EQCL feature filtering

So far, we’ve demonstrated using emodnet.wfs to request complete layers.

However, WFS services allows us to be selective with what features are returned from the EMODnet servers. For example: using CQL filters, we can send a query including a filtering clause that will be executed on the server and return only the subset of the information we request. This can make queries a lot faster and reduce the need for filtering data after download.

For example, let’s consider the case where we want to include the territorial sea boundaries in a habitat map.
One way to approach this would be to download the full layer containing all the European maritime boundaries from the human activities service and then filter the sf object returned for features representing Territorial Seas.

A more efficient approach would be to use a filter and request only the features representing Territorial Seas from the server. Let’s have a look at how to do that.

CQL & ECQL Query languages.

CQL (Common Query Language) is a plain-text language created for the OGC Catalog specification. GeoServer has adapted it to be an easy-to-use filtering mechanism. GeoServer actually implements a more powerful extension called ECQL (Extended CQL), which allows expressing the full range of filters that OGC Filter 1.1 can encode. ECQL is accepted in many places in GeoServer.

GeoServer supports the use of both CQL and ECQL in WFS requests and whenever the documentation refers to CQL, ECQL syntax can be used as well.

The examples provided here are only a small subset of filtering functionality available through Geoserver. Full references and further examples can be found in the following documentation:

ECQL Reference: reference for the syntax of the ECQL language.
Filter Function Reference: The OGC Filter Encoding specification provides a generic concept of a filter function. A filter function is a named function with any number of arguments, which can be used in a filter expression to perform specific calculations. This provides much richer expressiveness for defining filters.GeoServer provides many different kinds of filter functions, covering a wide range of functionality including mathematics, string formatting, and geometric operations.
CQL and ECQL Tutorial shows examples of defining filters.

Initialise EMODnet WFS client

First, let’s load the library and start a new WFS client with emodnet_init_wfs_client.

library(emodnet.wfs)

wfs <- emodnet_init_wfs_client(service = "biology")
#> Loading ISO 19139 XML schemas...
#> Loading ISO 19115 codelists...
#> ✔ WFS client created successfully
#> ℹ Service: "https://geo.vliz.be/geoserver/Emodnetbio/wfs"
#> ℹ Version: "2.0.0"

Inspecting attributes

To develop filters, we first need information about the attributes we can filter on. For example, we might want to know the data type and range or distribution of values of each attributes of a given layer.

Let’s look at existing layers.

emodnet_get_wfs_info(wfs)
#> # A tibble: 35 × 9
#> # Rowwise: 
#>    data_source service_name service_url   layer_name title abstract class format
#>    <chr>       <chr>        <chr>         <chr>      <chr> <chr>    <chr> <chr> 
#>  1 emodnet_wfs biology      https://geo.… mediseh_c… EMOD… "Coral … WFSF… sf    
#>  2 emodnet_wfs biology      https://geo.… mediseh_c… EMOD… "Coral … WFSF… sf    
#>  3 emodnet_wfs biology      https://geo.… mediseh_c… EMOD… "Cymodo… WFSF… sf    
#>  4 emodnet_wfs biology      https://geo.… Species_g… EMOD… "This d… WFSF… sf    
#>  5 emodnet_wfs biology      https://geo.… Species_g… EMOD… "This d… WFSF… sf    
#>  6 emodnet_wfs biology      https://geo.… Species_g… EMOD… "This d… WFSF… sf    
#>  7 emodnet_wfs biology      https://geo.… mediseh_h… EMOD… "Haloph… WFSF… sf    
#>  8 emodnet_wfs biology      https://geo.… mediseh_m… EMOD… "Maërl … WFSF… sf    
#>  9 emodnet_wfs biology      https://geo.… mediseh_m… EMOD… "Maërl … WFSF… sf    
#> 10 emodnet_wfs biology      https://geo.… mediseh_p… EMOD… "This d… WFSF… sf    
#> # ℹ 25 more rows
#> # ℹ 1 more variable: layer_namespace <chr>

Layer attribute names

To start, you might want to know the names of a layer’s attributes.

layer_attributes_get_names(wfs, layer = "mediseh_zostera_m_pnt")
#> [1] "id"       "country"  "the_geom"

All functions can also be used by providing a service name instead of a wfs object.

layer_attributes_get_names(service = "biology", layer = "mediseh_zostera_m_pnt")
#> ✔ WFS client created successfully
#> ℹ Service: "https://geo.vliz.be/geoserver/Emodnetbio/wfs"
#> ℹ Version: "2.0.0"
#> [1] "id"       "country"  "the_geom"

Layer attribute descriptions

The type of filtering you might want to apply will depend on the data type of each attribute.

