tile_writer

Slippy map tile generator for QGIS

About

This program is a slippy map tile generator for use with QGIS. All rendering is done by QGIS so what you see on your screen is what you'll get in your output tiles. Functionally it is quite similar to the QTiles plugin.

Advantages of this program:

• Renders the map a section at time. This allows even very large datasets to be rendered.
• Renders each zoom level separately. This means that scale-dependant feature visibility is honored.
• Does not exhibit the rendering issues that QTiles has at tile edges:
  • Does not truncate point icons
  • Labels don't shift
  • Patterned lines display correctly
• Fast: 20 minutes to render a complex 60km x 70km topographic map with roads, water, POIs, etc. from levels 5 through 15 on a 3.5GHz i7.
• Resistant to the QGIS bug that causes raster layers to sometimes not be displayed.
• Copious progress status display
• Simple Python script that can be easily modified to suit your needs.

Disadvantages of this program:

• No graphical interface.
• Cannot be made to stop early without killing QGIS.
• Rendering complex maps causes the QGIS GUI to temporarily freeze during some parts of the rendering process.
• Lacks some of the additional options that QTiles offers.
• Doesn't always clip the rendering to the specified area of interest. At lower zoom levels tiles will be rendered that are not within the area of interest.
• Might not work under QGIS 3.x. This program has been tested under QGIS 2.x. Reports indicate that it does not function on the 3.x series.

Comments are welcome, you can email SLIPPYsoftware@alephnull.net (remove the type of map to get the real address).

Files

readme.txt	This file
tile_writer.py	The main script
globalmercator.py	For latitude/longitude to tile coordinate conversions (Taken from GDAL2Tiles)
scales.xml	Scale presets for QGIS matching the tile zoom levels
LICENSE	License for this software

Usage

Prelimary steps:

• Create your map in QGIS
  • If you are using scale-dependant feature visibility see the section "Zoom levels" below

Running the script:

• Place tile_writer.py and globalmercator.py into the same directory
  (As packaged, this has already been done)
• In QGIS, open the Python console (Plugins → Python Console)
• Click on the Show editor icon in the Python Console (middle icon)
• Load the tile_writer.py script using the Open file icon (top icon on the right)
• Define the map's area of interest:
  • Create a new polygon vector layer
  • Make it editable
  • Add a polygon covering the area you want rendered
  • Save it to disk; the filename you chose should be put into the area_of_interest variable in the script.
  • Hide the layer
• Adjust the settings to suit your needs (see below).
• Note that all paths and filenames are relative to QGIS's current working directory.
  • If you run QGIS from the commandline then the directory you started it from is the working directory.
  • If you run QGIS from a graphical interface under Unix/Linux then the working directory is probably your home directory.
  • If you run QGIS under Windows drop me an email and let me know what QGIS's working directory is :^)
• Run the script using the Run script icon (the bottom icon)
• The script's progress will appear in the console
• An All done message will be printed when the script has finished

Note that QGIS may appear to have locked up while generating the regional tiles. If this happens simply be patient; complex maps may take several minutes to draw.

The script will avoid regenerating images whenever possible. This means that if you want to regenerate a map you should either specify a different output_path or first delete all of the tile directories (numbered) as well as the region tiles (e.g. "10_300_651_s16_b2.png").

Configuration variables

These are found near the top of tile_writer.py and should be changed as appropriate before running the script.

start_z

Minimum zoom level

Default value: 10

end_z

Maximum zoom level

Default value: 15

step

How many map tiles to place in each regional tile

(the actual number of map tiles per regional tile is step * step) Higher number speed up rendering (assuming enough RAM is available). Lower numbers decrease memory usage but increase rendering time.
Default value: 16

border

Number of extra map tiles to render along each edge of a regional tile

By rendering extra, unused border tiles we can avoid shifting labels, truncated images, and line-drawing inconsistancies at tile boundaries. If this value is set to 0 you will encounter rendering issues at tile boundaries.
Default value: 2

output_path

Directory to write the regional images and level subdirectories to

Default value: '.'

area_of_interest

Path of shapefile defining the area of interest

The extent of the shapefile is used to limit rendering to a particular area. Note that no clipping is done so at lower zoom levels tiles from outside the area of interest will be generated.
Default value: 'border.shp'

tile_format

Filename convention to follow

Can be set to either 'tms' or 'google'
Default value: 'tms'

Zoom levels

If you are using scale-dependant feature visibility you should use the scales listed below when deciding visibility. These scales can be found in the scales.xml file and installed in QGIS using: Settings → Options → Map Tools → Predefined Scales → Import from file (folder icon)

Level	Scale
1	1 : 295,829,355.45
2	1 : 147,914,677.73
3	1 : 73,957,338.86
4	1 : 36,978,669.43
5	1 : 18,489,334.72
6	1 : 9,244,667.36
7	1 : 4,622,333.68
8	1 : 2,311,166.84
9	1 : 1,155,583.42
10	1 : 577,791.71
11	1 : 288,895.85
12	1 : 144,447.93
13	1 : 72,223.96
14	1 : 36,111.98
15	1 : 18,055.99
16	1 : 9,028.00
17	1 : 4,514.00
18	1 : 2,257.00
19	1 : 1,128.50
20	1 : 564.25
21	1 : 282.12
22	1 : 141.06
23	1 : 70.53

Version history

v0.2.2	Clarified license, Documentation updates and corrections, Added Github repository (no code changes)
v0.2.1	Documentation updates, added scale presets file (no code changes)
v0.2	Now supports both TMS and Google naming conventions
v0.1	Initial release

Contact

Email:	SLIPPYsoftware@alephnull.net
	(remove the type of map to get the real address)
Website:	http://alephnull.net/software/gis/tile_writer.shtml
Github:	https://github.com/Alex-Kent/tile_writer

Legalese

tile_writer Version 0.2.2 2018-11-09
© 2018 Alexander Hajnal All rights reserved

This software is released under the terms of the Version 3 of the GNU Affero General Public License. See the LICENSE file for details. Contact me via email to negotiate licensing if you want to use this software under different a license.

Warranty

I make no warranty or representation, either express or implied, with respect the behavior of this script, its quality, performance, accuracy, merchantability, or fitness for a particular purpose. This script is provided 'as is', and you, by making use thereof, are assuming the entire risk. That said, I hope you this script useful. Have fun!