Open Portrayal Framework: Sharing Styles and Updating Tiles for a New Generation of Maps
Maps, i.e. the graphical representation of geospatial elements in our world, are rendered today dynamically by applying a specific style to some vector or raster data. These styles have been reproduced over and over in the past; mostly due to the lack of powerful style APIs and Web interfaces that supported discovery, management, and access of styles.
The Open Portrayal Framework in OGC Testbed 15 is now addressing this gap by developing a conceptual model for styles together with a series of new APIs for styles, images, maps and tiles handling. All APIs will be consistent with the emerging OGC API family of standards and reuse a common set of core elements. Both the style model and the APIs will be handed over to the OGC Standards Program in late 2019 for consideration and future development into OGC Standards. These new open APIs will allow consumers to easily discover, download, modify, and re-distribute styles using Visual Style Editors, and even share modified versions with others who can in turn apply these styles to their own data.
Maps, Tiles, and Images APIs will complement existing OGC Web Service interface standards such as WMS and WMTS with updated, open APIs. These APIs will make new imagery available to image archives, retrieve updates from tile servers, and access any type of map following the same underlying principles and technologies that have made the World Wide Web so successful.
To be prepared for offline situations, the Open Portrayal Framework efforts of Testbed 15 are exploring mechanisms to add style information together with corresponding style elements, such as symbols or sprites, to GeoPackage containers. Together with the new APIs, GeoPackage extensions, as well as style models, form the Open Portrayal Framework depicted in Figure 1.
In the Open Portrayal Framework, a style is an encoding with essential information about itself, e.g. title, abstract description of the style, thumbnail, and information about the data it can be applied to. These metadata elements allow users to share, discover, select, and apply a style. A style further contains a sequence of rules or instructions that define how mapping engines shall render vector or raster data or, in OGC speak, features and coverages. These rules are expressed in style encodings that are usually provided as files. OGC Testbed 15 will make use of OGC Styled Layer Descriptors and Mapbox styles. Styles can reference symbols, sprites, or fonts that are stored remotely.
Figure 1. Overview of the main Testbed 15 components used to exercise an Open Portrayal Framework
This figure outlines the interactions of three different users:
- The yellow user interacts with a styles server that is offering OGC API Styles. Using this API, the user discovers relevant styles, loads them into a visual style editor, modifies them, and eventually uploads the modified styles to the Style server.
- The blue user interacts with the emerging OGC API Images, OGC API Tiles, and OGC API Maps. These three APIs allow systems to add new imagery to an image archive, to update a tile set based on that new source image and then serve the updates as tiles to customers. This capability will become increasingly important as more and more imagery satellites join the hundreds already in orbit… perhaps even providing basemaps that are updated daily using open APIs.
- The green user at the bottom interacts with the GeoPackage builder, which retrieves the latest tiles from the tile server. The GeoPackage builder further loads corresponding styles and style resources such as symbols or sprites, and delivers a GeoPackage to the user. The OGC API Tiles allows quick updates to previously built GeoPackages by requesting only updated scenes from the tile server.