You can inspect attribute descriptions (metadata) for a given layer with layer_attribute_descriptions().

layer_attribute_descriptions(wfs, layer = "mediseh_zostera_m_pnt")
#>       name      type minOccurs maxOccurs nillable geometry
#> 1       id   integer         0         1     TRUE    FALSE
#> 2  country character         0         1     TRUE    FALSE
#> 3 the_geom     Point         0         1     TRUE     TRUE

Layer attribute summaries

You can get summaries of the values of each attribute with layer_attributes_summarise(). This function basically runs summary() on the attribute columns of the layer.

layer_attributes_summarise(wfs, layer = "mediseh_zostera_m_pnt")
#>     gml_id                id      country         
#>  Length:54          Min.   :0   Length:54         
#>  Class :character   1st Qu.:0   Class :character  
#>  Mode  :character   Median :0   Mode  :character  
#>                     Mean   :0                     
#>                     3rd Qu.:0                     
#>                     Max.   :0

Inspecting individual layer attributes

You can also inspect individual attributes which, in the case of categorical variables, can give more detailed information on the names and distribution of categories.

layer_attribute_inspect(
  wfs,
  layer = "mediseh_zostera_m_pnt",
  attribute = "country"
)
#> # A tibble: 7 × 3
#>   .            n percent
#>   <chr>    <int>   <dbl>
#> 1 Croazia      4  0.0741
#> 2 Francia      1  0.0185
#> 3 Italia      30  0.556 
#> 4 Libia        2  0.0370
#> 5 Slovenia     8  0.148 
#> 6 Spagna       8  0.148 
#> 7 Tunisia      1  0.0185

Layer attributes table

Finally, to enable full inspection and custom processing of attribute data, you can download the full set of attributes as a data.frame, excluding the geometry column. As the geometries are usually the largest column, this is much faster than downloading the full layer and can be useful in developing attribute filtering rules.

attr_tbl <- layer_attributes_tbl(wfs, layer = "mediseh_zostera_m_pnt")

attr_tbl
#> # A tibble: 54 × 3
#>    gml_id                      id country
#>    <chr>                    <int> <chr>  
#>  1 mediseh_zostera_m_pnt.1      0 Spagna 
#>  2 mediseh_zostera_m_pnt.2      0 Spagna 
#>  3 mediseh_zostera_m_pnt.3      0 Spagna 
#>  4 mediseh_zostera_m_pnt.4      0 Spagna 
#>  5 mediseh_zostera_m_pnt.5      0 Spagna 
#>  6 mediseh_zostera_m_pnt.6      0 Spagna 
#>  7 mediseh_zostera_m_pnt.7      0 Spagna 
#>  8 mediseh_zostera_m_pnt.8      0 Francia
#>  9 mediseh_zostera_m_pnt.9      0 Italia 
#> 10 mediseh_zostera_m_pnt.10     0 Italia 
#> # ℹ 44 more rows

Filtering by attributes

Filtering categorical attributes values using text comparisons

Once we know the values of a categorical attribute of a layer, we can use them to filter the features returned from our query. For example, let’s say we are only interested in results for France. From our previous interrogation of the attributes associated with the mediseh_zostera_m_pnt layer, we have seen that the attribute country contains the country.

We can use the name of the attribute and the value we require to construct our eqcl filter and pass it to our request using the cql_filter argument.

emodnet_get_layers(
  wfs,
  layers = "mediseh_zostera_m_pnt",
  cql_filter = "country = 'Francia'"
)
#> $mediseh_zostera_m_pnt
#> Simple feature collection with 1 feature and 3 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 4.84864 ymin: 43.37637 xmax: 4.84864 ymax: 43.37637
#> Geodetic CRS:  WGS 84
#>                    gml_id id country                 the_geom
#> 1 mediseh_zostera_m_pnt.8  0 Francia POINT (4.84864 43.37637)

