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move owc to its own repo (#24)
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‎owc_geopackage_extension.md

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# GeoPackage 1.0 Extension
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# GeoPackage-OWC
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**DRAFT**
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QGPKG offers read support for the GeoPackage-OWC extension. Goal of the extension is to store context and styling of a mapping project in a standardised way as part of a GeoPackage file. The specification is maintained at [this repository](https://github.com/GeoCat/geopackage-owc-spec).
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Extension follows template from Annex I of the OGC [GeoPackage 1.0 Specification](http://www.geopackage.org/).
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The extension introduces 3 tables;
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- owc_context; which stores the project context (layer order)
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- owc_style; which store the layer styling
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- owc_resource; stores additional resources (thumbnails, fonts, documentation)
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## Extension Title
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**OWS Context**
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## Introduction
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The main goal of the extension is to store context and styling of a mapping project as part of a GeoPackage file containing the data it refers to. The extension aims at similar use cases as presented in [The USGS GeoPackage Styling Experiment in Testbed 12](http://docs.opengeospatial.org/per/16-037.html), however the approach is a bit different.
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Approach: Table owc_context stores one or multiple OWS Context files that defines a set of layers on a set of data tables using certain styles. Each layer is encoded in an `ows:entry` element; each style is encoded **whithin** the layer, as a `ows:offering`, in the `owc:styleSet` element. Any data table or style can be referenced by a layer in an OWS Context multiple times. Table owc_style defines which styles are available. Resources from owc_resources can be referenced from metadata, styles or context, to enable inclusion of images, pdf, fonts etc.
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[OWS Context](http://www.owscontext.org/) 1.0 defines the capability to reference layers from OWS services and certain filetypes (gml/kml). In this extension we introduce the capability in OWS context to reference tables local to the Geopackage.
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The OWS Context specification is extended with an `offeringtype="gpkg"` and OWS-context is extended to support referencing resources local to the Geopackage by using `#table={table}[&{fieldname}={value}]`.
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```xml
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<owc:offering code="http://www.opengis.net/spec/owc-atom/1.0/req/gpkg">
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<owc:content type="application/x-sqlite" href="#table=MyPoints" />
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<owc:styleSet>
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<owc:name>simple_point</owc:name>
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<owc:title>Simple point</owc:title>
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<owc:content href="#table=owc_style&amp;name=simplePoint" type="application/sld+xml"/>
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</owc:styleSet>
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</owc:offering>
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```
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OWC offering MUST include a styleSet reference, referencing an external SLD file or an SLD from the owc_style table.
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## Extension Author
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Paul van Genuchten, Joana Simoes.
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## Extension Name or Template
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`owc`
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## Extension Type
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Extension of concepts
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## Applicability
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This extension applies to additional tables `owc_context`, `owc_resources`, `owc_style`.
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## Scope
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Read-write
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## Requirements
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### GeoPackage
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#### Extension tables
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An Extended GeoPackage with Context support MAY contain the following tables or views:
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**owc_context**
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| Column | type | Description |
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|----|-----|----|
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| name | varchar(30) unique | Context name |
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| abstract | text | Context abstract |
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| author | text | Comma separated list of authors |
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| language | text | based on RFC-3066 code|
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| mime_type | text NOT NULL | [mime type](http://www.iana.org/assignments/media-types/media-types.xhtml) of context ([application/atom+xml](https://portal.opengeospatial.org/files/?artifact_id=55183) / [application/json](https://github.com/opengeospatial/owscontext/tree/master/json) / [application/vnd.owc+xml](https://portal.opengeospatial.org/files/?artifact_id=55182)) |
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| content | text NOT NULL | Content of OWS_Context encoded as indicated in `mime_type` |
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**owc_style**
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| Column | type | Description |
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|----|-----|----|
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| name | varchar(30) * | |
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| abstract | text | |
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| mime_type | varchar(30) * | The [mime type](http://www.iana.org/assignments/media-types/media-types.xhtml) of the style ( [application/vnd.sld+xml](http://www.opengeospatial.org/standards/sld) / text/css / application/json) |
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| content | text NOT NULL | Content of the style encoded as indicated in `mime_type` |
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A unique index of name and mime_type allows clients to select the most optimal mime-type for rendering (some mime-types may support additional vendor parameters). It is recommended to at least provide one style as appliction/vnd.sld+xml.
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**owc_resource**
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| Column | type | Description |
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|----|-----|----|
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| name | varchar(30) unique | Name of resource (file name) |
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| mime_type | text NOT NULL | The [mime type](http://www.iana.org/assignments/media-types/media-types.xhtml) of the resource (image/png, application/pdf, ...) |
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| content | blob NOT NULL | Binary content of resource |
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### GeoPackage SQLite Configuration
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None
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### GeoPackage SQLite Extension
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None
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A GeoPackage-OWC example is provided [here](examples/multiple_layers.gpkg)

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