Simple Mapnik XML StyleSheet Generation

Mapnik XML Stylesheets

The normal choice for generating maps from OpenStreetMap data is Mapnik which is a very flexible image generator for maps.

To use Mapnik an XML based stylesheet needs to be generated which Mapnik will use to draw the parts of the map using information taken from a PostgreSQL database. The data is normally inserted into the database using osm2pgsql

The creation of the stylesheet is a complex task. The main OpenStreetMap web site originally used a hand-created stylesheet but has recently moved to a stylesheet that is generated from CSS type rules using the CartoCSS processor.

There are other methods of creating Mapnik XML stylesheets and in this context the Spreadnik pre-processor is relevant. This attempts to create a simple spreadsheet based way of defining what is to be drawn which is then converted into a Mapnik XML stylesheet.

This web page describes a new method of creating Mapnik XML stylesheets that is similar in some ways to Spreadnik in that it uses a spreadsheet, but simpler because it gives the user fewer choices although this does reduce the number of lines required.

Advantages / Disadvantages

There are a number of key advantages of using this spreadsheet method of generating the Mapnik stylesheet compared to manually generating one or using a tool that requires manual generation of the SQL statements.
Automatic generation of SQL statements
There is no need to understand the SQL syntax since the Perl script automatically generates an SQL statement that includes all columns that are required and tries to minimise the number of objects that are fetched.
Consistency of formatting of an object between zoom levels
Since the formatting uses only a small number of parameters the Perl script can ensure that the colours and formatting are consistent across zoom levels. This means that a particular road type will always be the same colour and have a width that scales across zoom levels.
Consistency between similar objects
With all of the objects of a similar type listed in a simple spreadsheet it is easy to ensure that there is consistency between them. This means that similar types of objects can appear in the same format at the same zoom level
Simplicity
By reducing the number of configurable options the whole process of customising the style sheet is simplified. This should make map generation available to more people.

There are also disadvantages of the spreadsheet method. Primarily because it is not possible to customise the SQL statements or to have drawing styles that are different to the ones that the spreadsheet allows or particularly complex.

Comparison

The default OpenStreetMap mapnik stylesheet has been converted into the spreadsheet format and maps for the UK generated from them. This is based on the CartoCSS formulation of the stylesheet with some small modifications (see below). The text files when exported from the spreadsheet amount to 48 kBytes and the Perl scripts are 61 kBytes which is much smaller than the 206 kBytes for the CartoCSS files which excludes the tools to generate the Mapnik stylesheet.

A map comparison is available that shows the standard stylesheet and the custom style sheet side-by-side.

There is also a map key that contains almost all of the features understood by this custom style sheet side-by-side with a rendering of the same information with the standard stylesheet.

Stylesheet Changes

While converting the CartoCSS stylesheet to a spreadsheet (in around October 2013) a number of small changes were made. The CartoCSS stylesheet has changed since then so some of these comments no longer apply.

The first set of changes are related to the limited options in the spreadsheet which has also imposed a consistency in formatting so that the following has changed (only visible at some zoom levels):

The second set of changes are stylistic changes to improve clarity (only visible at some zoom levels):

There are some parts of the implementation of the standard stylesheet that have not been debugged so there may be parts of the map that do not quite render properly. There are probably also some errors in the conversion that might not render correctly.

Spreadsheet Description

Each sheet contains a different type of data (for example: areas, lines, points) that has different columns to define what is to be drawn.

Each row within a sheet contains the information required to draw one type of object for a range of zoom levels.

Some information is common to all objects:

When drawing the objects they are drawn in the following order:

  1. Sheet order - Shapefiles are drawn before areas which are drawn before lines which are drawn before points.
  2. Layer order - The first layer on a sheet is drawn completely before the second layer before the third layer ...
  3. Reverse style order - The first style within a layer is drawn on top of the second on top of the third ... (or you can consider the last style is drawn first, then the second last, ...).
  4. Line order - Within a style only one line will be drawn for each object, if the first one matches (zoom and filters) then it is drawn, else if the second matches it is drawn, else if the third matches ... ; therefore the most restrictive filter should be placed before the least restrictive filters.

Objects are only drawn for the zoom levels which match the specified range, by using more than one line with the same LAYER, STYLE and FILTER for different zoom ranges the styling of an object can be changed at each zoom level.

