OGC Vector Tiles Pilot: WFS 3.0 Vector Tiles Extension Engineering Report

Publication Date: 2019-02-11

Approval Date: 2018-12-13

Submission Date: 2018-11-09

Reference number of this document: OGC 18-078

Reference URL for this document: http://www.opengis.net/doc/PER/vtp-wfs3

Category: Public Engineering Report

Editor: Panagiotis (Peter) A. Vretanos

Title: OGC Vector Tiles Pilot: WFS 3.0 Vector Tiles Extension Engineering Report

OGC Engineering Report

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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 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. Summary
2. References
3. Terms and definitions
- 3.1. Abbreviated terms
4. Overview
5. Vector Tiles API for WFS 3.0
6. Component Implementations
Appendix A: Abstract Test Suite
Appendix B: Revision History
Appendix C: Bibliography

1. Summary

Feature data tiling, colloquially referred to as 'vector tiling', is a data delivery method that allows for large vector feature datasets to be systematically split into subsets or tiles [1]. This engineering report (ER) presents an extension specification for publishing of vector tiles data through an Application Programming Interface (API) that conforms to the emerging version 3.0 of the Web Feature Service (WFS) standard. The core of the WFS 3.0 standard offers direct fine-grained access to geospatial information at the feature level. The WFS standard specifies discovery and query operations for web services that publish feature data. Extensions to the WFS 3.0 Core API offer other capabilities such as transaction operations.

Note	This engineering report interchangeably uses both 'tiled feature data' and the colloquial term 'vector tiles'.

1.1. Requirements & Research Motivation

The research presented in this engineering report has been motivated by the increasing adoption of vector tiling within the geospatial industry.

The engineering report addresses deliverable D001 of the Vector Tiles Pilot. The Vector Tiles Pilot Call for Participation (CFP) outlines the deliverable as follows.

"D001: WFS 3.0 Extension Engineering Report - A WFS 3.0 Extension written as a draft OGC standard. The report shall take into account the WFS extension approach documented in Testbed 13. The WFS 3.0 standard is written as a RESTful reusable OpenAPI set of components with responses in JSON and HTML."

1.2. Prior-After Comparison

To date, the only OGC web service standard that offered a tiling approach was the Web Map Tile Service (WMTS) standard. Recently, a tiling storage mechanism has also been incorporated into the OGC GeoPackage standard. In contrast however, the WFS standard and its extensions do not currently offer a tiling mechanism.

Recognizing the increasing adoption of vector tiling across the industry, OGC produced a vector tiling engineering report in the OGC Testbed-13 project [2]. The OGC Testbed-13 Vector Tiles Engineering Report described the evaluation of existing vector tiling solutions and reported on WFS vector tiles experimentation conducted during the testbed. Amongst other recommendations, the engineering report recommended the extension of the WFS standard for support vector tiling.

1.3. Recommendations for Future Work

Future work should explore:

The potential for asynchronous support for the Vector Tiles extension of WFS 3.0 using the OGC PubSub specification.
The development of Executable Test Suites for compliance testing of implementations of the Vector Tiles extension of WFS 3.0.

1.4. Document contributor contact points

All questions regarding this document should be directed to the editor or the contributors:

Contacts

Name	Organization
Panagiotis (Peter) A. Vretanos (editor)	CubeWerx Inc.
Clemens Portele	interactive instruments GmbH
Kalimar Maia	Mapbox
Brian Davidson	Mapbox
Luis Bermudez	Open Geospatial Consortium
Gobe Hobona	Open Geospatial Consortium
Andrea Aime	GeoSolutions
Chia-Cheng (Ricky) Lin	GIS.FCU
Jerome St-Louis	Ecere

Name

Organization

Panagiotis (Peter) A. Vretanos (editor)

CubeWerx Inc.

Clemens Portele

interactive instruments GmbH

Kalimar Maia

Mapbox

Brian Davidson

Mapbox

Luis Bermudez

Open Geospatial Consortium

Gobe Hobona

Open Geospatial Consortium

Andrea Aime

GeoSolutions

Chia-Cheng (Ricky) Lin

GIS.FCU

Jerome St-Louis

Ecere

1.5. Foreword

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.

Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.

2. References

The following normative documents are referenced in this document.

