Wichtig

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3.6. OGC API Features

OGC API Features (OAPIF) is the first implementation of the new generation of OGC protocols. It is described by the OGC API - Features - Part 1: Core document.

The API can be reached on typical installations via http://localhost/qgisserver/wfs3

Here is a quick informal summary of the most important differences between the well known WFS protocol and OAPIF:

  • OAPIF is based on a REST API

  • OAPIF must follow the OPENAPI specifications

  • OAPIF supports multiple output formats but it does not dictate any (only GeoJSON and HTML are currently available in QGIS OAPIF) and it uses content negotiation to determine which format is to be served to the client

  • JSON and HTML are first class citizens in OAPIF

  • OAPIF is self-documenting (through the /api endpoint)

  • OAPIF is fully navigable (through links) and browsable

Wichtig

While the OGC API Features implementation in QGIS can make use of the MAP parameter to specify the project file, no extra query parameters are allowed by the OPENAPI specification. For this reason it is strongly recommended that MAP is not exposed in the URL and the project file is specified in the environment by other means (i.e. setting QGIS_PROJECT_FILE in the environment through a web server rewrite rule).

Bemerkung

The API endpoint provides comprehensive documentation of all supported parameters and output formats of your service. The following paragraphs will only describe the most important ones.

3.6.1. Resource representation

The implementation of OGC API Features in QGIS Server currently supports the following resource representation (output) formats:

  • HTML

  • JSON

The format that is actually served will depend on content negotiation, but a specific format can be explicitly requested by appending a format specifier to the endpoints.

Supported format specifier extensions are:

  • .json

  • .html

Additional format specifier aliases may be defined by specific endpoints:

  • .openapi: alias for .json supported by the API endpoint

  • .geojson: alias for .json supported by the Features and Feature endpoints

3.6.2. Endpoints

The API provides a list of endpoints that the clients can retrieve. The system is designed in such a way that every response provides a set of links to navigate through all the provided resources.

Endpoints points provided by the QGIS implementation are:

Name

Path

Beschreibung

Landing Page

/

General information about the service and provides links to all available endpoints

Conformance

/conformance

Information about the conformance of the service to the standards

API

/api

Full description of the endpoints provided by the service and the returned documents structure

Collections

/collections

List of all collections (i.e. ‚vector layers‘) provided by the service

Collection

/collections/{collectionId}

Information about a collection (name, metadata, extent etc.)

Funktionalitäten

/collections/{collectionId}/items

List of the features provided by the collection

Feature

/collections/{collectionId}/items/{featureId}

Information about a single feature

Similar to WFS-T (transactional Web Feature Service), it is possible to add, update and delete features (CRUD). The respective requests are described on „/api“.

Landing Page

The main endpoint is the Landing Page. From that page it is possible to navigate to all the available service endpoints. The Landing Page must provide links to

  • the API definition (path /api link relations service-desc and service-doc),

  • the Conformance declaration (path /conformance, link relation conformance), and

  • the Collections (path /collections, link relation data).

../../../_images/server_wfs3_landing_page.png

Abb. 3.24 Server OAPIF landing page

API Definition

The API Definition is an OPENAPI-compliant description of the API provided by the service. In its HTML representation it is a browsable page where all the endpoints and their response formats are accurately listed and documented. The path of this endpoint is /api.

The API definition provides a comprehensive and authoritative documentation of the service, including all supported parameters and returned formats.

Bemerkung

This endpoint is analogue to WFS’s GetCapabilities

Collections list

The collections endpoint provides a list of all the collections available in the service. Since the service „serves“ a single QGIS project the collections are the vector layers from the current project (if they were published as WFS in the project properties). The path of this endpoint is /collections/.

../../../_images/server_wfs3_collections.png

Abb. 3.25 Server OAPIF collections list page

Collection detail

While the collections endpoint does not provide detailed information about each available collection, that information is available in the /collections/{collectionId} endpoints. Typical information includes the extent, a description, CRSs and other metadata.

The HTML representation also provides a browsable map with the available features.

../../../_images/server_wfs3_collection.png

Abb. 3.26 Server OAPIF collection detail page

Features list

This endpoint provides a list of all features in a collection knowing the collection ID. The path of this endpoint is /collections/{collectionId}/items.

The HTML representation also provides a browsable map with the available features.

Bemerkung

This endpoint is analogue to GetFeature in WFS 1 and WFS 2.

../../../_images/server_wfs3_features.png

Abb. 3.27 Server OAPIF features list page

Feature detail

This endpoint provides all the available information about a single feature, including the feature attributes and its geometry. The path of this endpoint is /collections/{collectionId}/items/{itemId}.

The HTML representation also provides a browsable map with the feature geometry.

../../../_images/server_wfs3_feature.png

Abb. 3.28 Server OAPIF feature detail page

3.6.3. Pagination

Pagination of a long list of features is implemented in the OGC API through next and prev links, QGIS server constructs these links by appending limit and offset as query string parameters.

URL example:

http://localhost/qgisserver/wfs3/collection_one/items.json?offset=10&limit=10

Bemerkung

The maximum acceptable value for limit can be configured with the QGIS_SERVER_API_WFS3_MAX_LIMIT server configuration setting (see: Environment variables).

