer
Saving a raster layer
Reprojecting a layer
Batch format conversion
Batch reprojection
Loading vector layers into SpatiaLite
Loading vector layers into PostGIS
2
2. Data Management
2. Data Management
Introduction
Joining layer data
Cleaning up the attribute table
Configuring relations
Joining tables in databases
Creating views in SpatiaLite
Creating views in PostGIS
Creating spatial indexes
Georeferencing rasters
Georeferencing vector layers
Creating raster overviews (pyramids)
Building virtual rasters (catalogs)
3
3. Common Data Preprocessing Steps
3. Common Data Preprocessing Steps
Introduction
Converting points to lines to polygons and back – QGIS
Converting points to lines to polygons and back – SpatiaLite
Converting points to lines to polygons and back – PostGIS
Cropping rasters
Clipping vectors
Extracting vectors
Converting rasters to vectors
Converting vectors to rasters
Building DateTime strings
Geotagging photos
4
4. Data Exploration
4. Data Exploration
Introduction
Listing unique values in a column
Exploring numeric value distribution in a column
Exploring spatiotemporal vector data using Time Manager
Creating animations using Time Manager
Designing time-dependent styles
Loading BaseMaps with the QuickMapServices plugin
Loading BaseMaps with the OpenLayers plugin
Viewing geotagged photos
5
5. Classic Vector Analysis
5. Classic Vector Analysis
Introduction
Selecting optimum sites
Dasymetric mapping
Calculating regional statistics
Estimating density heatmaps
Estimating values based on samples
6
6. Network Analysis
6. Network Analysis
Introduction
Creating a simple routing network
Calculating the shortest paths using the Road graph plugin
Routing with one-way streets in the Road graph plugin
Calculating the shortest paths with the QGIS network analysis library
Routing point sequences
Automating multiple route computation using batch processing
Matching points to the nearest line
Creating a routing network for pgRouting
Visualizing the pgRouting results in QGIS
Using the pgRoutingLayer plugin for convenience
Getting network data from the OSM
7
7. Raster Analysis I
7. Raster Analysis I
Introduction
Using the raster calculator
Preparing elevation data
Calculating a slope
Calculating a hillshade layer
Analyzing hydrology
Calculating a topographic index
Automating analysis tasks using the graphical modeler
8
8. Raster Analysis II
8. Raster Analysis II
Introduction
Calculating NDVI
Handling null values
Setting extents with masks
Sampling a raster layer
Visualizing multispectral layers
Modifying and reclassifying values in raster layers
Performing supervised classification of raster layers
9
9. QGIS and the Web
9. QGIS and the Web
Introduction
Using web services
Using WFS and WFS-T
Searching CSW
Using WMS and WMS Tiles
Using WCS
Using GDAL
Serving web maps with the QGIS server
Scale-dependent rendering
Hooking up web clients
Managing GeoServer from QGIS
10
10. Cartography Tips
10. Cartography Tips
Introduction
Using Rule Based Rendering
Handling transparencies
Understanding the feature and layer blending modes
Saving and loading styles
Configuring data-defined labels
Creating custom SVG graphics
Making pretty graticules in any projection
Making useful graticules in printed maps
Creating a map series using Atlas
11
11. Extending QGIS
11. Extending QGIS
Introduction
Defining custom projections
Working near the dateline
Working offline
Using the QspatiaLite plugin
Adding plugins with Python dependencies
Using the Python console
Writing Processing algorithms
Writing QGIS plugins
Using external tools
12
12. Up and Coming
12. Up and Coming
Introduction
Preparing LiDAR data
Opening File Geodatabases with the OpenFileGDB driver
Using Geopackages
The PostGIS Topology Editor plugin
The Topology Checker plugin
GRASS Topology tools
Hunting for bugs
Reporting bugs
13
Index
Index

Loading vector layers into PostGIS

PostGIS is the spatial add-on to the popular PostgreSQL database. It's a server-style database with authentication, permissions, schemas, and handling of simultaneous users. When you want to store large amounts of vector data and query them efficiently, especially in a multicomputer networked environment, consider PostGIS. This works fine for small data too, but many users find its configuration too much work when SpatiaLite may be better suited.