3. Terms and definitions

For the purposes of this report, the definitions specified in Clause 4 of the OWS Common Implementation Standard OGC 06-121r9 shall apply. In addition, the following terms and definitions apply.

feature

abstraction of real world phenomena [ISO 19101-1:2014]

feature collection; collection
```
a set of features from a dataset
```

tile

a rectangular representation of geographic data, often part of a set of such elements, covering a spatially contiguous extent which can be uniquely defined by a pair of indices for the column and row along with an identifier for the tile matrix (adapted from OGC 07-057r7)

tiling scheme

a scheme that defines how space is partitioned into individual tiles. It defines the coordinate reference system, the geometric properties of a tile, which space a uniquely identified tile occupies, and reversely which unique identifier corresponds to a space satisfying the geometric properties to be a tile.

tileset

a series of tiles of a given tiling scheme containing encoded vector features from one or more feature collections

3.1. Abbreviated terms

API Application Programming Interface
MVT Mapbox Vector Tiles
VTP Vector Tiles Pilot
WFS Web Feature Service
WMTS Web Map Tile Service

4. Overview

Clause 5 describes two Vector Tile APIs for WFS 3.0; a 'Direct tile access path API' and a 'Feature access path API'. Both are primarily designed to support the portrayal or visualization use case. The first API, like the WMTS, is based on tiled feature data being organized into regularly gridded tiles arranged into a pyramid of tile matrices at a fixed set of resolutions (i.e. tile matrix set). The latter API treats features the way the WFS has always treated features and views vector tiles as simply another output format that a server may support. The feature access path API is designed to leverage the flexible query capabilities of the WFS to generate a vector tile response on the server side leaving the styling to be done on the client side.

In Clause 6 each participant describes the components that they developed and tested for the Vector Tiles Pilot. WFS 3.0 servers implementing one or both of the APIs described in Clause 5 were provided by the following participants:

CubeWerx
Ecere
GeoSolutions
interactive instruments GmbH
Mapbox

Clients able to interact with these servers were provided by the following participants:

Ecere
GIS FCU
Mapbox
GeoSolutions

The following table summarizes the result of the client and server interactions i.e. Technology Integration Experiments (TIE):

Table 1. WFS TIE Matrix
CLIENTS	SERVERS
	CubeWerx	Ecere	GeoSolutions	interactive instruments	Mapbox
Ecere	Connects	Connects	Connects	Connects	Connects
GIS FCU	Connects	Connects	Connects	Connects
Mapbox	Connects	Connects	Connects	Connects	Connects

5. Vector Tiles API for WFS 3.0

5.1. Introduction

OGC Web Feature Service (WFS) is that standard mostly used for disseminating and querying vector feature data on the Web. WFS 3.0 is a complete rewrite of previous versions, focusing on a simple RESTful core specified as reusable OpenAPI components with responses in JavaScript Object Notation (JSON) and Hypertext Markup Language (HTML).

Feature data tiling (colloquially referred to as vector tiling) is a proven mechanism for optimally storing, delivering and visualizing vector data from arbitrary large data sets, used to achieve high performance in Geographic Information Systems (GIS) for several decades. Multi-resolution tile sets leverage the fact that a typical client is either visualizing a large geospatial extent at low resolution, or a limited geospatial extent at high resolution. Furthermore, tiled feature data enables efficient caching of data and provides clients with the option of displaying the best currently available data while awaiting requests for more refined tiles.

The Mapbox Vector Tile (MVT) specifications is a way to encode vector tiles which has garnered widespread support and adoption and has brought vector tiles to mainstream web mapping applications.

Extending WFS 3.0 to support vector tiles, including those based on the MVT specification and GeoJSON Vector Tile (JVT) output formats, is an evolution of the standard that will help deliver geospatial data in a modern optimized mechanism.

This section provides two interfaces that can be used for accessing vector tiles through WFS 3.0 in Mapbox Vector Tile format. The first interface focuses on direct access to the tiles as defined by a tiling scheme, while the second one focuses on features and is more similar to a typical WFS 3.0 request.

Requirements Class: Core

http://www.opengis.net/spec/wfs/3.0/vt/req/core

Target type

Token

Requirement 1

http://www.opengis.net/spec/wfs/3.0/vt/req/core/direct-tile-access-path-api

Requirement 2

http://www.opengis.net/spec/wfs/3.0/vt/req/core/feature-access-path-api

The relationship between WFS feature collections and tiling schemes is one-to-many. That is to say that tiles for a specific WFS feature collection can be offered using one or more tiling schemes.

5.2. Direct tile access path API

5.2.1. Introduction

This interface focuses on accessing individual tiles directly. It provides resources describing supported tiling schemes, as well as templated paths to access individual tiles.

It assumes the following:

The tiling scheme ('tile matrix set' to use WMTS terminology) defines how space is partitioned into individual tiles.
In the context of the pilot, as in WMTS, all tiling schemes definitions uniquely identify tiles using a three-part identifier made up of {level} (zoom), {row} (vertical) and {col} (column: horizontal).
The level parameter references a particular zoom level of a fixed spatial resolution, akin to the typical use of an individual 'tile matrix' in WMTS.
With the exception of the GNOSISGlobalGrid implemented by Ecere, all other tiling schemes were regular grids of tiles.

