Wednesday, June 27, 2018

What About Vector Tiles in GeoPackage?

I periodically get questions about whether GeoPackage can/will/should support vector tiles. Without getting into the benefits or drawbacks of vector tiles, let's just say it is an emerging capability that is getting a lot of community interest. As it turns out, supporting vector tiles in GeoPackage is the easy part. We already have a precedent for handling types other than features and raster tiles - the tiled gridded coverage extension.

The real issue is that OGC has not yet adopted an approach for vector tiling. What is the content encoding for a vector tile? Is it Protocol Buffers (as per Mapbox Vector Tiles - MVT)? GeoJSON? Something else? OGC members are reluctant to support a vector tiling approach that is only used with GeoPackage. (Supporting one-off solutions often has a poor cost-benefit story.) OGC simply needs to make a decision and a community standard would suffice.

Once OGC makes a decision, it is a straightforward matter to support it in GeoPackage. We would create a new extension that would define a new content type that depends on the tiles option. I don't think there would be any other ancillary tables like were needed for tiled gridded coverages, but if there are we can add those. This would get us to a community extension which will satisfy some people. If we want an official (OGC adopted) extension then we have to go through the normal process of OAB approval, open comment period, TC vote, etc. This process will probably go smoothly as long as we have proof of implementation.

UPDATE: After talking with a few people, I went ahead and drafted a community extension based on the MVT approach.

Wednesday, June 20, 2018

We Want Your (Technical) Feedback

Earlier this month, GeoPackage 1.2.1 (a corrigendum release) was approved by the OGC Technical Committee. In this release, we focused on clarifications and consistency. Look for publication and formal announcement soon. Until this occurs, you can review the release notes. 

I thought things would slow down after this but I was wrong. There has been a bunch of testing related to the Geospatial to the Edge Plugfest and this testing has revealed some areas of the standard that need to be refined. Some of these things are quite technical and I would like to get community feedback before we do anything rash. The following items have GitHub issues and corresponding pull requests. Please let us know if you think we are doing something wrong or if we can do better.

  1. Support for views. We've said for the beginning that GeoPackage is supposed to support views where appropriate. We clarified this as part of version 1.2.0 and 1.2.1 but we need to do more because of our requirement for primary keys on user defined tables. The concept of primary keys does not exist for views in SQLite so we are now proposing that if there is no primary key column, then the first column must be primary-key-like, containing unique integers.
    Update: a competing approach would add a column to "gpkg_contents" to name the primary key column for a particular set of contents. I like this idea (more easily discoverable) but it may be too late to add even an optional column to that table.
  2. Enforcing alignment between the CRSs in gpkg_contents, gpkg_geometry_columns, and gpkg_tile_matrix_set. I do not see how a GeoPackage can possibly be interoperable when the CRSs are not in alignment (how would a client decide which one to pick?) and I don't think anyone is doing this now, but since the question has come up more than once, we are taking the opportunity to tighten things up.
    Update: this issue was resolved.
  3. Ensuring that feature geometries fall within the extents in the Well Known Binary (WKB) values. These extents are used as an optimization for spatial querying and indexing. If they are not valid (which is apparently happening on occasion), incorrect results will be returned.
    Update: This is still being discussed.
Since we are introducing new requirements as part of these changes, they are penciled in as part of a possible 1.3.0 release. We have no timeline for when this might occur but figure some time in 2019. As always, remain committed to maintaining reverse compatibility as much as possible. We will continue to avoid introducing requirements that have the potential to break existing GeoPackages unless those GeoPackages couldn't possibly work as designed.

Friday, June 1, 2018

Where We Are with Feature Styling and Portrayal

The GeoPackage SWG is very interested in solving the portrayal issue - how to encode style/symbology for GeoPackage feature data so that it can be portrayed or rendered in a client application. It is the #1 thing I hear about and I am committed to getting there. While proprietary solutions exist, we want something that will be interoperable. It is just taking a while and we need to be patient because there are a lot of moving parts. 

SWG members have asked for three things in a candidate solution:
  1. SQL table-based (since GeoPackage is an SQLite encoding)
  2. Consistent across the OGC baseline (so that implementers can build a single basic approach for the baseline, as opposed to a GeoPackage-specific implementation)
  3. Loose coupling of the features and their styles (unlike KML, for example)
OGC hasn't nailed down a solution for #1 and #2 yet, but it is in progress. The Styled Layer Descriptor SWG has awoken from its decade-long slumber and is in the process of producing a v2 which is basically a total rewrite. In this version, they will implement a core and extensions model and support multiple encodings, resolving two major gaps in the original version. I was one of the contributors to a document called the Portrayal Concept Development Study Engineering Report which discusses these concepts in great detail. I hope for this document to be published in the next month or so. This will also be a hot topic at the OGC Technical Committee meetings in Ft. Collins next week.

To address #3, I believe the way to go is to harmonize GeoPackage and OWS Context so that the two work seamlessly with each other. I have written a discussion paper on the topic. I will present this paper to the GeoPackage SWG next week and if all goes well, it will be published by OGC later this month. I believe the concept is ready to go except for the style encoding. Once that is resolved, we should be able to standardize this pretty quickly. By the way, as far as I know, this would be the first time OGC adopts a standard that is an extension for two different standards simultaneously.