In the object definitions in the spreadsheets the database columns must be enclosed in square brackets where they are used (for example: [highway]) and the text values that they are compared against should use single or double quotes (for example: "motorway" or 'motorway').

In the object definitions the additional styles defined by the "*-STYLE" column will override any automatically generated styles (for example: line widths can be fixed not automatically scale in the "lines" sheet). The styles should be appropriate for the symboliser type and consist of a name, an equals sign and a value; if there are multiple style options then they should be separated by a semi-colon (for example: "stroke-width=1 ; opacity=0.5").

Shapefiles Sheet

These are for reading data from a file, generally used for low zoom (to avoid database accesses) or for coastlines.

The columns for the shapefile sheet are the simplest possible to allow implementation of the standard OSM shapefiles.

Each row is implemented as a separate layer so the rules for layer names described above does not apply here.

Areas Sheet

These are areas or polygons in the data (for example: landuse, natural, buildings) and they can be filled with a plain colour or a symbol and/or have a line drawn round them and/or have a label placed in them.

There are four sections for each area object drawn in this order (bottom to top as seen in the final image):

  1. A plain colour fill controlled by the MIN-ZOOM-FILL, MAX-ZOOM-FILL, FILL-COLOUR and FILL-STYLE columns.
  2. A pattern fill controlled by the MIN-ZOOM-SYMBOL, MAX-ZOOM-SYMBOL and FILL-SYMBOL columns.
  3. A border around the object controlled by the MIN-ZOOM-LINE, MAX-ZOOM-LINE, LINE-COLOUR and LINE-STYLE columns.
  4. A label for the area controlled by the MIN-ZOOM-LABEL, MAX-ZOOM-LABEL, LABEL-COLOUR, LABEL-SIZE, LABEL-STYLE and LABEL columns.

A label is only drawn on objects that are "large enough" which is automatically controlled by additional filtering on the 'way_area' database column. The labels are drawn after the lines so that they are not obscured.

Lines Sheet

These are linear objects (for example: highway, railway, boundaries) and they can be drawn with a casing (normally used on roads) and/or an underlay (a solid colour for under a dashed line) and/or a fill colour or pattern, and can optionally have tunnel and/or bridge markings and/or oneway markings and/or one or two labels along them.

There are six sections for each line object which will be drawn in order (bottom to top as seen in the final image):

  1. A casing controlled by the MIN-ZOOM, MAX-ZOOM, MIN-ZOOM-CASING and CASING-COLOUR columns and with a width controlled by the WIDTH-Z18 column (see below).
    • If the line is on a bridge and the zoom is more than MIN-ZOOM-TUNNEL-BRIDGE then the casing colour is changed to BRIDGE-CASING-COLOUR.
    • If the line is in a tunnel and the zoom is more than MIN-ZOOM-TUNNEL-BRIDGE then the casing colour is changed to TUNNEL-CASING-COLOUR and drawn as a dashed line.
  2. A fill controlled by the MIN-ZOOM, MAX-ZOOM, FILL-COLOUR, FILL-STYLE, FILL-PATTERN and WIDTH-Z18 (see below) columns.
    • If there is an UNDERLAY-COLOUR then this is drawn before the fill to allow a dashed line to be drawn above it.
    • If the line is in a tunnel and the zoom is more than MIN-ZOOM-TUNNEL-BRIDGE then the fill colour is changed to TUNNEL-FILL-COLOUR and drawn as a dashed line.
    • If there is a FILL-PATTERN selected then this is used instead of the FILL-COLOUR.
  3. Direction arrows (using a specific arrow pattern) controlled by the MIN-ZOOM, MAX-ZOOM and MIN-ZOOM-ONEWAY columns.
  4. A label drawn along the line controlled by the MIN-ZOOM, MAX-ZOOM, MIN-ZOOM-LABEL, LABEL-COLOUR, LABEL-SIZE, LABEL-STYLE and LABEL columns (if SHIELD-SYMBOL is not set).
  5. A shield drawn on the line controlled by the MIN-ZOOM, MAX-ZOOM, MIN-ZOOM-LABEL, LABEL-COLOUR, LABEL-SIZE, LABEL-STYLE and LABEL columns (if SHIELD-SYMBOL is set).
  6. A second label drawn on the line controlled by the MIN-ZOOM, MAX-ZOOM, MIN-ZOOM-LABEL2, LABEL2-COLOUR, LABEL2-SIZE, LABEL2-STYLE and LABEL2 columns.

