=== Layer

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


.Layer attributes
[options="header",cols="m,d,d,d"]
|====================
| 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. 
|====================

.Layer 
====
[source,xml]
----
<Layer name="layer_1">
  <StyleName>style_1</StyleName>
  <StyleName>style_2</StyleName>

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