OGC API – Maps Sprint 2020: Summary Engineering Report

This OGC Engineering Report (ER) documents the results and recommendations from a code sprint that was held from 28 to 29 July 2020 to advance the development of the draft OGC API – Maps Standard. An Application Programming Interface (API) is a standard set of documented and supported functions and procedures that expose the capabilities or data of an operating system, application, or service to other applications (adapted from ISO/IEC TR 13066-2:2016). The OGC API - Maps Sprint was an online virtual event. The sprint was sponsored by Ordnance Survey.

This Engineering Report (ER) summarizes the main achievements of the OGC API – Maps Sprint. The sprint served to accelerate development of the draft OGC API – Maps Standard. The objectives of the code sprint were to:

Part of the motivation for holding the sprint was:

The draft OGC API - Maps standard describes an API that presents data as maps by applying a style. The draft standard enables a client application to request maps as images. This includes the ability to specify or change parameters such as the size of an image and coordinate reference systems at the time of request. The draft standard supports the creation of implementer-friendly API definitions that are easily understandable by developers with little or no geospatial experience. These maps can be retrieved as images of any size generated on-the-fly or in a tiled structure (if used alongside the draft OGC API - Tiles standard).

A server that implements the draft OGC API - Maps standard provides information about available maps, produces them, and answers queries regarding their content. The draft standard comes from the OGC’s concerted effort to create modular, resource-oriented API standards that use OpenAPI for describing interfaces that offer geographic information over the web. These OGC APIs are collectively known as the OGC API family of draft and approved standards. The draft OGC API - Maps standard addresses use cases similar to those addressed by the OGC Web Map Service (WMS) standard.

The sprint produced very valuable results. Participants identified areas for improving the draft standard, allowing the editor of the draft standard to record their recommendations. In some cases, the specification editor was able to fix issues in real time as they those issues were being identified.

The following normative documents are referenced in this document.

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.

The code sprint was held to focus on the draft OGC API – Maps standard. Sprint participants prototyped implementations of the draft OGC API - Maps - Part 1: Core standard, identifying missing requirements, and documenting these requirements so that the draft standard could be improved.

An OGC Code Sprint is a collaborative and inclusive event driven by innovative and rapid programming with minimal process and organization constraints. OGC sprints are designed to support the development of new applications and open standards. OGC Code Sprints experiment with emerging ideas in the context of geospatial standards, help improve interoperability of existing standards by experimenting with new extensions or profiles, and are used as a proof of concept for other OGC Innovation Program initiatives, or support OGC Standards Program activities.

This code sprint was held as part of a series in the third quarter of 2020. The other code sprints included:

The focus of the sprint was on support of development of the draft OGC API – Map Standard. Implementations of this draft standard were deployed in each participant’s own infrastructure in order to build a solution with the architecture shown below in Figure 1.

As illustrated above, the sprint architecture was designed with the view of enabling client applications to connect to different servers that implement OGC API - Maps. The servers were provisioned with data and styling rules, enabling them to render the data onto digital maps that were then sent to client applications in response to a GET request.

Multiple organizations provided servers, API implementations, and capabilities during the event. The remainder of this section describes each of the implementations.

The Centre for Ecological Research and Forestry Applications (CREAF) at the Autonomous University of Barcelona (UAB) deployed an instance of the MiraMon Map Server that implements support for the draft OGC API - Maps standard. The server is implemented as a CGI application encoded in C language as a part of the MiraMon Geographic Information System (GIS) & Remote Sensing (RS) suite and is interoperable with other vendors’ clients. The user interface of the MiraMon suite is shown on Figure 2.

CubeWerx deployed an implementation of OGC API - Maps and other OGC APIs, CubeWerx server ("cubeserv"). The CubeWerx server was provided in the C programming language. The server exposes an interactive API description created from the OpenAPI definition document which enables both users and client applications to query the server. The server was updated in real time based on feedback from other participants using the services during the sprint. The API definition presented by the CubeWerx server is shown in Figure 3.

Ecere initiated the development of server-side maps rendering capabilities, deployed at an end-point supporting OGC API - Maps, and a number of other OGC APIs. The implementation was done in Ecere’s GNOSIS Map Server product which leverages the open-source cross-platform Ecere SDK and eC programming language. An example output from Ecere’s GNOSIS Map Server is shown in Figure 4.

Ordnance Survey’s OpenMapLocal product was among the maps published through Ecere’s development server. A screenshot of a visualization of the data is shown in Figure 5. The OpenMapLocal product was exposed as a collection of maps through an OGC API - Maps interface. Ecere also worked on implementing client support for visualizing maps accessed through OGC API - Maps in GNOSIS Cartographer.

