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

Making pretty graticules in any projection

A graticule is a set of reference lines on a map that help orient a map reader. They are often set at, and labeled, with the coordinates. The tricky part about using graticules, however, is projections. If you don't make them correctly, instead of smooth curves between the line intersections, you get awkward unusual shapes (mostly straight lines). The default QGIS graticule creator is not projection-friendly, so in this recipe, you'll see an add-on processing algorithm that does this. This recipe is about ensuring you get nice, smooth, and properly-labeled graticules.

Getting ready

You don't really need much for this recipe other than a bounding box and a coordinate interval that you want to space the lines at. Usually, these will be in Latitude, Longitude WGS 84 (EPSG:4326), and decimal degrees, respectively, since the whole point of a graticule is to add reference lines that help orient a user.

How to do it…

  1. Start by downloading a Processing Toolbox algorithm specifically for this task called Lines Graticule:
    1. Open the Processing Toolbox.
    2. Go to Scripts | Tools | Get scripts from on-line scripts collection:
      How to do it…
    3. In the Not Installed section, check the box for the Lines Graticule algorithm.
    4. Click on the OK button to install the algorithm.

      Tip

      Every time that you use a tool, it's good to check for updates.

    You will see something like the following screenshot:

    How to do it…
  2. Now that you've downloaded the algorithm, open it by navigating to Scripts | Vector (it's called Lines graticule though the code is actually pygraticule.py):
    How to do it…
  3. You can fill in the parameters by hand if you know them or use the … button to get values from your existing project.
  4. For now, you can use the defaults that will make a graticule for the whole world. The outputs are determined by outfile and graticule. These parameters are optional, you can choose to pick one, both, or neither. If you want a GeoJSON file, set the outfile. If you want a shapefile, set the graticule (if you want the results to autoload afterwards, make sure that the second output is set to temporary or a real file, just not blank). Refer to the Help tab for details about each parameter. There are two really important values to control the graticule:
    1. The spacing value denotes how often to draw a line (when doing world-scale maps, 20 or 30 degrees works well).
    2. The density value denotes how often to put nodes:
      How to do it…

      Tip

      The more nodes, the smoother the curves; however, you get a bigger file that takes longer to make. Picking the right density may require trial and error to find the largest density before you notice the lines stop curving smoothly for a given map scale.

  5. Once you've chosen your settings, click on Run.
  6. After it runs, a vector layer should get loaded with the results. This won't look all that exciting, just straight lines making a grid.
  7. The real magic is to now enable projection on-the-fly with one of the many decent world-wide projections such as "World Robinson (EPSG:54030):
    How to do it…
  8. (Optional) If it doesn't look like the image, but instead still has straight lines that are oddly spaced, you need to disable the QGIS rendering simplification:
    1. Pick the layer from Properties | Rendering.
    2. Make sure that Simplify geometry is disabled:
    How to do it…
  9. (Bonus) Generate a vector grid from Vector | Research tools. The difference looks like the following:
    How to do it…

How it works…

Graticules are basically line layers (though sometimes they are also polygons). If you draw a grid with nodes only at the points where two lines intersect, you can easily see how distorting the grid will lead to blocky shapes. The key to smooth graticules is adding additional line nodes in between the intersections (that is, increase the node density).

It's important to note that, when using projections that don't cover the whole world (for example, polar or stereographic projections), pick bounding box values that fall within the projection limits; otherwise, you may get errors when trying to reproject.

There's more…

The primary advantages of graticules in the main map canvas are that you can use them as references while working in QGIS, include them in web and digital maps, and have full control of the labels and symbology. The method used here differs from other graticule (grid) tools in QGIS because it focuses on putting Latitude/Longitude lines with smooth curves as references into any projection. Other grid tools focus more on making regular squares across a map to subdivide a region.

The main advantages of the print composer method (next recipe) are its ability to make multiple coordinate systems easily and to add tick marks around the outside edge of a map. Tick marks are what you commonly see on navigation-oriented maps, such as USGS Topo quads, and other printed maps.

See also

Lines graticule (aka Pygraticule) can also be used as a pure Python script; for updates and more information, refer to https://github.com/wildintellect/pyGraticule.

To learn how to write your own processing toolbox algorithms, refer to the Writing processing algorithms recipe in Chapter 11, Extending QGIS.