I.  Abstract

This Engineering Report describes an extension for storing Releasable Basemap Tiles (RBT) in GeoPackage developed during an OGC Code Sprint initiative sponsored by the US Army Geospatial Center (AGC) which took place in March 2024. During this initiative, participants Ecere Corporation, Compusult Limited and TechMaven Geospatial, assembled into this single extension document draft extensions developed in previous OGC initiatives for storing vector tiles, semantic annotations and portrayal styles in GeoPackages. Minor updates and clarifications to the requirements of these draft extensions were made based on additional experimentation performed during the sprint. Additional requirements and recommendations were also introduced for this extension, including requirements specific to RBT content, a recommendation to include fonts used by styles, requirements to support compressed vector tiles, as well as requirements further restricting how to store vector tiles in the context of this extension to improve interoperability.

III.  Preface

This Engineering Report describes an extension for storing Releasable Basemap Tiles (RBT) in GeoPackage developed during an OGC Code Sprint initiative sponsored by the US Army Geospatial Center (AGC) which took place in March 2024. The participants in this initiative included Ecere Corporation, Compusult Limited and TechMaven Geospatial.

This extension was written with the intent to be adapted into a National System for Geospatial Intelligence (NSG) standard.

This RBT GeoPackage extension was assembled from multiple draft GeoPackage extensions developed in previous OGC initiatives including the Vector Tile Pilots (Phase 1+extension and Phase2), Testbed 15 and Testbed 16, which at the time of the initiative, had not yet been adopted as OGC standards. These extensions had been developed primarily by Image Matters LLC, in collaboration with the other participants of these initiatives, which performed several Technology Integration Experiments to validate interoperability across implementations from different technology vendors. In addition to a significant re-organization of the content, minor changes and clarifications were made to the technical details (such as some of the SQLite tables) sourced from these extensions as part of this initiative.

Since these different source draft specifications may still eventually evolve into multiple approved OGC standards and conformance classes, this specification maintains a separation into conceptual classes of requirements so as to facilitate maintaining the alignment of these individual extensions with this candidate NSG standard during further OGC standardization efforts.

For the purpose of improving interoperability, this extension defines additional requirements, beyond what would be specified in these more general and flexible OGC GeoPackage extensions, which further restrict the nature and encoding of content stored in an RBT GeoPackage.

From the perspective of this RBT extension, the entirety of this document describes a single GeoPackage extension and requirements class, consisting of mandatory requirements and optional recommendations.

An annex of this report includes details of the implementation by participants of both producers and consumers (viewers) for RBT content stored in GeoPackages using this extension. The annex also presents challenges and lessons learned during the initiative. Participants produced GeoPackages from RBT content and portrayal information provided by AGC as Mapbox Vector Tiles and TileJSON stored in MBTiles, as well as Mapbox / MapLibre styles. Participants also performed Technology Integration Experiments testing consumption of RBT GeoPackages produced by the other implementations. An additional annex defines an Abstract Test Suite for the extension, and another annex provides suggestions for future related work.

1.  Scope

This Engineering Report describes an extension for storing Releasable Basemap Tiles (RBT) in GeoPackage developed during an OGC Code Sprint initiative.

This extension regroups requirements specific to storing RBT data products in GeoPackages, requirements for storing vector tiles in GeoPackages, requirements for defining semantic annotations in GeoPackages, as well as requirements to include portrayal information in GeoPackages.

In addition to the extension itself, this Engineering Report includes details of three implementations developed during the code sprint, including Technology Integration Experiments demonstrating interoperability between them.

2.  Conformance

This extension defines of a single mandatory conformance class, but is broken down into conceptual requirement classes which may eventually correspond to individual standardized OGC GeoPackage extensions.

The requirements apply to either of these two standardization target types:

Conformance with this extension shall be checked using all the relevant tests specified in Annex A (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be achieved to claim conformance are specified in the NSG Compliance Testing Policies and Procedures and the NSG Compliance Testing web site.

In order to conform to this candidate NSG data product standard, a software implementation producing a compliant data product shall implement the core requirement classes defined in this extension.

All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.

3.  Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

Jeff Yutzler: OGC 12-128r19, OGC® GeoPackage Encoding Standard. Open Geospatial Consortium (2024). http://www.opengis.net/doc/IS/geopackage/1.4.0.

Joan Masó, Jérôme Jacovella-St-Louis: OGC 17-083r4, OGC Two Dimensional Tile Matrix Set and Tile Set Metadata. Open Geospatial Consortium (2022). http://www.opengis.net/doc/IS/tms/2.0.0.

4.  Terms and definitions

This document uses the terms defined in OGC Policy Directive 49, which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this document and OGC documents do not use the equivalent phrases in the ISO/IEC Directives, Part 2.

