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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

Importing data from text files

Data can be imported from text files, providing some additional about how the geometry information is stored in the text. This recipe shows you how to create a new points layer, based on a text file.

How to do it…

  1. Select the Add delimited text layer menu entry from the Layer menu. You will see a dialog like the following one:
    How to do it…
  2. In the upper field, enter the path to the elev_lid792_randpts.csv file in the sample dataset. That file contains a points layer as text.
  3. Once you enter the file path or select it in the file browser that can be opened by clicking on the Browse button, the fields in the lower part of the dialog will be filled, as shown in the following screenshot:
    How to do it…

    Note

    We are using a CSV file that has values separated by commas, so you must select the CSV option in the Format field.

    The X field and Y field drop-down lists will be populated with the fields that are available, which are described in the first line of the text file. Select X for X field and Y for Y field. Now, QGIS knows how to create the geometries and has enough information to create a new layer from the text file.

  4. Enter a name for the layer in the Layer name field and click on OK. The layer will be added to the QGIS project, as shown in the following screenshot:
    How to do it…
  5. No information about the CRS is contained in the text file or entered in the parameters dialog, so it must be added manually. In this case, the CRS used is EPSG:3358. To set this as the CRS of the layer, right-click on the layer name and select Set layer CRS:
    How to do it…
  6. In the CRS selection dialog, select the EPSG:3358 CRS and click on OK. The layer now has the correct CRS.

How it works…

Data is read from the text file and processed to create geometries. All the fields in the table (all data in a row in the text file) are also added, including the ones used to create the geometries, as you will see by right-clicking on the layer and selecting Open attribute table, as shown in the following screenshot:

How it works…

Along with the CSV file, this file may contains a CSVT file, which describes the types of the fields. This is used by QGIS to set the appropriate type for the attributes table of the layer. If the CSVT file is missing, as in our example's case, QGIS will try to figure out the type based on the values for each field.

There's more…

Layers created from text files are not restricted to point files. Any geometry can be created from the text data. However, if it is not a point, instead of selecting two columns, you must place all the geometry information in a single one and enter a text representation of the geometry. QGIS uses the Well-Known Text (WKT) format, which is a text markup language for vector geometries, to describe geometries as strings. Here is an example of a very simple CSV file with line features and two attributes:

geom,id,elevation
LINESTRING(0 1, 0 2, 1 3),1,50
LINESTRING(0 -1, 0 -2, 1 -3),2,60
LINESTRING(0 1, 0 3, 5 4),3,70

See also

End of Section 5
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