Publication Date: 2019-02-11

Approval Date: 2018-12-13

Submission Date: 2018-11-14

Reference number of this document: OGC 18-083

Reference URL for this document: http://www.opengis.net/doc/PER/vtp-wmts1

Category: Public Engineering Report

Editor: Panagiotis (Peter) A. Vretanos

Title: OGC Vector Tiles Pilot: WMTS Vector Tiles Extension Engineering Report

OGC Engineering Report

COPYRIGHT

Copyright © 2019 Open Geospatial Consortium. To obtain additional rights of use, visit http://www.opengeospatial.org/

WARNING

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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1. Summary

The tiling of feature data is an approach that can be used to optimize the delivery vector feature data over the web to create maps. The approach provides a pre-defined shape (i.e. tile) to package vector data. Tiling of vector data enables faster map loads (due to reduced size) and offer flexible styling on the client side with modern, easy-to-use tools.

This Engineering Report (ER) describes the work done by participants during the Vector Tiles Pilot (VTP) to add Mapbox and GeoJSON vector tile support to Web Map Tile Servers. A summary of other work done in the VTP is presented in the VTP Summary Engineering Report [1].

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

1.1. Requirements & Research Motivation

The Web Map Tile Service (WMTS) standard currently offers predefined tiles in a fixed set of graphical representations with no provision, from the perspective of a client, for adding additional styles or even dynamically styling tiles on the fly. However, some implementations, like GeoServer, can be configured to allow style selection, dynamic styling and even on-the-fly content filtering, at the price of either extra storage or no caching).

This ER addresses this deficiency by exploring the addition of tiled feature data support to a WMTS as another supported output format. This addition to WMTS follows lessons learnt from the recent OGC Testbed-13 initiative [2]. Tiled feature data can be efficiently transmitted to the client where the data can be styled in a manner chosen by the client using a number of easy-to-use tools before being finally rendered onto the display.

1.2. Prior-After Comparison

The following Standards Working Groups (SWGs) are most directly involved in OGC work related to map rendering and would be interested in the work described in this ER:

  • Web Mapping Service 1.4 SWG (also responsible for WMTS)

  • Styled Layer Descriptor & Symbology Encoding SWG

The GeoPackage SWG, being involved in work related to packaging formats that support map rendering in Denied, Degraded, Intermittent or Limited bandwidth (DDIL) environments would also be directly interested in the work described in this ER.

The following SWGs, are involved in work that is redefining the underlying architecture of OGC Web Services (OWS). As such, they would also be interested in the work described in this ER as it explores how far the new paradigms can be extended to support the resource type of most interest to OGC members:

  • Web Feature Service/Filter Encoding Specification SWG

  • Web Processing Service SWG

1.3. Recommendations for Future Work

Having developed a set of abstract test suites in the Vector Tiles Pilot, future work should look to develop executable test suites for compliance testing of implementations of this specification.

Further investigation is also needed towards allowing WMTS servers and clients to operate in denied, degraded, intermittent or limited bandwidth (DDIL) networks. The DDIL clause in this document discusses some avenues of investigation.

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

CubeWerx Inc.

Kalimar Maia

Mapbox

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.

  • dataset

    identifiable collection of data [ISO 19115-1:2014]
Note
 The use of 'collection' in the definition from ISO 19115-1 is broader than the use of the term 'collection' in this specification. See the definition of 'feature collection'.

  • distribution

    represents an accessible form of a dataset [W3C DCAT]

EXAMPLE: a downloadable file, an RSS feed or a web service that provides the data.

  • feature

    abstraction of real world phenomena [ISO 19101-1:2014]
Note
 If you are unfamiliar with the term 'feature', the explanations in the W3C/OGC Spatial Data on the Web Best Practice document may help, in particular the section on Spatial Things, Features and Geometry.

  • feature collection; collection

    a set of features from a dataset
Note
 In this specification, 'collection' is used as a synonym for 'feature collection'. This is done to make, for example, URI path expressions shorter and easier to understand for those that are not geo-experts.

  • coordinate reference system

    coordinate system that is related to the real world by a datum [ISO 19111]

  • coordinate system

    set of mathematical rules for specifying how coordinates are to be assigned to points [ISO 19111]

  • feature info

    information related to a particular pixel of a map that refers to the geographic data portrayed on that area

  • layer

    basic unit of geographic information that may be requested as a map from a server

  • map

    portrayal of geographic information as a digital image file suitable for display on a computer screen

  • portrayal

    graphical presentation of information to humans

  • procedure oriented architectural style

    platform-independent design approach that is focused on operations, their parameters and their results, that can be defined in an abstract level specification. Concrete platform-dependent specifications can be derived from the abstract level, allowing, for example, KVP or SOAP messaging.

  • resource oriented architectural style

    platform-independent design approach that is focused on resources, representations and actions, that can be defined in an abstract level specification. Concrete platform-dependent specifications can be derived form the abstract level, allowing, for example, a RESTful architecture.

  • theme

    a group of layers that can be nested hierarchically

  • tile

    a tessellated 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)

  • tile matrix

    a collection of tiles for a fixed scale

  • tile matrix set

    a collection of tile matrices defined at different scales

3.1. Abbreviated terms

Table 1. Abbreviations

JPEG

Joint Photographic Experts Group (image format)

JVT

GeoJSON Vector Tile

MVT

Mapbox Vector Tile

PNG

Portable Network Graphics (image format)

REST

Representational State Transfer

SLD

Styled Layer Descriptor

WFS

Web Feature Service

WMTS

Web Map Tile Service

4. Overview

Clause 5 describes the client and server implementations deployed for the Vector Tiles Pilot.