The width of the line fill is set by the value of WIDTH-Z18 when the zoom is 18; for smaller zooms the width halves for each change of 2 in the zoom level and for larger zooms the width doubles for each change of 2 in the zoom level. The width of the casing is generally 1.5 times the width of the filled section but always between 1 and 3 pixels wider.

Points Sheet

These are point objects (for example: barriers, amenities) which are drawn with a symbol and/or one or two labels.

There are three sections for each point:

  1. A symbol for the object controlled by the MIN-ZOOM-SYMBOL, MAX-ZOOM-SYMBOL and SYMBOL columns.
  2. A label for the object controlled by the MIN-ZOOM-LABEL, MAX-ZOOM-LABEL, LABEL-COLOUR, LABEL-SIZE, LABEL-STYLE and LABEL columns.
  3. A second label for the object controlled by the MIN-ZOOM-LABEL2, MAX-ZOOM-LABEL2, LABEL2-COLOUR, LABEL2-SIZE, LABEL2-STYLE and LABEL2 columns.

In addition the same symbol and label(s) can be used for polygons if the POLYGON column specifies this is to happen. This allows the same map marking for an object (e.g. A shop) that has been marked with a point and one that has been marked with a polygon.

Generating The Stylesheet

The sheets must be exported from the spreadsheet in TAB separated format without quotes to files named after each of the sheet names (for example: shapefiles.txt, areas.txt ...).

The Mapnik stylesheets (one for each zoom level and one containing all information) are generated by running the Perl script called "mapnik.pl".

Generated Mapnik Stylesheet

The Mapnik stylesheet that is generated is split up into sections for each zoom level of the final map and each zoom level has its own set of Layers and Styles. This is in contrast to the OpenStreetMap Mapnik stylesheet that has traditionally mixed zoom levels within a single Layer.

The advantage of generating a new Layer for each zoom level for the same set of objects is that an optimised SQL statement can be used which is automatically generated by the Perl script. This means that the minimum set of data can be extracted from the database since all rules will match the current zoom which is not always the case if one Layer is used for multiple zoom levels. This also means that the minzoom and maxzoom attributes on the Layer can be used instead of in each rule which simplifies the file.

Each Layer also uses multiple styles (in general) and the cache-features attribute on the Layer to optimise the performance with re-using the same object information in each layer.

When generating the map tiles Mapnik will process the objects read from the database in the following order:

By using multiple Styles within the Layer the rules for drawing lines can ensure that road casings (borders) are drawn before the line fill which is drawn before the oneway arrows which are drawn before the names and/or references. This avoids needing to select the data from the database multiple times for casing, fill, oneway and then text which has traditionally been done by the OpenStreetMap Mapnik stylesheet.

For the Layers that are used for lines the group-by attribute is used and the layer tag of the object in the database is used to order the objects. In this case the processing of objects is slightly different as shown below:

This means that if a single Layer is used for roads and railways then they will be drawn in the correct order with the lower layers completely drawn first before the upper layers. The OpenStreetMap Mapnik stylesheet has for a long time used more than 20 Layers to draw roads and bridges in the correct order (but not tunnels on different layers) which means searching the database more times.

Status

This software is no longer maintained (since August 2014). Tiles for the UK are still generated from it on a weekly basis.

This method of creating a Mapnik stylesheet is easier than the Carto based stylesheet used on the main OSM map. Maintaining feature parity with that stylesheet is very complex so this stylesheet is no longer updated.

There are still, in my opinion, advantages of this custom style which is why it is still used. These advantages are: replacement of shields on roads with text and drawing of rights of ways over other highways.

Download

The complete source code to create your own styles can be browsed in the Subversion viewer which also has a list of the latest changes.

The source code can be downloaded from the Subversion repository with a command like the following:

svn co http://gedanken.org.uk/svn/spreadsheet-mapnik/trunk spreadsheet-mapnik

The current stylesheet use on this website (just the stylesheet and icons, not the perl scripts) can be downloaded as a tar file.