OpenHistoricalMap

OpenHistoricalMap logo

OpenHistoricalMap (website) is an open historical geographic information system based on OpenStreetMap software. This page discusses ongoing integration of OpenHistoricalMap (OHM) content into various Wikimedia projects and further plans for expansion.

Project overview

OHM is an ambitious spinoff of OSM that broadens the scope to include information about geography in the past (as well as in the present). OHM is more research-oriented than OSM, placing greater emphasis on reliable sources while retaining OSM's emphasis on local knowledge and ground truth.

Consistent with Wikimedia movement values, OHM is a nonprofit, educational project that prioritizes the collaborative dissemination of knowledge by a worldwide community of volunteers. Many OHM contributors are also longtime Wikimedians. The Wikimaps User Group serves as an informal liaison between the Wikimedia movement and the OSM community; OHM is in many ways an outgrowth of this collaboration.

OHM is open data powered by open source software. It is the first of only a few projects that have completely forked the full OSM software stack. OHM tends to take a conservative approach, departing from upstream OSM projects only when necessary to avoid extra maintenance overhead. Some components are original to OHM, so the development team is much more comfortable making significant changes to them. OHM has chosen to place the vast majority of its content in the public domain under a CC0 dedication, except for some mostly CC BY-SA content from OSM as indicated in individual database records. Given the more restrictive ODbL license that OSM currently uses, there are ongoing discussions about how to facilitate the transfer of historical content from OSM to OHM for safekeeping.

OHM is a charter project of OpenStreetMap U.S. (OSMUS), a 501(c)(3) nonprofit organization based in the United States. As fiscal sponsor, OSMUS accepts donations on behalf of OHM and promotes the project through conferences and tie-ins with other charter projects. OSMUS is a local chapter of the UK-based OpenStreetMap Foundation. This affiliation ensures contributors' access to important resources such as aerial imagery that have been donated to the OSM project.

The worldwide OHM community is distributed over a variety of communication channels, especially the public OHM forum and dedicated OHM channels on OSM's Discord server and Slack workspace. OHM's development team consists of a few part-time contractors through the GreenInfo Network consultancy, who are familiar with some parts of the software stack. On occasion, former developers from Development Seed are retained when expertise is needed in some less familiar areas. OHM participated in the 2024 Google Summer of Code through the OSMF.

Existing integration

OAuth

The OHM website allows users to log in using their Wikimedia SUL credentials via OAuth. This login can also be reused on the OHM forum.

Linked data

A map of the Intramuros of Manila in 1900 along with an expert from the corresponding article on the English Wikipedia.

OHM's inspector sidebar integrates Wikipedia article extracts and/or Wikimedia Commons images for some 64,000 features (example).

Wikidata has extensive usage of OHM, employing OpenHistoricalMap relation ID (P8424) with nearly 5,000 instances as of 2024. This property links Wikidata items to OHM features, enhancing historical geographic data representation within Wikimedia. A SPARQL query against the Wikidata Query Service reveals all current usages of this property.

QLever, one of the contenders to replace Blazegraph in the Wikidata Query Service, queries OHM data with daily replication. QLever supports federated queries among Wikidata, Wikimedia Commons, OSM, and OHM, aided by the more than 70,000 Wikidata QIDs embedded in OHM metadata.

The OSM-Wikidata Map Framework supports OHM's Overpass instance, QLever's OHM dataset, and Wikidata's OHM property as alternative sources. [1][2]

Wikimaps Warper

Wikimaps Warper facilitates tracing Wikimedia Commons maps in OHM. [3]

Wikibase

OHM's technical and community documentation lives on the OpenStreetMap Wiki, which is powered by MediaWiki. The OSM Wiki includes a Wikibase instance for "data items" that describe the database schema. OHM's extensions to this schema are also represented by data items.

GeoHack

The GeoHack page for several Wikipedia language editions links to OHM alongside OSM, but it is up to the user to manually adjust the time slider to reflect the time period they were reading about on Wikipedia. There is a proposal to modify GeoHack to pass a date specified by the article through to OHM. [4]

Story maps

Aoppo has developed Globstory, an experimental user script that highlights keywords in a Wikipedia article on an embedded OHM map.

Double Map navigates an OHM globe by entries featured in the English Wikipedia's selected anniversaries section. (Click the book icon in the upper-right corner to activate the OHM basemap.)

Opportunities for further integration

OAuth

Wikimaps Warper could allow users to log in using their OHM credentials.

The Wikipedia Library

Some OHM contributors rely very heavily on The Wikipedia Library (TWL) for access to subscription databases, which they use to contribute simultaneously to OHM and Wikimedia projects. OHM would see more sustained growth and improvements in quality from expanded access to TWL. TWL could provide streamlined access to mappers, or OHM's editors could make it easier to contribute to both OHM and Wikidata at the same time, helping them maintain TWL access under the current rules.

Imports

Since OHM and Wikidata are both largely in the public domain, both projects could bulk-import data from each other. OHM has seen some undocumented imports of places from Wikidata. Unfortunately, OHM has had to revert or scale back some of these imports. Wikidata suffers from rampant conflation of human settlements with administrative areas or governmental authorities, affecting the accuracy of names and dates, which are of critical importance to OHM. The OSM community has been actively working to clean up Wikidata's coverage of human settlement items, particularly in the United States, but the dates on these items remain a source of concern.