Clause 6 presents a discussion on some of the potential requirements for a Vector Tiles extension to WMTS.

5. WMTS Implementations

A number of WMTS services and client applications took part in the Pilot. The following subsections describe those services and client applications.

5.1. Pilot Architecture

To support experimentation, an architecture was designed to cover various client-server relationships that are common in OGC use cases as illustrated in Figure 1.

VT Architecture
Figure 1. Vector Tiles Pilot Architecture

The architecture therefore addresses the following client-server relationships to simultaneously enable tiled feature data across the relevant suite of OGC standards.

  • Desktop Client → Web Feature Service (WFS) 3.0

  • Web Client → Web Map Tile Service (WMTS) 1.0

  • Mobile Client → GeoPackage 1.2

The following sections focus only on WMTS, as it is the subject of this engineering report.

5.2. Servers

The pilot included WMTS services from CubeWerx, Ecere, GeoSolutions and MapBox. The following subsections describe the services.

5.2.1. CubeWerx

The CubeWerx WMTS service implements version 1.0 of the WMTS standard. The service offers the following operations:

  • GetCapabilities

  • GetTile

  • GetFeatureInfo

The service also offers the following vendor-specific operations:

  • GetDescription

  • GetLegendGraphic

  • GetTileJson

  • GetAccessibility

  • GetAssociations

Server deployment

The CubeWerx WMTS has been deployed on an Amazon instance at the following endpoint:

It offers the standard spherical mercator (smerc) tile matrix set and supports the following tile formats:

  • image/x-jpegorpng

  • image/jpgpng

  • image/png

  • image/jpeg

  • application/vnd.mapbox-vector-tile

The complete capabilities document of the server can be found here:

Implementation Efforts for Adding MVT Support to WMTS

No changes were necessary to the WMTS standard. It simply advertises that vector tiles data is offered as another available tile format. To add Mapbox Vector Tile (MVT) specification support, the server:

  • integrated the third-party protobuf 3.6.0 and protobuf-c 1.3.0 libraries to handle the Google Protobuf encoding requirements.

  • implemented the generation of MVTs as per the Mapbox Vector Tile specification [3].

Some of the challenges encountered during this implementation were:

  • supporting the various geometry types and property value types

  • implementing proper (and efficient) simplification and clipping of geometries

  • honoring any filters and scale rules that may be present in the specified style

  • making the buffer zone size configurable

  • making the ratio of internal resolution to display resolution configurable

  • making whether or not properties should be emitted configurable

  • implementing a low-level mechanism that will generate an MVT tile of any size, in any coordinate system, at any resolution, with any number of layers, from any supported source

  • implementing a higher-level tile-oriented mechanism that generates vector tiles in accordance with the WMTS-defined tile-matrix sets

  • implementing GeoJSON Vector Tile support in parallel

  • performing appropriate testing

Testing

In order to test the CubeWerx MVT implementation, a bespoke test client that is based on OpenLayers was deployed at the following endpoint:

This client is hardcoded to test the CubeWerx server specifically.

5.2.2. Ecere

The Ecere WMTS service implements version 1.0 of the WMTS standard. The service offers the following operations:

  • GetCapabilities

  • GetTile

Server deployment

The Ecere WMTS has been deployed at the following endpoint:

Tiles for any layers are generated on-the-fly for any supported encoding and tiling scheme from a GNOSIS tiled data store.

It supports the following vector tile formats:

The complete capabilities document of the server can be found here:

The service supports the following Tile Matrix Sets:

  • GlobalCRS84Pixel - EPSG:4326 (geographic); Based on the WMTS Well Known Scale Set with convenient pixels/degrees values (not a quad tree, resolution does not double at each level)

  • GlobalCRS84Scale - EPSG:4326 (geographic); Based on the WMTS Well Known Scale Set with convenient scale denominator values (not a quad tree, resolution does not double at each level)

  • GoogleCRS84Quad - EPSG:4326 (geographic); Based on the WMTS Well Known Scale Set, a quad tree with convenient scale denominator values

  • GoogleMapsCompatible - EPSG:3857 (spherical pseudo-Mercator projection); Based on the WMTS Well Known Scale Set compatible with GoogleMaps and others (also a quad tree)

  • GNOSISGlobalGrid - EPSG:4326 (geographic); A global grid with fewer columns of tiles closer to the pole than at the equator, approximating equal area tiles, a quad-tree except for starting with eight 90deg x 90deg tiles at level 0, and tiles touching a pole splitting in 3 rather than 4 [http://docs.opengeospatial.org/per/17-041.html#_global_gnosis_tiling_scheme_adapted_to_polar_regions]

Implementation Efforts for Adding MVT Support to WMTS

No changes were necessary to the WMTS specification. MVT was simply included as an additional output format offered by the WMTS. Support for encoding vector features according to the MVT specification, using Google Protocol Buffers, was implemented during the course of this pilot within the Ecere’s GNOSIS software libraries and had to be integrated within Ecere’s GNOSIS Map Server.

Testing

Mapbox vector tiles generated by Ecere’s WMTS were tested using 'vtvalidate', which performs the following tests [1]:

  • Tiled feature data consistent with the Mapbox Vector Tile specification - Version 2.

  • Read tile layer(s) and feature(s)

  • Decode properties

  • Decode geometries

Tiles were also tested successfully with OGR software, which required a fix for an extent greater than 16384, as well as with QGIS 3.2.3.

Support for visualizing Mapbox vector tiles was implemented in Ecere’s visualization client (see section below), which was also used to test the tiles delivered by the WMTS service.

The Ecere WMTS content was also successfully rendered by Compusult, thereby confirming a successful Technology Integration Experiment (TIE).