Mapnik - The Missing Manual

3.6. Layer

A layer renders data from a data source using one or more styles.

Table 21. Layer attributes
Attribute	Value	Default	Description
`buffer-size`	float	0
`cache-features`	bool	false	On layers with multiple styles: read features from data source only once and cache them in memory. Improves perforance, but at the cost of using more memory, so off by default.
`clear-lable-cache`	bool	false	Clear the placement cache from previous layers, so that texts, points and markers may overlap with those from previous layers.
`group-by`	string	none	On layers with multiple styles these styles are processed one by one in the given order by default. When giving a group-by property, feature elements will be sorted by this property, and styles will be processed in order for each distinct group value. E.g with `group-by=z` and `z` having values from `-1` to `1`, first all features with `z=-1` will be processed using all styles, then all with `z=0`, and finally all with `z=-1`.
`minimum-scale-denominator`	float	0	Minimum map scale above which this layer should be rendered.
`maximum-extent`	4x float		Maximum exent for which features should be processed.
`maximum-scale-denominator`	float	max. float	Maximum map scale up to which this layer should be rendered.
`name`	string	none	Name by which the layer can be refeenced, e.g. for error messaes.
`queryable`	bool	false	unused?
`srs`	proj4 srs string	`+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs`	Spatial reference system
`status`	boolean	true	Whether this layer should actually be processed or not. Caution: when set to `false` in XML the complete `<Layer>` will simply be skipped on reading the style file, and no layer object will be created for it. So you won’t be able to enable such a layer with `Layer.set_status(true)` using API code later.

Example 13. Layer

<Layer name="layer_1">
  <StyleName>style_1</StyleName>
  <StyleName>style_2</StyleName>

  <DataSource>
    [...]
  </DataSource>
</Layer>