The above shows the most basic text comparisons we can make use the equality operator =. If we want to match more than one value, we can use the operator IN instead and provide a list of values to compare to. For example, to request both France and Spain, we could use the following filter:

emodnet_get_layers(
  wfs,
  layers = "mediseh_zostera_m_pnt",
  cql_filter = "country IN ('Francia', 'Spagna')"
)
#> $mediseh_zostera_m_pnt
#> Simple feature collection with 9 features and 3 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: -4.167154 ymin: 36.71226 xmax: 4.84864 ymax: 43.37637
#> Geodetic CRS:  WGS 84
#>                     gml_id id country                   the_geom
#> 1  mediseh_zostera_m_pnt.1  0  Spagna  POINT (-2.61314 36.71681)
#> 2  mediseh_zostera_m_pnt.2  0  Spagna POINT (-3.846598 36.75127)
#> 3  mediseh_zostera_m_pnt.3  0  Spagna POINT (-3.957785 36.72266)
#> 4  mediseh_zostera_m_pnt.4  0  Spagna POINT (-4.039712 36.74217)
#> 5  mediseh_zostera_m_pnt.5  0  Spagna POINT (-4.100182 36.72331)
#> 6  mediseh_zostera_m_pnt.6  0  Spagna POINT (-4.167154 36.71226)
#> 7  mediseh_zostera_m_pnt.7  0  Spagna POINT (-1.268366 37.55796)
#> 8  mediseh_zostera_m_pnt.8  0 Francia   POINT (4.84864 43.37637)
#> 9 mediseh_zostera_m_pnt.54  0  Spagna  POINT (3.291868 42.29132)

It is the same as using the OR filter:

emodnet_get_layers(
  wfs,
  layers = "mediseh_zostera_m_pnt",
  cql_filter = "country='Francia' OR country='Spagna'"
)
#> $mediseh_zostera_m_pnt
#> Simple feature collection with 9 features and 3 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: -4.167154 ymin: 36.71226 xmax: 4.84864 ymax: 43.37637
#> Geodetic CRS:  WGS 84
#>                     gml_id id country                   the_geom
#> 1  mediseh_zostera_m_pnt.1  0  Spagna  POINT (-2.61314 36.71681)
#> 2  mediseh_zostera_m_pnt.2  0  Spagna POINT (-3.846598 36.75127)
#> 3  mediseh_zostera_m_pnt.3  0  Spagna POINT (-3.957785 36.72266)
#> 4  mediseh_zostera_m_pnt.4  0  Spagna POINT (-4.039712 36.74217)
#> 5  mediseh_zostera_m_pnt.5  0  Spagna POINT (-4.100182 36.72331)
#> 6  mediseh_zostera_m_pnt.6  0  Spagna POINT (-4.167154 36.71226)
#> 7  mediseh_zostera_m_pnt.7  0  Spagna POINT (-1.268366 37.55796)
#> 8  mediseh_zostera_m_pnt.8  0 Francia   POINT (4.84864 43.37637)
#> 9 mediseh_zostera_m_pnt.54  0  Spagna  POINT (3.291868 42.29132)

Other text comparisons include text pattern matching using operator LIKE. For example, to request maritime boundaries for countries starting with an S, we can use the following filter:

emodnet_get_layers(
  wfs = wfs,
  layers = "mediseh_zostera_m_pnt",
  cql_filter = "country LIKE 'S%'",
  reduce_layers = TRUE
)
#> Simple feature collection with 16 features and 3 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: -4.167154 ymin: 36.71226 xmax: 13.73725 ymax: 45.58252
#> Geodetic CRS:  WGS 84
#> First 10 features:
#>                      gml_id id  country                   the_geom
#> 1   mediseh_zostera_m_pnt.1  0   Spagna  POINT (-2.61314 36.71681)
#> 2   mediseh_zostera_m_pnt.2  0   Spagna POINT (-3.846598 36.75127)
#> 3   mediseh_zostera_m_pnt.3  0   Spagna POINT (-3.957785 36.72266)
#> 4   mediseh_zostera_m_pnt.4  0   Spagna POINT (-4.039712 36.74217)
#> 5   mediseh_zostera_m_pnt.5  0   Spagna POINT (-4.100182 36.72331)
#> 6   mediseh_zostera_m_pnt.6  0   Spagna POINT (-4.167154 36.71226)
#> 7   mediseh_zostera_m_pnt.7  0   Spagna POINT (-1.268366 37.55796)
#> 8  mediseh_zostera_m_pnt.39  0 Slovenia  POINT (13.73725 45.56948)
#> 9  mediseh_zostera_m_pnt.40  0 Slovenia  POINT (13.70701 45.58252)
#> 10 mediseh_zostera_m_pnt.41  0 Slovenia  POINT (13.71894 45.54535)

CQL/ECQL filters can also include any of the filter functions available in GeoServer, including multiple string functions which greatly increases the power of CQL expressions.

