3.6. Feições OGC API

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.

Aqui está um rápido resumo informal das diferenças mais importantes entre o bem conhecido protocolo WFS e a OAPIF:

  • OAPIF é baseado em uma API REST

  • OAPIF deve seguir as OPENAPI especificações

  • 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 e HTML são cidadãos de primeira classe na OAPIF

  • OAPIF é autodocumentação (através do endpoint /api)

  • OAPIF é totalmente navegável (através de links) e buscável

Importante

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).

Nota

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. Representação de recursos

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:

Nome

Caminho

Descrição

Landing Page

/

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

Conformidade

/conformidade

Informações sobre a conformidade do serviço com as normas

API

/api

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

Coleções

/coleções

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

Coleção

/collections/{collectionId}

Informações sobre uma coleção (nome, metadados, extensão etc.)

Feições

/collections/{collectionId}/items

Lista das feições fornecidas pela coleção

Feição

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

Informações sobre uma feição única

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

Fig. 3.23 Página principal do servidor OAPIF

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.

Nota

This endpoint is analogue to WFS’s GetCapabilities

Lista de coleções

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

Fig. 3.24 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

Fig. 3.25 Página de detalhes da coleta do Servidor OAPIF

Lista de feições

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.

Nota

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

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

Fig. 3.26 Página de lista de recursos do servidor OAPIF

Detalhe da feição

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

Fig. 3.27 Página de detalhes do servidor OAPIF

3.6.3. Paginação

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/oapif/collection_one/items.json?offset=10&limit=10

Nota

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

3.6.4. Feature filtering

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

Filtro de data e hora

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.

Exemplo de URL:

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

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

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

http://localhost/qgisserver/oapif/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/oapif/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:

A ordem dos elementos separados por vírgula é:

  • Lower left corner, WGS 84 longitude

  • Lower left corner, WGS 84 latitude

  • Upper right corner, WGS 84 longitude

  • Upper right corner, WGS 84 latitude

Nota

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

Filtros de atributos

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/oapif/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/oapif/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/oapif/collection_one/items.json?sortby=name&sortdesc=1

3.6.6. Seleção de atributo

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

URL example:

retorna apenas o atributo nome

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

3.6.7. Personalizar as páginas HTML

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: Substituição de modelos). 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:

Funções de modelo personalizado

  • 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

  • estático( caminho ): retorna a URL completa para o caminho estático especificado. Por exemplo: “estático(“/style/black.css” )” com um caminho raiz “http://localhost/qgisserver/oapif” retornará “http://localhost/qgisserver/oapif/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( texto ): Retorna o texto de entrada com todas as novas linhas substituídas por “<br>” tags

Substituição de modelos

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.