Table of Contents for
QGIS: Becoming a GIS Power User

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition QGIS: Becoming a GIS Power User by Alexander Bruy Published by Packt Publishing, 2017
  1. Cover
  2. Table of Contents
  3. QGIS: Becoming a GIS Power User
  4. QGIS: Becoming a GIS Power User
  5. QGIS: Becoming a GIS Power User
  6. Credits
  7. Preface
  8. What you need for this learning path
  9. Who this learning path is for
  10. Reader feedback
  11. Customer support
  12. 1. Module 1
  13. 1. Getting Started with QGIS
  14. Running QGIS for the first time
  15. Introducing the QGIS user interface
  16. Finding help and reporting issues
  17. Summary
  18. 2. Viewing Spatial Data
  19. Dealing with coordinate reference systems
  20. Loading raster files
  21. Loading data from databases
  22. Loading data from OGC web services
  23. Styling raster layers
  24. Styling vector layers
  25. Loading background maps
  26. Dealing with project files
  27. Summary
  28. 3. Data Creation and Editing
  29. Working with feature selection tools
  30. Editing vector geometries
  31. Using measuring tools
  32. Editing attributes
  33. Reprojecting and converting vector and raster data
  34. Joining tabular data
  35. Using temporary scratch layers
  36. Checking for topological errors and fixing them
  37. Adding data to spatial databases
  38. Summary
  39. 4. Spatial Analysis
  40. Combining raster and vector data
  41. Vector and raster analysis with Processing
  42. Leveraging the power of spatial databases
  43. Summary
  44. 5. Creating Great Maps
  45. Labeling
  46. Designing print maps
  47. Presenting your maps online
  48. Summary
  49. 6. Extending QGIS with Python
  50. Getting to know the Python Console
  51. Creating custom geoprocessing scripts using Python
  52. Developing your first plugin
  53. Summary
  54. 2. Module 2
  55. 1. Exploring Places – from Concept to Interface
  56. Acquiring data for geospatial applications
  57. Visualizing GIS data
  58. The basemap
  59. Summary
  60. 2. Identifying the Best Places
  61. Raster analysis
  62. Publishing the results as a web application
  63. Summary
  64. 3. Discovering Physical Relationships
  65. Spatial join for a performant operational layer interaction
  66. The CartoDB platform
  67. Leaflet and an external API: CartoDB SQL
  68. Summary
  69. 4. Finding the Best Way to Get There
  70. OpenStreetMap data for topology
  71. Database importing and topological relationships
  72. Creating the travel time isochron polygons
  73. Generating the shortest paths for all students
  74. Web applications – creating safe corridors
  75. Summary
  76. 5. Demonstrating Change
  77. TopoJSON
  78. The D3 data visualization library
  79. Summary
  80. 6. Estimating Unknown Values
  81. Interpolated model values
  82. A dynamic web application – OpenLayers AJAX with Python and SpatiaLite
  83. Summary
  84. 7. Mapping for Enterprises and Communities
  85. The cartographic rendering of geospatial data – MBTiles and UTFGrid
  86. Interacting with Mapbox services
  87. Putting it all together
  88. Going further – local MBTiles hosting with TileStream
  89. Summary
  90. 3. Module 3
  91. 1. Data Input and Output
  92. Finding geospatial data on your computer
  93. Describing data sources
  94. Importing data from text files
  95. Importing KML/KMZ files
  96. Importing DXF/DWG files
  97. Opening a NetCDF file
  98. Saving a vector layer
  99. Saving a raster layer
  100. Reprojecting a layer
  101. Batch format conversion
  102. Batch reprojection
  103. Loading vector layers into SpatiaLite
  104. Loading vector layers into PostGIS
  105. 2. Data Management
  106. Joining layer data
  107. Cleaning up the attribute table
  108. Configuring relations
  109. Joining tables in databases
  110. Creating views in SpatiaLite
