From OpenStreetMap Wiki
Jump to navigation Jump to search
OpenHistoricalMap logo.svg source
Shepherd atlas.jpeg
all things related to the source of both an object's geometry and its properties. Edit this description in the wiki page. Edit this description in the data item.
Group: annotations
Used on these elements
may be used on nodesmay be used on waysmay be used on areas (and multipolygon relations)may be used on relations
Documented values: 32
See also
Status: de facto

In OpenHistoricalMap (OHM) the source key can apply to any object, can be enumerated, can have sub-keys and is used as a keyspace suffix for other keys to describe the source of any property of the mapped object.

See also: OHM TagInfo for source


There are several important reasons for tagging OHM objects as thoroughly as possible:

  1. Object geometry validation and verification: Consistent with academic sourcing, OHM users are encouraged to cite as many sources as possible to validate the assertion of what has been mapped. This will also enable conflict resolution, if possible. And, if not, please see the next point.
  2. Enabling other mappers: rarely will any mapper be be able to map all of the features for any single map. Leaving source tags behind will help enable expansion or addition to that map from related or similar sources.
  3. Being a good neighbor. In the worlds of academia and linked open data, contributors may be reluctant to share data if it is not properly sourced
  4. License validation. If OHM asserts that something is CC0 or available in the public domain, it is difficult to prove that without citing an appropriate source with that license.
  5. Supporting >1 representation of an object: This is mainly an edge case. OHM intentionally violates the "One feature, one OSM element" principle for the purpose of tracking changes over time. That principle could also be applied to having multiple versions of the same object at the same time, such as when there is disput about historical ground truth. A primary means for allowing this type of multiplicity of representation is the source tag.

When to use

The source tag should be used almost anywhere possible in OHM. This means using it on nodes, ways, areas, and relations, not to mention on the keys that describe those elements.

The only exception would be otherwise untagged nodes that are part of a properly-sourced way or relation. Those nodes are fine without specific source tags and adding tags to them would also make editing more difficult, as tagged nodes are often represented differently and less noticeably in editors.

In general, the source key is used to describe an object's geometry. To identify the source of any key, use tag:source=*, following the same conventions outlined below.

source by itself can refer to either a short name of the source or a direct link to the actual source, if available online. The rationale for this non-specific requirement is to encourage the use of some source tag without unnecessarily slowing down the mapper.

There is no agreed standard for short text citations yet. Please see the discussion below for more details. In the meantime, please use as little text as you can while maintaining the ability to clearly identify the source described by the text.

The base source key is flexible

As mentioned above, the most important part of having a source tag is just having a source tag on every object.

Given that, OHM mappers have used a variety of methods for satisfying this base requirement, all of which are reasonable, valuable, and much, much better than not tagging at all, as shown in the table below. Thinking of source as an "umbrella" key may be helpful.

Encouraged OHM source subkeys
key approach example notes
source plain name source="Shepherd's Historical Atlas, 1923" optionally complemented with source:url
source URL of source map source= optionally complemented with source:name
source URL wiki project page source= great way to refernece an OHM project wiki page, but hides the specific source
source URL of wiki-based citation source= points to a specific source using wiki formatting
source list of multiple sources source="Shepherd's Historical Atlas, 1923;; OHM wiki Newcastle project page" faster for the mapper, less ready for machine parsing; perhaps less humand readable; perhaps more?

Where further clarification or refining of a source description is desired, or possible, mappers are encouraged to add whatever level of additional source tagging, as described in the sections below.

Encouraged subkeys

Encouraged OHM source subkeys
key subkey description example
source name text of a source's description source:name="Shepherd's Historical Atlas, 1923"
source url url for a specific source source:url=
source tiles url for the tiles used in tracing; can be plugged directly into iD or JOSM source:tiles={z}/{x}/{y}.png

Adding sources to object property keys

Identifying the sources used for a geometry's attributes can help other mappers build from the same or complementary contemporary sources.

Note that a historical entity might be depicted on a map, but described in a separate, non-map source like a photo, a newspaper ad or article, a business directory, or even a painting.

These object key source keys should only be used when they differ from the base source keys. There is no need to redundantly tag every key with the same source.

OHM key:source subkey examples
key subkey description example
start_date source:[subkey] descriptor for the source of the start_date start_date:source:name="Stanford Daily, 12 October, 1941"
name source:[subkey] url for a specific source name:source:url=

Enumerating sources

Where more than 1 source is used for either a geometry or an attribute – which is encouraged – a number can be added in between source and any subkeys"


  • start_date:source:3:name="Acme Co. Grand Opening newspaper ad, London Times, 16 March 1823"
  • source:2:name="1940 USGS Map of San Francisco"
  • Enumerated source keys in OHM Taginfo: source:2, source:3

Sourcing dataset imports

External GIS datasets often contain a variety of metadata that OHM mappers may want to consider for inclusion in OHM. There is no prohibition on importing this data, but there are some general principles that are encouraged:

  • DO import, as a minimum: a unique identifier for that import, such as dataset:id=* to enable a direct connection between an object or an attribute in OHM and an external source. The dataset key should be a unique key already not in use by a separate import or project.
  • Consider NOT importing: anything that lacks historical significance or is specific to the generation of the GIS data. Fields like: AREA, LENGTH, CREATED_ON, CREATED_BY, LASTEDIT, etc., should generally be omitted from an import.

When your source has a SOURCE (or equivalent) field

This can be a bit tricky and mappers should feel free to consult the community for a best solution. In general, if the source's SOURCE (or some other field that indicates where they got their information) is essentially no different from the bibliography of a source text or article, mappers may want to omit this source.

In cases where there is source information that is extremely proximate or obviously valuable to other mappers, then this field should be considered for inclusion. For example, survey-level descriptions or pointers to specific statements of reference points in historical terms.

Always be sure to add an imported dataset's citation to the OHM Contributors wiki page.

Sourcing other datasets

Other datasets may have significantly longer source citations and might be richer sources of the provenance of data contained in OHM. Examples of these datasets include Pleiades, the World Historical Gazetteer, WIkidata, Europeana, etc. In these cases, mappers should consider including tags like: [dataset]:id.

Always be sure to add an imported dataset's citation to the OHM Contributors wiki page.

Sourcing your own assessments

Invariably, when mapping historical places, the mapper will be faced with uncertainty. Given the choice of mapping or not mapping, OHM mappers are encourage to map and tag how they arrived at their conclusions. There are two key considerations for this type of sourcing:

  • Date uncertainty: please add a start_date:edtf or end_date:edtf designator. The EDTF date format is designed to support uncertainty, vaugeness, and ranges when attempting to assess a time for some event.
  • Other uncertainty: At a minimum, please be sure to add all of the sources used to reach your assessment. Then, optionally, add something like: name:source="listed as a saloon in source:1 and a church in source:2 in order to alleviate a reviewer from figuring out how you resolved the discrepancy.

Source citation format

There are a variety of formats for citing maps and other sources. OHM does not have a preferred format [at this time]. Formats for consideration when mapping include:

OHM editor tagging support

Both iD and JOSM allow the use of free-form tagging in cases where there are no presets or templates available.

Software for tracking and managing sorces

Tracking and managing sources can be difficult for anyone working in depth on any type of research. As such, there are some popular tools that help with this challenge.

Some Wikipedians swear by the built-in Citoid tool or the third-party Zotero service to manage possible citations while conducting research. Nothing stops a mapper from also using Zotero (which is standard-issue in academia). You could use it to generate all the metadata tags and paste them en masse into iD’s raw tag editor

See also