OGC RBT Follow-up 2

Last week, I wrote about the Releasable Basemap Tiles Sprint and how tiles were used as an enabling technology to efficiently deliver feature data in a GeoPackage. Here, I will focus on the other half of the problem, how to visualize the feature data once it has been delivered from the GeoPackage to the client. Proper portrayal requires both styles (how lines, polygons, and text are drawn) and symbols (reusable graphic elements to be used for points and fills). We have two basic challenges[1]:

1. Architecture – we want to retain flexibility. In some environments, the styles and symbols must be delivered with the data so that the system can operate in a Disconnected, Intermittent, or Limited (DIL) environment. In other cases, the styles and symbols will be provided separately by another system. A naming scheme must allow clients to use either architecture seamlessly.
2. Interoperability – there is no perfect standard for conveying styles or symbols. They all have strengths and weaknesses[2]. We want to support commonly used standards and specifications in a way that is discoverable by clients.

The RBT Engineering Report[3] includes a section that includes a draft styling specification. As with the tiled feature data approach, the approach here divides requirements into conformance classes. The engineering report presents three conformance classes.

1. The base class establishes the schemas for four new tables needed to describe the content.
2. A second class is for fonts, which need their own table.
3. An additional class is for Mapbox GL, the styling standard used in the code sprint.

There is a lot of power in the work we have done. Sprite sheets can be used to deliver multiple symbols in the same file. The URI mechanism will allow styles and symbols to be packaged with the GeoPackage or through another system. We do have some additional work to do before we are ready to adopt this work as a standard beyond the administrative work of standards-writing:

1. A conceptual model for styling and symbology metadata, something for our tables to map to that will encourage interoperability with other emerging standards such as OGC API – Styles.
2. Additional conformance classes for other styling formats, including SLD/SE, CartoSym CSS, and CartoSym JSON, or at least an interoperable way to express which formats are being used.

[1] See https://geopackage.blogspot.com/2018/06/where-we-are-with-feature-styling-and.html for further discussion on this topic.

[2] Obligatory XKCD reference.

[3] The draft is here but a final version will be added here upon publication.