Open Historical Map/Tags

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OpenHistoricalMap (OHM) tagging is based on OpenStreetMap (OSM) tagging, and as such, almost any standard practice for OSM tagging will apply to OHM, as well.

Those familiar with OSM tagging practices know that they are firm, but not always binding, guidance. Going with the crowd is the greatest guarantee of tagging success.

OHM has some additional strongly recommended tags that may supersede OSM's conventions. These have been put in place to help with the time-based rendering of information, to provide for finer-grained information about sources of the map data, or to clarify any licensing concerns.

Although OHM has been around for a while, it is always evolving, and as such, tagging conventions are up for discussion, which is encouraged on the Discussion tab of this page or in any of our various online forums.

As part of this evolution, it is helpful to think of what works now, what is in consideration to work in the near future, and what we aspire to support, but are not sure when we'll be able to.

Foundational OHM Tags

All of these tags fall into the category of what works now. The following tags should be considered the bare minimum for enabling rich interaction and collaborative mapping. These tags should be able to help others quickly identify where the map traces came from and provide a foundation for a conversation about mapping that poi, way, or relation.

  • name=* - the most basic identifier of all! Same as OSM.
  • start_date=YYYY-MM-DD; YYYY-MM; YYYY - this MUST be in one of these formats. OHM only supports strict ISO 8601 compliant dates. In the future, we hope to support the updated 8601 standard, ISO 8601-2:2019 which will enable such terminology as "about," "around," "circa," and other more nuanced date formats.
  • end_date=YYYY-MM-DD; YYYY-MM; YYYY - same as for start_date.
  • source:url=https://yoursource.org - just a plain old source=*, but it doesn't help people find your information quickly. Please use source=* as your first choice.
  • license=* - without this tag, the assumption is that the license is CC0.

In addition, the following tags are highly encouraged, in order to enable connection to other data sources across the Internet and to make OHM map data available to others.

  • wikipedia=* - this will help with an improved object inspector.
  • wikidata=* - this is critical for open data queries of OHM.
  • source:wms=* - this enables another editor to continue mapping the exact same source you have mapped, just by plugging this URL into iD or JOSM.

The following tags are in need of clarification on how to best use them, but will also enrich the depiction of data on OHM.

  • Date modifiers: as_of=*, construction_start=* - not everything has a clearly-defined "start" date. Many construction projects have taken years. Other specifics are harder to nail down. A building can be on a map published in 1830, and have no known start date. So, as of 1830, we know it was there.
  • Images: image=*, image:date=* - we are working to include images in the object inspector.
  • Additional information: info=* - this is to provide links to other deep or rich sources of info about that particular way. This information might be complementary and beyond the level of detail in Wikipedia, or be relevant for a particular time in history.

Differences from OSM Tagging

Things to keep in mind that are different from OSM:

  • Tagging license=* at all.
  • Tagging source=* at the object and not changeset level. We want to keep the source of data as close as possible to the data itself. And, the source needs to be easily recoverable.

Where to Tag

Ideally, tagging should take place on the highest level object possible. And, in this case, relations are your friend.

For example, there's a circular way in London's Regent's Park. Originally, it enclosed a private nursery. Later, it was home to the Royal Botanic Society. After that, it merged into the Park. Mapping each of those entities as a separate relation, with its own time intervals, enables the way itself to remain unchanged.

Known Issues

  • Multiple sources, or images, or names for a single way.

Provisional OHM Tagging conventions

These tags are generally accepted but are potentially subject to improvements/changes.

Experimental OHM Tagging conventions

Mappers are using these tags as they feel their way towards standard ways of doing things. You are encouraged to review these to see if any might serve your mapping needs, and to enter data on your own experimental tagging.

Import Datasets (proposed)

Imports are encouraged on OHM, as opposed to OSM, where they are approached (wisely!) with caution. Imports on OHM, however, should be tagged appropriately in order to recover any potential changes to that data that may want to be harvested for possible improvements in the original dataset, or to properly handle citation requirements (e.g., in a CC-BY licensing of the imported dataset.

  • dataset=* (proposed) - this would help unify and tie together all related OSM objects for a particular import. This should be some sort of unique identifier to a particular source, data, and user driving the import. e.g. "Sea_His_Soc_2024_02_29_jeffmeyer" or maybe even something briefer.
  • citation=* (proposed) - to enable on-map citation of the data source

Representation of change in historical road networks

Several different approaches have been experimented with.

  • representing only road segments that no longer exist, leaving OSM to represent segments that are still present. this has some limitations, as there is not currently an accepted way to link across OHM and OSM and it is therefore hard to use the data. additionally, if relations are required to represent relationships and concepts, this approach does not work well.
  • representing complete roads regardless of duplication of OSM. Duplication is unfortunate, but then if classifications have changed, such duplication is probably necessary. it makes building relations easier

if there are changes over time that do not change the geometry (classification, surfaces, etc.), several approaches have been suggested

  • multiple ways sharing nodes, with different tagging/start & end dates on each individual way. may be challenging to edit
  • a single way, with relations carrying the varying tags and dates. An example of this approach, contributed by Leon Karcher, is this way and set of relations: https://openhistoricalmap.org/way/198180780

Representation of Troop Movements & Formations

There is currently discussion on this on the Open Historical Map/Projects/American Civil War page

Related Conversations