Brad Hards, a participating developer, extended an existing OpenSphere plugin to support OGC API - Maps. OpenSphere is a pluggable, single-page geospatial web application that supports 2D and 3D views. OpenSphere supports binding to many popular servers and formats. Other features include animation of both raster and vector data, import and export of various formats, and saving files and layers between sessions. Figure 6 shows the OpenSphere application displaying a map retrieved from the Ecere implementation of OGC API - Maps.

One of the prototype implementations, developed by OGC staff, demonstrated how to create an OGC API - Maps facade or proxy interface on top of a WMS. The WMS underneath the facade was implemented using GeoServer. Development of the prototype facade was used as the basis for developing a basic guide for developers that use the Sprint Framework (spring.io) - a popular framework for Java developers. The prototype implementation published a map, shown in Figure 7, created from the Ordnance Survey’s OS Open Zoomstack product.

The sprint showed how JSON could be used successfully used to describe maps. In addition to JSON-based messaging, another significant difference between OGC API – Maps and a WMS instance was demonstrated during the sprint to be the ability to retrieve a complete map through a REST call without any query parameters. However, this capability raised the question of whether the standard should specify rules for default values of query parameters.

Early on in the sprint the participants agreed that an additional link relation type is needed to support the binding of client applications to server implementations of the OGC API – Maps draft standard. A number of link relation types were proposed during the sprint. The expectation is that upon approval of the standard, any new link relation types would be registered with the OGC Naming Authority (OGC-NA) in the OGC Definitions Service and ultimately with the Internet Assigned Numbers Authority (IANA).

Regarding styles and legends, the sprint participants discussed the need to specify a path for retrieving a legend. The role of styles and legends was also shown to be as important for OGC API – Maps as it has been for the WMS. This is an area where the emerging draft OGC API – Styles Standard will need to complement the functionality offered by OGC API – Maps [1].

The participants discussed a use case involving the use of custom styles sent to an implementation of OGC API – Maps. The participants explored how the following approaches, described in Issue#42 , could support such a use case:

There was acknowledgement across the sprint that the issue of posting styles was broader than just the Maps API, and that posting styles applies to the general posting of private resources. Therefore, the participants agreed that the issue should be addressed by the draft OGC API – Common Standard.

Another issue that was identified was whether the server should describe the spatial extents of each CRS, or whether that information should simply be inferred from the definitions of the CRS. If the spatial extent is inferred from the definition of the CRS referenced by identifier, then there would an expectation that the client application has access to the CRS definitions. Further, there was discussion about whether the CRS of the bounding box coordinates should be specified in a parameter called ‘crs’ or one called ‘bbox-crs’.

A copy of the draft standard was annotated with feedback from sprint participants. The participants expect that the editor of the draft standard will work with the SWG to revise the draft standard according to the feedback provided. The rest of the discussion was captured on the Gitter platform. A screenshot of the Gitter channel is shown below. The Gitter channel can be found at https://gitter.im/opengeospatial/OGC-API-Sprint-July-2020

A screenshot of the GitHub repository is shown below. The GitHub repository can be found at https://github.com/opengeospatial/OGC-API-Sprint-July-2020

The sprint successfully demonstrated the ability of OGC API – Maps to publish digital maps, as well as the developer-friendliness of the draft standard. The sprint was the first to focus solely on the development of the draft OGC API – Maps Standard. Previous sprints and initiatives worked on both OGC API – Maps and OGC API – Tiles before the SWG’s decision to separate the requirements into two independent standards [2]. The successful implementation and demonstration of the draft standard during the sprint was therefore a significant achievement.

The participants demonstrated that the OGC API pattern can indeed address the needs of the map making, map publishing and map user communities. The participants envisage that as the draft standard continues to be developed, additional National Mapping Agencies (NMAs) will begin to familiarize themselves with the draft standard and the many benefits the draft standard could offer to them. There is, however, more work to be done in the SWG. Therefore the sprint reported in this document should be considered the first in a series of sprints that will be needed to move the draft standard closer towards approval by the OGC Membership.

[1] Portele, C.: OGC Testbed-15: Styles API Engineering Report. 19-010r2,Open Geospatial Consortium, http://docs.opengeospatial.org/per/19-010r2.html (2019).

[2] Pau, J.M.: OGC Testbed-15: Maps and Tiles API Engineering Report. OGC 19-069,Open Geospatial Consortium, http://docs.opengeospatial.org/per/19-069.html (2020).