WFS filters — wfs_filter • ffm

Utility functions to construct XML or CQL queries. These functions are the backend of the filter argument in the filter capabilities of all ffm functions that interact with a WFS (e.g., bkg_admin, bkg_clc or bkb_geonames).

Usage

wfs_filter(
  ...,
  filter = NULL,
  bbox = NULL,
  poly = NULL,
  predicate = "intersects",
  geom_property = "geom",
  default_crs = 25832,
  lang = NULL
)

Arguments

...

Used to construct CQL filters. Dot arguments accept an R-like syntax that is converted to CQL queries internally. These queries basically consist of a property name on the left, an aribtrary vector on the right, and an operator that links both sides. If multiple queries are provided, they will be chained with AND. The following operators and their respective equivalents in CQL and XML are supported:

R	CQL	XML
`==`	`=`	`PropertyIsEqualTo`
`!=`	`<>`	`PropertyIsNotEqualTo`
`<`	`<`	`PropertyIsLessThan`
`>`	`>`	`PropertyIsGreaterThan`
`>=`	`>=`	`PropertyIsGreaterThanOrEqualTo`
`<=`	`<=`	`PropertyIsLessThanOrEqualTo`
`%LIKE%`	`LIKE`	`PropertyIsLike`
`%ILIKE%`	`ILIKE`
	`%in%`	`IN`

To construct more complex queries, you can use the filter argument to pass CQL queries directly. Also note that you can switch between CQL and XML queries using options(ffm_query_language = "xml"). See also wfs_filter.

filter

A character string containing a valid CQL or XML filter. This string is appended to the query constructed through .... Use this argument to construct more complex filters. Defaults to NULL.

bbox

An sf geometry or a boundary box vector of the format c(xmin, ymin, xmax, ymax). Used as a geometric filter to include only those geometries that relate to bbox according to the predicate specified in predicate. If an sf geometry is provided, coordinates are automatically transformed to ESPG:25832 (the default CRS), otherwise they are expected to be in EPSG:25832.

poly

An sf geometry. Used as a geometric filter to include only those geometries that relate to poly according to the predicate specified in predicate. Coordinates are automatically transformed to ESPG:25832 (the default CRS).

predicate

A spatial predicate that is used to relate the output geometries with the object specified in bbox or poly. For example, if predicate = "within", and bbox is specified, returns only those geometries that lie within bbox. Can be one of "equals", "disjoint", "intersects", "touches", "crosses", "within", "contains", "overlaps", "relate", "dwithin", or "beyond". Defaults to "intersects".

geom_property

Name of the geometry property included in the WFS. In most cases, this is "geom", but there are some exceptions.

default_crs

A WFS defines a default CRS in which coordinates for spatial filtering have to be provided. For BKG services, this is usually EPSG:25832. All sf objects provided through bbox or poly are first transformed to this CRS before creating the query.

lang

Query language to use for constructing the query. One of "cql" and "xml". By default, almost all ffm functions use CQL because it is simpler and less prone to errors. However, CQL is limited in terms of query size. Especially when providing a poly, URLs can become so long that the WFS server will decline them. XML can be a valid alternative to construct large queries. Additionally, some services like the one used by bkg_geonames only support XML. If NULL, defaults to getOption("ffm_query_language").

Value

A CQL query or an XML query depending on the lang argument.

Examples

# CQL and XML support mostly the same things
wfs_filter(ags %LIKE% "05%", lang = "cql")
#> ags LIKE '05%' 
wfs_filter(ags %LIKE% "05%", lang = "xml")
#> <fes:Filter>
#>   <fes:PropertyIsLike wildCard="%" singleChar="_" escapeChar="\">
#>     <fes:ValueReference>ags</fes:ValueReference>
#>     <fes:Literal>05%</fes:Literal>
#>   </fes:PropertyIsLike>
#> </fes:Filter>

bbox <- c(xmin = 5, ymin = 50, xmax = 7, ymax = 52)
wfs_filter(bbox = bbox, lang = "cql")
#> intersects(geom, POLYGON ((5 50, 7 50, 7 52, 5 52, 5 50))) 
wfs_filter(bbox = bbox, lang = "xml")
#> <fes:Filter>
#>   <fes:Intersects>
#>     <fes:ValueReference>geom</fes:ValueReference>
#>     <gml:Polygon srsName="urn:ogc:def:crs:EPSG::25832" gml:id="file21851c6e9d5.geom.0">
#>       <gml:exterior>
#>         <gml:LinearRing>
#>           <gml:posList>5 50 7 50 7 52 5 52 5 50</gml:posList>
#>         </gml:LinearRing>
#>       </gml:exterior>
#>     </gml:Polygon>
#>   </fes:Intersects>
#> </fes:Filter>

# Using `filter`, more complex queries can be built
wfs_filter(ars %LIKE% "%0", filter = "regierungs NOT IS NULL")
#> ars LIKE '%0' AND regierungs NOT IS NULL 

wfs_filter(
  filter = "<fes:Not>
    <fes:PropertyIsNull>
      <fes:ValueReference>aussprache</fes:ValueReference>
    </fes:PropertyIsNull>
  </fes:Not>",
  lang = "xml"
)
#> <fes:Filter>
#>   <fes:Not>
#>     <fes:PropertyIsNull>
#>       <fes:ValueReference>aussprache</fes:ValueReference>
#>     </fes:PropertyIsNull>
#>   </fes:Not>
#> </fes:Filter>