Getting ready

Pick a vector layer and load it in QGIS. You will also need to have a working copy of Postgres/PostGIS running, a PostGIS database created, and an account that allows table creation.

Tip

BostonGIS maintains a decent tutorial on installation for Windows, and getting a PostGIS set up for everyone. You can find this at http://www.bostongis.com/?content_name=postgis_tut01#316.

You should configure QGIS to be aware of your database and its connection parameters by creating a new database item in the PostGIS load dialog or by right-clicking on PostGIS in the Browser tab and selecting New Connection:

You can find more information about PostGIS at http://docs.qgis.org/2.8/en/docs/user_manual/working_with_vector/supported_data.html#postgis-layers.

How to do it…

Now that you can connect to a PostGIS database, you are ready to try importing data:

  1. Open DB Manager from the Database menu.
  2. Expand the PostGIS item to list your databases. Expand the database that you want to connect to, and you should be prompted to authenticate (if you haven't saved your password in the settings).
  3. Expand the list and select the Public schema.

    Tip

    In general, unless you are performing advanced work and understand how Postgres schemas work, place your layers in the Public schema. This is the default that everyone expects.

  4. Click on the following import layer icon:
    How to do it…
  5. A dialog will pop up, providing you with import options.

    Tip

    SQL databases are usually case insensitive, so you can use all lowercase. Also, never use spaces or special characters in table names; this can just lead to headaches later. An occasional underscore is okay.

  6. Select the layer to import from the drop-down list.
  7. Fill in a name for the new table.
  8. Check whether schema is set to public.
  9. In most cases, the only thing left to do is check the Create spatial index checkbox:
    How to do it…

How it works…

QGIS converts your geometries to a format that is compatible with PostGIS, and inserts it, along with importing the attributes. Afterwards, it updates the metadata views in PostGIS to register the geometry column and build the spatial index on it. These two post-processes make the database table appear as a spatial layer to QGIS and speed up the loading of data from the table when panning and zooming.

There's more…

The options presented in the dialog are not all the options that are available. If you need more control or advanced options present, you'll likely be looking at the command-line tools: shp2pgsql (a graphical plugin for pgadmin3 is available on some platforms) and ogr2ogr. The shp2pgsql tool generally only handles shapefiles. If you have other formats, ogr2ogr can handle everything that QGIS is capable of loading. You can also use these tools to develop batch import scripts.

To import large or complicated CSV or text files, you sometimes will need to use the pgadmin3 or psql command-line interface to Postgres.

Need even more control? Then, consider scripting. OGR and Postgres both have very capable Python libraries.

Another option is using the OpenGeo Suite plugin, which has some additional options, such as allowing importing multiple layers into a single table or into one table per layer. To learn more about this, including how to install it, refer to http://qgis.boundlessgeo.com/static/docs/intro.html.

What happens if this fails? Databases can be really picky sometimes:

  • It could be character encoding (accents, non-Latin languages), which requires specifying the encoding.
  • It could be picky about mixing multilayers with regular layers. Multilayers is when you have several separate geometries that are part of one record. For example, Hawaii is actually many islands. So, if you only have one row representing Hawaii, you need to cram all the island polygons into one geometry field. However, if you mix this with North Dakota that is just a polygon, the import will fail. If you have this problem, you'll need to perform the import on the command-line using ogr2ogr and its new feature, -nlt PROMOTE_TO_MULTI, which converts all single items to multi-items, to fix this.
  • Depending on your original source, you may have a mix of points, lines, and polygons. You'll either need to convert this to a Geometry Collection, or you need to split each type of geometry into a separate layer. Geometry Collections are currently poorly supported in many GIS viewers, so this is only recommended for advanced users.

See also

For more information on PostGIS installation and setup, refer to http://postgis.net/install.

For a more in-depth text on using PostGIS, there are many books available, including Packt Publishing's PostGIS Cookbook.

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