3.6.4. Objektfiltrierung

The features available in a collection can be filtered/searched by specifying one or more filters.

Date and time filter

Collections with date and/or datetime attributes can be filtered by specifying a datetime argument in the query string. By default the first date/datetime field is used for filtering. This behavior can be configured by setting a „Date“ or „Time“ dimension in the QGIS Server ► Dimension section of the layer properties dialog.

The date and time filtering syntax is fully described in the API Definition and also supports ranges (begin and end values are included) in addition to single values.

URL examples:

Returns only the features with date dimension matching 2019-01-01

http://localhost/qgisserver/wfs3/collection_one/items.json?datetime=2019-01-01

Returns only the features with datetime dimension matching 2019-01-01T01:01:01

http://localhost/qgisserver/wfs3/collection_one/items.json?datetime=2019-01-01T01:01:01

Returns only the features with datetime dimension in the range 2019-01-01T01:01:01 - 2019-01-01T12:00:00

http://localhost/qgisserver/wfs3/collection_one/items.json?datetime=2019-01-01T01:01:01/2019-01-01T12:00:00

Bounding box filter

A bounding box spatial filter can be specified with the bbox parameter:

The order of the comma separated elements is:

  • Lower left corner, WGS 84 longitude

  • Lower left corner, WGS 84 latitude

  • Upper right corner, WGS 84 longitude

  • Upper right corner, WGS 84 latitude

Bemerkung

The OGC specifications also allow a 6 item bbox specifier where the third and sixth items are the Z components, this is not yet supported by QGIS server.

URL example:

http://localhost/qgisserver/wfs3/collection_one/items.json?bbox=-180,-90,180,90

If the CRS of the bounding box is not WGS 84, a different CRS can be specified by using the optional parameter bbox-crs. The CRS format identifier must be in the OGC URI format:

URL example:

http://localhost/qgisserver/wfs3/collection_one/items.json?bbox=913191,5606014,913234,5606029&bbox-crs=http://www.opengis.net/def/crs/EPSG/9.6.2/3857

Attribute filters

Attribute filters can be combined with the bounding box filter and they are in the general form: <attribute name>=<attribute value>. Multiple filters can be combined using the AND operator.

URL example:

filters all features where attribute name equals „my value“

http://localhost/qgisserver/wfs3/collection_one/items.json?attribute_one=my%20value

Partial matches are also supported by using a * („star“) operator:

URL example:

filters all features where attribute name ends with „value“

http://localhost/qgisserver/wfs3/collection_one/items.json?attribute_one=*value

3.6.5. Feature sorting

It is possible to order the result set by field value using the sortby query parameter.

The results are sorted in ascending order by default. To sort the results in descending order, a boolean flag (sortdesc) can be set:

http://localhost/qgisserver/wfs3/collection_one/items.json?sortby=name&sortdesc=1

3.6.6. Attribute selection

The feature attributes returned by a Features list call can be limited by adding a comma separated list of attribute names in the optional properties query string argument.

URL example:

returns only the name attribute

http://localhost/qgisserver/wfs3/collection_one/items.json?properties=name

3.6.7. Customize the HTML pages

The HTML representation uses a set of HTML templates to generate the response. The template is parsed by a template engine called inja. The templates can be customized by overriding them (see: Template overrides). The template has access to the same data that are available to the JSON representation and a few additional functions are available to the template:

Custom template functions

  • path_append( path ): appends a directory path to the current url

  • path_chomp( n ): removes the specified number „n“ of directory components from the current url path

  • json_dump( ): prints the JSON data passed to the template

  • static( path ): returns the full URL to the specified static path. For example: „static( „/style/black.css“ )“ with a root path „http://localhost/qgisserver/wfs3“ will return „http://localhost/qgisserver/wfs3/static/style/black.css“.

  • links_filter( links, key, value ): Returns filtered links from a link list

  • content_type_name( content_type ): Returns a short name from a content type, for example „text/html“ will return „HTML“

  • nl2br( text ): Returns the input text with all newlines replaced by „<br>“ tags

  • starts_with( string, prefix ): returns true if a string begins with the provided string prefix, false otherwise

Template overrides

Templates and static assets are stored in subdirectories of the QGIS server default API resource directory (/usr/share/qgis/resources/server/api/ on a Linux system), the base directory can be customized by changing the environment variable QGIS_SERVER_API_RESOURCES_DIRECTORY.

A typical Linux installation will have the following directory tree:

/usr/share/qgis/resources/server/api/
└── ogc
    ├── schema.json
    ├── static
    │   ├── jsonFormatter.min.css
    │   ├── jsonFormatter.min.js
    │   └── style.css
    └── templates
        └── wfs3
            ├── describeCollection.html
            ├── describeCollections.html
            ├── footer.html
            ├── getApiDescription.html
            ├── getFeature.html
            ├── getFeatures.html
            ├── getLandingPage.html
            ├── getRequirementClasses.html
            ├── header.html
            ├── leaflet_map.html
            └── links.html

To override the templates you can copy the whole tree to another location and point QGIS_SERVER_API_RESOURCES_DIRECTORY to the new location.