  111. Creating views in PostGIS
  112. Creating spatial indexes
  113. Georeferencing rasters
  114. Georeferencing vector layers
  115. Creating raster overviews (pyramids)
  116. Building virtual rasters (catalogs)
  117. 3. Common Data Preprocessing Steps
  118. Converting points to lines to polygons and back – QGIS
  119. Converting points to lines to polygons and back – SpatiaLite
  120. Converting points to lines to polygons and back – PostGIS
  121. Cropping rasters
  122. Clipping vectors
  123. Extracting vectors
  124. Converting rasters to vectors
  125. Converting vectors to rasters
  126. Building DateTime strings
  127. Geotagging photos
  128. 4. Data Exploration
  129. Listing unique values in a column
  130. Exploring numeric value distribution in a column
  131. Exploring spatiotemporal vector data using Time Manager
  132. Creating animations using Time Manager
  133. Designing time-dependent styles
  134. Loading BaseMaps with the QuickMapServices plugin
  135. Loading BaseMaps with the OpenLayers plugin
  136. Viewing geotagged photos
  137. 5. Classic Vector Analysis
  138. Selecting optimum sites
  139. Dasymetric mapping
  140. Calculating regional statistics
  141. Estimating density heatmaps
  142. Estimating values based on samples
  143. 6. Network Analysis
  144. Creating a simple routing network
  145. Calculating the shortest paths using the Road graph plugin
  146. Routing with one-way streets in the Road graph plugin
  147. Calculating the shortest paths with the QGIS network analysis library
  148. Routing point sequences
  149. Automating multiple route computation using batch processing
  150. Matching points to the nearest line
  151. Creating a routing network for pgRouting
  152. Visualizing the pgRouting results in QGIS
  153. Using the pgRoutingLayer plugin for convenience
  154. Getting network data from the OSM
  155. 7. Raster Analysis I
  156. Using the raster calculator
  157. Preparing elevation data
  158. Calculating a slope
  159. Calculating a hillshade layer
  160. Analyzing hydrology
  161. Calculating a topographic index
  162. Automating analysis tasks using the graphical modeler
  163. 8. Raster Analysis II
  164. Calculating NDVI
  165. Handling null values
  166. Setting extents with masks
  167. Sampling a raster layer
  168. Visualizing multispectral layers
  169. Modifying and reclassifying values in raster layers
  170. Performing supervised classification of raster layers
  171. 9. QGIS and the Web
  172. Using web services
  173. Using WFS and WFS-T
  174. Searching CSW
  175. Using WMS and WMS Tiles
  176. Using WCS
  177. Using GDAL
  178. Serving web maps with the QGIS server
  179. Scale-dependent rendering
  180. Hooking up web clients
  181. Managing GeoServer from QGIS
  182. 10. Cartography Tips
  183. Using Rule Based Rendering
  184. Handling transparencies
  185. Understanding the feature and layer blending modes
  186. Saving and loading styles
  187. Configuring data-defined labels
  188. Creating custom SVG graphics
  189. Making pretty graticules in any projection
  190. Making useful graticules in printed maps
  191. Creating a map series using Atlas
  192. 11. Extending QGIS
  193. Defining custom projections
  194. Working near the dateline
  195. Working offline
  196. Using the QspatiaLite plugin
  197. Adding plugins with Python dependencies
  198. Using the Python console
  199. Writing Processing algorithms
  200. Writing QGIS plugins
  201. Using external tools
  202. 12. Up and Coming
  203. Preparing LiDAR data
  204. Opening File Geodatabases with the OpenFileGDB driver
  205. Using Geopackages
  206. The PostGIS Topology Editor plugin
  207. The Topology Checker plugin
  208. GRASS Topology tools
  209. Hunting for bugs
  210. Reporting bugs
  211. Bibliography
  212. Index