This document also uses terms defined in the OGC Standard for Modular specifications (OGC 08-131r3), also known as the ‘ModSpec’. The definitions of terms such as standard, specification, requirement, and conformance test are provided in the ModSpec.

For the purposes of this document, the following additional terms and definitions apply.

tiling scheme consisting of a set of tile matrices defined at different scales covering approximately the same area and having a common coordinate reference system.

[SOURCE: OGC 17-083r4]

a URI identifying a class of geospatial data whose component data layers conform to a particular logical schema

A registry of geodataclasses would ideally resolve these URIs to metadata including these schemas. A geodataclass serves multiple purposes, including the ability to easily identify, using a simple identifier comparison, relevant datasets for a particular purpose, the compatibility of datasets as inputs for processes, the compatibility of the output of a process with the input of another process, as well as the compatibility of portrayal style information with a particular dataset (see also https://github.com/opengeospatial/styles-and-symbology/issues/12).

a specification developed by Mapbox for encoding tiled vector data using protocol buffers.

tilesets of foundational geospatial data developed by the US Army Geospatial Center for public release, including vector tilesets of cultural and physical features, hillshaded elevation data and optional imagery

experiments performed between different implementations of a specification or standard to test and demonstrate interoperability between them, and in the case of TIEs performed in the context of an initiative to validate a draft specification or standard, to validate the soundness and completeness of the requirements specified therein

5.  Overview

This Engineering Report describes an extension for storing Releasable Basemap Tiles (RBT) in GeoPackage developed during the March 2024 OGC Code Sprint initiative sponsored by the US Army Geospatial Center (AGC).

The report is organized into four main sections:

and three annexes:

6.  Releasable Basemap Tiles GeoPackage Extension

7.  Vector Tiles

CREATE TABLE tiles_physical(
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   zoom_level INTEGER NOT NULL,
   tile_column INTEGER NOT NULL,
   tile_row INTEGER NOT NULL,
   tile_data BLOB NOT NULL,
   UNIQUE (zoom_level, tile_column, tile_row)
)

CREATE TABLE gpkgext_vt_layers (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   table_name TEXT NOT NULL REFERENCES gpkg_contents (table_name),
   name TEXT NOT NULL,
   description TEXT,
   minzoom INTEGER,
   maxzoom INTEGER,
   attributes_table_name TEXT,
   geometry_dimension INTEGER
)

CREATE TABLE gpkgext_vt_fields (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   layer_id INTEGER REFERENCES gpkgext_vt_layers,
   name TEXT NOT NULL,
   type TEXT
)

CREATE TABLE gpkgext_content_types (
   content_id INTEGER REFERENCES gpkg_contents,
   media_type TEXT,
   encoding TEXT
)

8.  Semantic Annotations

CREATE TABLE gpkgext_semantic_annotations (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   type TEXT NOT NULL, title TEXT NOT NULL,
   description TEXT,
   uri TEXT
)

CREATE TABLE gpkgext_sa_reference (
   table_name TEXT NOT NULL,
   key_column_name TEXT NOT NULL,
   key_value INTEGER NOT NULL,
   sa_id INTEGER NOT NULL REFERENCES gpkgext_semantic_annotations,
   UNIQUE(table_name, key_column_name, key_value, sa_id)
)

9.  Styling

CREATE TABLE gpkgext_styles (
   id INTEGER PRIMARY KEY,
   style TEXT NOT NULL,
   description TEXT,
   uri TEXT,
   UNIQUE(uri)
)

CREATE TABLE gpkgext_stylesheets (
   id INTEGER PRIMARY KEY,
   style_id INTEGER REFERENCES gpkgext_styles,
   format TEXT NOT NULL,
   stylesheet BLOB NOT NULL,
   UNIQUE(style_id, format)
)

CREATE TABLE gpkgext_symbols (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   uri TEXT,
   symbol TEXT NOT NULL,
   title TEXT NOT NULL,
   description TEXT
)

CREATE TABLE gpkgext_symbol_images (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   symbol_id INTEGER NOT NULL REFERENCES gpkgext_symbols,
   content_id INTEGER NOT NULL REFERENCES gpkgext_symbol_content,
   width INTEGER,
   height INTEGER,
   offset_x INTEGER,
   offset_y INTEGER,
   pixel_ratio INTEGER
)

CREATE TABLE gpkgext_symbol_content (
   id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
   format TEXT NOT NULL,
   content BLOB NOT NULL,
   uri TEXT NOT NULL
)

CREATE TABLE gpkgext_fonts(
   id INTEGER PRIMARY KEY,
   name TEXT UNIQUE,
   font BLOB,
   glyphs BLOB
)