Tracked in Phabricator:
Task T207705 open

In the other direction, OHM's chronology relations and other relation types can help Wikidata source dates for a variety of features, including administrative areas, railroad stations, and buildings. Imports of this data would complement imports from Wikipedia infoboxes, which are somewhat less robust due to that project's freeform structure. There may be some dataloss when importing some features from OHM to Wikidata, due to Wikibase's lack of support for the Extended Date/Time Format (EDTF) standard.

The OSM ↔ Wikidata matcher's developer has expressed a desire to add support for OHM in the future. [5]

Kartographer

Wikipedia articles routinely incorporate OSM-based maps via the Kartographer extension, such as in infoboxes. Most of these maps serve as locator maps in articles that cover the present day. By contrast, in an article about a historical person, place, or event, an OSM-based map would be anachronistic and potentially misleading. An OHM-based map would provide more accurate historical and geographic context. With nascent but growing coverage of precolonial societies, OHM has the potential to help Wikipedia counter systemic bias in its use of maps. Moreover, Wikipedia's maps need stability as the world changes. The January 2025 Southern California wildfires destroyed many notable historic structures, whose Wikipedia articles can no longer use {{mapframe}}'s default feature highlighting functionality, because the corresponding OSM feature has been deleted or is at risk of being deleted. OHM would be able to store these features permanently.

Constantin-Helen's Church in Kyiv from OHM superimposed on a present-day OSM-based map

Multiple presentation modes are possible. With the addition of a time slider, the map could simulate a country's territorial evolution, a city's growth, or troop movements on a battlefield. An interactive slider or onion skinning control could allow the reader to compare a city before and after a natural disaster or comprehend the disappearance of a glacier due to climate change. Alternatively, a mashup of OSM and OHM data can provide a sense of scale. For example, the French Wikipedia's Pillier Sainte-Irène article shows the structure in relation to the present-day neighborhood.

Tracked in Phabricator:
Task T390395 open

As a first step, Kartographer should link to OHM in the "External maps" list when viewing a map in full screen mode. This would make OHM more accessible to help counter any anachronisms in the embedded map. However, any overlay on the map would not persist after the reader follows the link. For that, the embedded map would need to depict OHM data.

Kartographer's Kartotherian-based backend, Wikimedia Maps, currently produces raster tiles, which are unsuitable for historical maps. After all, it would be impractical to generate a whole world's worth of tiles for each day in history or prehistory. By contrast, OHM produces an official vector tileset that contains a more efficient representation of history, as well as stylesheets to render the tiles. Kartographer could integrate these resources or something similar hosted on Wikimedia infrastructure.

Because Wikimedia Maps is only available to Wikimedia projects, some third-party wikis have developed an alternative to Kartographer capable of displaying OSM- and OHM-based vector maps. This gadget is installed at the OSM Wiki and AARoads Wiki (example). There is no direct equivalent to the geoline and geoshape services that Kartographer relies on. Instead, these wikis rely on geodata from the Wikimedia Commons API. Existing methods for exporting OSM features to Wikimedia Commons can easily be adapted to OHM using OHM's Overpass instance. A similar user script has been proposed for the English Wikipedia (March 2025) due to the relative ease of deploying a user script compared to getting a proper extension approved by the WMF engineering team.

Unicode

OHM tracks the evolution of writing systems through toponymy. Many classical and indigenous languages have limited Unicode support. The Wikimedia Foundation (WMF) could use its membership in the Unicode Consortium to facilitate improved encoding for these orthographies. The Unicode Technical Committee takes into account evidence of usage in OHM (example).

IIIF

Both Wikimedia Commons and OHM have expressed interest in adopting the International Image Interoperability Framework (IIIF), though it is not currently on either project's official technical roadmap.

Cloud Services

OHM currently relies on a combination of Amazon Web Services (AWS) and Hetzner for hosting and computing, plus GitHub Actions and GitHub Pages for some static publishing. OHM previously standardized on AWS, but in 2024, some services were offloaded to a paid Hetzner machine to reduce costs. The development team does not have a dedicated site reliability engineeer (SRE).

Cloud VPS could be desirable for reducing OHM's operational cost while maintaining high uptime. OHM's development team has poured significant effort into making OSM's software stack and OHM's tile generator run on AWS. Migrating the core services to another environment may be a nontrivial effort, but Cloud VPS could be a plausible alternative to Hetzner for relatively isolated services such as the tile generator, Nominatim, and Overpass.

Long-term ideas

In the long term, establishing OHM as a Wikimedia sister project (while remaining affiliated with OSM) could streamline integration with Wikimedia platforms and yield mutual benefits for both Wikimedia projects and OHM. This integration could expand access to historical geographic data and increase visibility for OHM. The various integrations proposed on this page can establish a working relationship necessary for considering OHM as a sister project.

Challenges

OHM's current development team is very small. It may be possible to tap into the much larger ecosystem of OSM developers to work on compelling projects. OHM's relative obscurity has hindered outreach to OSM developers in the past. Any association with the Wikimedia movement would improve visibility.

WMF built out OSM-based map infrastructure several years ago but currently has no expertise in hosting or developing OSM.

Get involved

If you'd like to get involved with developing this integration, consider joining the OHMWiki room on Matrix to connect with the community and participate in ongoing discussions.

See also

• WikiHistoricalMap
• Wikihistorymap

Further reading

• Meyer, Jeff (18 October 2025). OpenHistoricalMap: A Linked, Attribution- & Integration-Friendly Map Complement to Wikimedia. WikiConference North America 2025. New York City. Retrieved 19 October 2025 – via Google Slides. 

External links

• Official website
• Project documentation