Hunting for bugs

While QGIS developers do their best to make every QGIS release as stable as possible, sometimes you may encounter bugs or even crashes. To get them fixed in the future, it is necessary to inform the developers about issues.

This recipe shows you how to perform basic debugging and collect information that will help developers understand the problem better and help to fix it.

Getting ready

As the QGIS development process is very quick, bugs that are present in older versions are very likely already fixed in the latest version. So, it is necessary to ensure that you have the most recent QGIS version. If you use the development version of QGIS (so called "nightly" builds), upgrade to the last available build. If you prefer stable releases, then ensure that you have the latest stable version.

How to do it…

  1. Repeat the same actions again using the same data and settings to ensure that this is not an accidental error.
  2. Test your vector data (if any) with geometry checking tools to ensure that data is valid and has no geometry errors. If the data has geometry errors, then try to reproduce the bug with valid data.
  3. Check whether the same error happens with other data to ensure that this is not related to the specific dataset.
  4. If the error happens only on some specific features, extract them into a separate layer and make a small self-containing test dataset that allows you to reproduce bug. The same approach should be used if the dataset is large.
  5. Sometimes, errors may be caused by third-party plugins. Disable all plugins and try to reproduce the error. If you cannot reproduce the bug with the disabled plugins, probably this bug is somehow related to some plugin. To find this problematic plugin, activate the plugins one by one and try to reproduce the error.
  6. Look in the QGIS message log, it may contain useful debug and/or error messages that are related to your problem:
    How to do it…
  7. To open the Log Messages window, click on the Messages button located in the right corner of the QGIS status bar.
  8. If QGIS crashes, try to create a backtrace and/or collect debug messages (refer to the following sections). This will be extremely useful if your bug is not reproducible on the developer's computer.

Creating a backtrace under Linux

Under Linux, QGIS automatically tries to use gdb to produce a backtrace when crashed.

Note

To see the backtrace, it is necessary to start QGIS from the terminal emulator.

If you see no backtrace after the crash, this may mean that the possibility to connect debugger to the running processes is disabled in your distribution (for example, Ubuntu after version 10.10). This behavior is controlled by the ptrace_scope sysctl value. If it equals to 1 ptrace calls from external processes are not allowed. A value that equals to 0 allows external processes to examine memory of the other process.

In such cases, to enable a backtrace creation, temporarily open the root shell and execute the following command:

echo 0 > /proc/sys/kernel/yama/ptrace_scope

If you want to enable a backtrace creation permanently, you need to edit the /etc/sysctl.d/10-ptrace.conf file as root, and set the value to 0. Then, run as root to reload sysctl settings, as follows:

sysctl -p

After this, repeat the steps to reproduce the crash, copy the backtrace, and attach it to your bug report or e-mail.

Capturing debug output with DebugView under Windows

DebugView is a small program for the Windows operating system that allows you to view and save the debug output of programs. With its help, you can easily get the QGIS debug output and add it to your bug report.

Note

Note that you will see no debug output if your QGIS compiled without debugging support. Official packages from the OSGeo4W installer and the QGIS standalone installer are built with the debugging output.

To get the debug output with DebugView, follow these steps:

  1. Download DebugView from the Microsoft site at https://technet.microsoft.com/en-us/sysinternals/bb896647.aspx.
  2. Extract the archive to some folder on your hard drive and launch Dbgview.exe.
  3. Start QGIS and perform the actions that lead to a crash or an error:
    Capturing debug output with DebugView under Windows
  4. Save the log to a file using the Save button on the DebugView toolbar.
  5. Attach the saved file to your bug report or e-mail.

    Also, if QGIS crashes, it produces a minidump file (usually these files are created in your Temp directory and have the tmp.mdmp extension), as shown in the following screenshot:

    Capturing debug output with DebugView under Windows
  6. This file should also be attached to the bug report, as it allows developers to understand the problem better even if they cannot reproduce the crash on their computers.

How it works…

A backtrace is a summary of program functions that are still active. In other words, it shows all nested functions and calls from the program's start to the crash point. With the help of a backtrace, developers can isolate place where the bug is.

There's more…

If you have access to computers with different operating systems, it would be good to check whether this error is reproduced in different environments.

Almost all modern computers and laptops have enough performance to run virtual machines. The snapshots feature is available in the most popular virtual machines. You can have a clean and up-to-date system with recent QGIS for testing and debugging purposes.

See also