For example, we can request countries that contain l anywhere, including the first letter. To make the request case independent, we can turn country names to lowercase and then use a like comparison to a lowercase l:

emodnet_get_layers(
  wfs = wfs,
  layers = "mediseh_zostera_m_pnt",
  cql_filter = "strToLowerCase(country) LIKE '%l%'",
  reduce_layers = TRUE
)
#> Simple feature collection with 40 features and 3 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: 11.71682 ymin: 33.07783 xmax: 14.4714 ymax: 45.72451
#> Geodetic CRS:  WGS 84
#> First 10 features:
#>                      gml_id id country                  the_geom
#> 1   mediseh_zostera_m_pnt.9  0  Italia POINT (13.71831 45.70017)
#> 2  mediseh_zostera_m_pnt.10  0  Italia POINT (13.16378 45.72451)
#> 3  mediseh_zostera_m_pnt.11  0  Italia POINT (13.35982 45.70508)
#> 4  mediseh_zostera_m_pnt.12  0  Italia POINT (12.26722 45.25975)
#> 5  mediseh_zostera_m_pnt.13  0  Italia   POINT (12.31285 45.362)
#> 6  mediseh_zostera_m_pnt.14  0  Italia POINT (12.53509 45.53269)
#> 7  mediseh_zostera_m_pnt.15  0  Italia  POINT (12.48269 45.5065)
#> 8  mediseh_zostera_m_pnt.16  0  Italia POINT (12.77334 43.96751)
#> 9  mediseh_zostera_m_pnt.17  0  Italia POINT (13.62604 43.50809)
#> 10 mediseh_zostera_m_pnt.18  0  Italia POINT (13.46398 43.60892)

For more details on string functions, consult the full list in the geoserver function reference user documentation.**

Filtering numeric variables

A number of additional comparison operators, such as =, <>, >, >=, <, <= , arithmetic operators +, -, *, / as well as comparison and math functions can be used for filtering numeric variables.

This time we’ll use the mediseh_posidonia_nodata.

We can inspect again, using some of our interrogative functions to get information on layer attributes.

layer_attributes_get_names(
  wfs,
  layer = "mediseh_posidonia_nodata"
)
#> [1] "id"       "km"       "the_geom"

layer_attribute_inspect(
  wfs,
  layer = "mediseh_posidonia_nodata",
  attribute = "km"
)
#>     Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
#>   0.2371   6.0306  16.6190  40.1771  42.8958 472.9613

We can see that values range between 0.23 and ~473 with a mean of ~40.

Let’s say we are interested in values greater than 400. We can use the operator > to create our filter:

emodnet_get_layers(
  wfs = wfs,
  layers = "mediseh_posidonia_nodata",
  cql_filter = "km > 400",
  reduce_layers = TRUE
)
#> Simple feature collection with 4 features and 3 fields
#> Geometry type: MULTICURVE
#> Dimension:     XY
#> Bounding box:  xmin: -1.572609 ymin: 35.24538 xmax: 28.33243 ymax: 38.68017
#> Geodetic CRS:  WGS 84
#>                         gml_id id       km                       the_geom
#> 1   mediseh_posidonia_nodata.8  0 456.3759 MULTICURVE (LINESTRING (2.3...
#> 2 mediseh_posidonia_nodata.227  0 464.6573 MULTICURVE (LINESTRING (26....
#> 3 mediseh_posidonia_nodata.228  0 472.9613 MULTICURVE (LINESTRING (26....
#> 4 mediseh_posidonia_nodata.229  0 463.8207 MULTICURVE (LINESTRING (27....

