Publication Date: 2020-07-08

Approval Date: 2020-06-26

Submission Date: 2020-05-01

Reference number of this document: OGC 19-082r1

Reference URL for this document: http://www.opengis.net/doc/PER/vtp2-D001

Category: OGC Public Engineering Report

Editor: Sergio Taleisnik

Title: OGC Vector Tiles Pilot 2: Tile Set Metadata Engineering Report

OGC Public Engineering Report

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Copyright © 2020 Open Geospatial Consortium. To obtain additional rights of use, visit http://www.opengeospatial.org/

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This document is not an OGC Standard. This document is an OGC Public Engineering Report created as a deliverable in an OGC Interoperability Initiative and is not an official position of the OGC membership. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an OGC Standard. Further, any OGC Public Engineering Report should not be referenced as required or mandatory technology in procurements. However, the discussions in this document could very well lead to the definition of an OGC Standard.

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Table of Contents

1. Subject

The OGC Vector Tiles Pilot 2: Tile Set Metadata Engineering Report (ER) describes a conceptual model for Tile Set Metadata that provides information about the intended usage of a Tile Set as well as the origin, security level, tiling scheme, layers and feature properties contained within. In this ER, a tile set is a series of tiles containing data and following a common tiling scheme.

The metadata is intended to facilitate retrieval of tile sets and describes the major characteristics of tile sets without actually accessing the tiles nor the content contained in a tile. Such a process could be time consuming when there are a large number of tiles in a tile set.

Additionally, this ER summarizes the discussions about Tile Set Metadata among the VTP2 participants, and draws up conclusions and recommendations for future work on the subject.

Finally, this ER describes the Technology Integration Experiments (TIEs) performed to test the prototype implementation of the proposed Tile Set Metadata Model on API endpoints, client applications, and GeoPackages.

2. Executive Summary

Tile sets are a well-established method for structuring, storing, and accessing manageable packages of tiled data, both offline and online. As the use of tile sets increases and the need for seamless transitions between online and offline environments increases, the need for efficient access to the tile set metadata becomes more critical.

The objective of VTP2 was to deliver a consistent, interoperable online/offline architecture for vector tiles based on feature servers, tile servers and GeoPackage.

The metadata activities of VTP2 can be grouped into three major components:

  • Designing the Tile Set Metadata Model.

  • Capturing and summarizing discussions and recommendations.

  • Describing TIEs implementing the Tile Set Metadata Model.

The Tile Set Metadata Model built in VTP2 is based on requirements established in the Call For Participation (CFP) and then further refined through the course of the Pilot. The initial metadata requirements specified in the CFP were to:

  • Develop a metadata model for vector tiles and stored tile caches.

  • Extend the National System for Geospatial Intelligence (NSG) Metadata Foundation (NMF).

  • Design the metadata to describe fundamental aspects of the tiles, such as date, creator, source, etc.

  • Specify metadata elements that describe the tiling scheme (tile matrix set).

  • Define how space is partitioned into individual tiles, substitution variables to identify tiles using a three-part identifier such as {level} (zoom), {row} (vertical) and {col} (column: horizontal).

  • Provide elements for Style names or identifiers and metadata to describe multi-layer vector tiles.

  • Provide other elements required to support the Vector Tiles Filter language [1].

  • Provide a standard resource path structure to consistently present simple metadata for vector tiles as an extension to tile and feature servers, standalone tile caches and GeoPackage tables.

Note
 The term tile cache was used in the VTP2 CFP as a synonym for tile set repositories. The use of this term was subject to discussion throughout the Pilot and reflected by this ER.

Since the CFP was defined a year before its release, many requirements evolved in order to cope with the developments carried out since then. Additionally, conversations throughout the Pilot further refined these requirements: The requirement to extend the NMF was changed to limiting the NMF elements implemented to only those considered of value to the tile set use cases presented in this Pilot, and the requirement for describing tile and tile set metadata was changed to exclusively describing tile set metadata.

The metadata model defined in this Pilot can be divided into three main groups:

  • Describing fundamental aspects of the TileSet (representation of a tile set) such as names, identifiers, dates, owners, origin, security levels, etc.

  • Describing the tiling scheme by storing the Tile Matrix Set information of the TileSet. This helps identify the geographic extent, the level of detail and the tiling scheme.

  • Describing the layers and feature properties stored within, in order to understand the use and application domain of the TileSet.

A novel approach to store and access Tile Set Metadata was implemented. The creation of an additional resource to an OGC Application Programming Interface (API) was discarded for many technical reasons. This led to consensus that all the Tile Set Metadata should be stored in a stand-alone file that would be located in the root folder of the offline repository.

2.1. Business Value

The main benefit of transitioning from raster tiles to vector tiles has been the possibility of flexible map styling and the reduction of storage space required for maps, the latter allowing for maps being stored on devices with lower storage capacity as well as requiring lower bandwidth communications for transmission. In some cases, depending on the file format, maps structured using tiled vector/feature data can be 20 to 30 times smaller than the same maps represented by raster tiles for a similar level of detail. This reduction enables the possibility of storing a large number of maps (i.e.,tile sets) into secure and lightweight removable media devices.

The VTP2 initiative implemented a scenario (Figure 1) in which a tile set repository (such as GeoPackage) is being used by a humanitarian relief convoy in the middle of the desert. The convoy has limited to no connectivity and is only supported by a group of interconnected systems working and communicating with each other. The repositories are generated in Command Post Computing Environments (CPCE) and comprise tile sets, styles, maps and routes served by National-level Services and Enterprise-level Services, which communicate with each other through implementations of OGC APIs for styles, tiles, images and routes.