Notes:

An implementation could either generate tiles on-the-fly or serve an existing pre-generated set of tiles.
A tiling scheme does not need to be a quad-tree tile pyramid, e.g. GlobalCRS84Pixel and GlobalCRS84Scale Well Known Scale Sets do not have a constant factor of 2 between zoom levels.
In the context of the pilot, only a read-only interface has been considered.

Requirement 1

http://www.opengis.net/spec/wfs/3.0/vt/req/core/direct-tile-access-path-api

An implementation asserting support of the Direct Tile Access Path API shall support the behavior described in Clause 5.2 of the Vector Tiles extension of WFS 3.0 .

5.2.2. Preliminaries

OPTIONS & HEAD method

"Cross-Origin Resource Sharing (CORS) is a mechanism that uses additional HTTP headers to tell a browser to let a web application running at one origin (domain) have permission to access selected resources from a server at a different origin" ^[1]. It is recommended that the OPTIONS & HEAD methods be available on each resource in order to support CORS preflight request and to be able to determine the supported representations, methods and response headers.

Content negotiation

According to the WFS specification (clause 7.8): "…any server that supports multiple encodings will have to support a mechanism to mint encoding-specific URIs for resources in order to express links, for example, to alternate representations of the same resource. This document does not mandate any particular approach for how this is supported by the server." Although it lists f as a parameter that is used on many paths in this email, this is but one possibility. Other methods, such as extensions, may also be used. It is up to the server.

OGC Tile Matrix Set Standard Candidate (17-083)

In general, this is the schema that should be used for the definition of a tiling scheme. However, in this pilot the following changes were made:

the coordinates for the topLeftCorner, as well as the lowerCorner and upperCorner of the boundingBox, are changed from a string to an array of double-precision floating point numbers
adding a "links" object to the tilingScheme definition so that templates, for accessing the tiles, can be included

   "links" : [
      {
         "rel" : "tiles",
         "type" : "application/geo+json",
         "title" : "Tile in GeoJSON. The link is a URI template where {level}/{row}/{col} is the tile based on the tiling scheme.",
         "href" : "https://services.interactive-instruments.de/vtp/daraa/tiles/default/{level}/{row}/{col}?f=json",
         "templated" : "true"
      },
      {
         "rel" : "tiles",
         "type" : "application/vnd.mapbox-vector-tile",
         "title" : "Mapbox Vector Tile. The link is a URI template where {level}/{row}/{col} is the tile based on the tiling scheme.",
         "href" : "https://services.interactive-instruments.de/vtp/daraa/tiles/default/{level}/{row}/{col}?f=mvt",
         "templated" : "true"
      }
   ]

In the content that follows this clause, any mention of OGC 17-083 should be interpreted to mean OGC 17-083 with the changes proposed above.

The following is a partial example description for the tiling scheme based on the GoogleMapsCompatible Well Known Scale Set:

   {
     "identifier": "smerc",
     "title": "Spherical Mercator projection with quad-tree zoom levels, compatible with Google Maps®, Bing Maps®, etc.",
     "boundingBox": {
       "lowerCorner": [
         -20037508.34278924,
         -20037508.34278924
       ],
       "upperCorner": [
         20037508.34278924,
         20037508.34278924
       ],
       "crs": "http://www.opengis.net/def/crs/EPSG/0/3857"
     },
     "supportedCRS": "http://www.opengis.net/def/crs/EPSG/0/3857",
     "wellKnownScaleSet": "http://www.opengis.net/def/wkss/OGC/1.0/GoogleMapsCompatible",
     "tileMatrices": [
       {
         "identifier": "0",
         "scaleDenominator": 559082264.028717,
         "tileWidth": 256,
         "tileHeight": 256,
         "matrixWidth": 1,
         "matrixHeight": 1,
         "topLeftCorner": [
           -20037508.3427892,
           20037508.3427892
         ]
       },
       {
         "identifier": "1",
         "scaleDenominator": 279541132.014358,
         "tileWidth": 256,
         "tileHeight": 256,
         "matrixWidth": 2,
         "matrixHeight": 2,
         "topLeftCorner": [
           -20037508.3427892,
           20037508.3427892
         ]
       },
    ...

Recommendations for further changes to OGC 17-083

The following were noted during the pilot as further recommendations to the Tile Matrix Set Candidate Standard:

Supporting tiling schemes with a variable number of columns would enable supporting global grids making adjustment for latitude regions, such as Ecere’s GNOSIS Global Grid as well as the CDB tiling grid. This was initially suggested in Testbed-13 (http://ogc.standardstracker.org/show_request.cgi?id=518) and discussed by the Web Map Service (WMS) Standards Working Group (SWG) at Technical Committee (TC) meetings in 2018. An example of how this could be described can be found here[http://maps.ecere.com/hms/tiles/GNOSISGlobalGrid?f=json] with a 'variableWidths' parameter.

$Ecere GGG1$

Figure 1. GNOSIS Global Grid at the South Pole