To request a range we can use filter functions between and and.

emodnet_get_layers(
  wfs = wfs,
  layers = "mediseh_posidonia_nodata",
  cql_filter = "km between 40 and 400",
  reduce_layers = TRUE
)
#> Simple feature collection with 120 features and 3 fields
#> Geometry type: MULTICURVE
#> Dimension:     XY
#> Bounding box:  xmin: -2.1798 ymin: 30.26623 xmax: 34.59127 ymax: 45.35511
#> Geodetic CRS:  WGS 84
#> First 10 features:
#>                         gml_id id        km                       the_geom
#> 1   mediseh_posidonia_nodata.1  0 291.50323 MULTICURVE (LINESTRING (27....
#> 2   mediseh_posidonia_nodata.2  0  75.37950 MULTICURVE (LINESTRING (23....
#> 3   mediseh_posidonia_nodata.4  0 110.34480 MULTICURVE (LINESTRING (19....
#> 4  mediseh_posidonia_nodata.13  0  66.99746 MULTICURVE (LINESTRING (9.1...
#> 5   mediseh_posidonia_nodata.5  0 222.44651 MULTICURVE (LINESTRING (8.6...
#> 6   mediseh_posidonia_nodata.6  0  58.72822 MULTICURVE (LINESTRING (6.8...
#> 7   mediseh_posidonia_nodata.7  0 192.50519 MULTICURVE (LINESTRING (6.4...
#> 8  mediseh_posidonia_nodata.10  0  65.74354 MULTICURVE (LINESTRING (-1....
#> 9  mediseh_posidonia_nodata.11  0 138.68819 MULTICURVE (LINESTRING (4.8...
#> 10 mediseh_posidonia_nodata.17  0 184.05959 MULTICURVE (LINESTRING (10....

Using `OR` and `AND` statements

Filter conditions can also be a logical combination of other conditions using AND, OR or NOT. We showed previously how we could use a list of potential values to match an attribute to. This could also be achieved using an OR statement.

For example, the following query returns features where country is either Baltic Sea or Bulgaria.

wfs <- emodnet_init_wfs_client(service = "geology_seabed_substrate_maps")
emodnet_get_layers(
  wfs = wfs,
  layers = "seabed_substrate_1m",
  cql_filter = "country='Baltic Sea' OR country='Bulgaria'",
  reduce_layers = TRUE
)

Let’s say we also want to restrict the features returned those having shape_length > 1. We can include an additional condition which must be met using an AND statement.

filter_sf1 <- emodnet_get_layers(
  wfs = wfs,
  layers = "seabed_substrate_1m",
  cql_filter = "country='Baltic Sea' OR country='Bulgaria' AND shape_length>1",
  reduce_layers = TRUE
)
filter_sf1

The query returns features where country is Baltic Sea or Bulgaria but we can see that the minimum value is below the minimum we set with our filter. That’s because the AND filter is added only to the country='Bulgaria', so while only features with shape_length > 1 are returned where country is also Bulgaria, the filter is not applied to shape_length where country is Baltic Sea.

unique(filter_sf1$country)

min(filter_sf1$shape_length)

To add the AND filter to both OR filters, we can use a parenthesis:

filter_sf2 <- emodnet_get_layers(
  wfs = wfs,
  layers = "seabed_substrate_1m",
  cql_filter = "(country='Baltic Sea' OR country='Bulgaria') AND shape_length>1",
  reduce_layers = TRUE
)

Now the minimum value is indeed above 1. However, we only get features where country is 'Bulgaria'. That’s because the single Baltic Sea feature had a shape_length value smaller than 1 so is filtered out.

min(filter_sf2$shape_length)
unique(filter_sf2$country)

Finally filtering for shape_length < 1 returns features with shape_length values smaller than 1 for both country values.

filter_sf3 <- emodnet_get_layers(
  wfs = wfs,
  layers = "seabed_substrate_1m",
  cql_filter = "(country='Baltic Sea' OR country='Bulgaria') AND shape_length<1",
  reduce_layers = TRUE
)

max(filter_sf3$shape_length)
unique(filter_sf3$country)

Advanced use

There is more that can be accomplished by using the EMODnet WFS services than downloading data. The emodnet.wfs package is built on top of the ows4R library, meaning that all the functionalities of this package are available for emodnet.wfs. The ows4R returns a special type of R object called R6. You can learn more in Hadley Wickham’s chapter on R6 Objects of the Advance R book.

For instance: it is not efficient to read a large dataset into R just and later subset part of it. This requires longer waiting times and more bandwidth usage, and in very large datasets it would simply not be possible. For instance, all the occurrences data available through the EMODnet Biology portal are stored in one table: These are approximately 30 millions rows! In this case, we suggest you access the EMODnet Biology occurrence data through the download toolbox or the eurobis R package instead.