Open Historical Map/OHM Basics

From OpenStreetMap Wiki
Jump to navigation Jump to search

Welcome to Open Historical Map

This page is here to help you get oriented. If you come here from OpenStreetMap (OSM), much will be similar.

For Absolute Beginners

If you don't have any experience with OSM, then there are introductions and tutorials available. Editing OSM and editing OHM is so similar that the OSM tutorials are fine for learning the basics.

The basic elements are covered in Beginners_Guide_1.3.

There are some good tutorials on LearnOSM and on TeachOSM.


When considering starting out in Open Historical Map, it can be really helpful to think about specific projects. If you decide on a project, it's a good idea to create a page for it under Open Historical Map/Projects. In fact, you should probably visit that page and see some of the things people have chosen to do. Some projects are really large. For example, the Open Historical Map/Projects/American Civil War project is huge. As of this writing the folks working on it have chosen to focus on smaller sub projects.

It is recommended that individuals starting projects add some form of contact information, perhaps a link to their personal wiki talk page, to the project page. This facilitates coordination if others wish to get involved in these projects.


You will need to create a login for Open Historical Map (OHM). OHM does not share OSM's user database. You can use your OSM login if you like, as long as no one else has taken it.


Open Historical Map works with the same editors as OSM.

ID & Potlatch

ID and Potlatch are accessed on the OHM page the same way as on the OSM page.


JOSM takes slightly more work.

Changing Server Settings

You must change the API settings in JOSM preferences to set the OHM server API connection parameter (uncheck the default): use Please note, https: and not http. You will need to set the username and login to match your OHM account.

JOSM OHM Preferences.png

You may also edit using JOSM by choosing "Edit with Remote Control" in the dropdown menu next to the "Edit"-button on the website. Ensure that Remote Control is enabled under Preferences in JOSM and that JOSM is running. The api will be connected automatically. Built in firewalls in your computer's operating system may need to be adjusted. The procedure varies from system to system.

Using an alternate Preferences file

You can set up an alternate JOSM preferences.xml file and run JOSM from the command line if you wish to avoid changing the server preferences repeatedly.

Mac & Linux

By default JOSM puts its preferences file in a subdirectory, .josm. You can clone this directory and run JOSM from the command line. For example, copy .josm to .josm-ohm; edit the preferences.xml file and review & possibly change the settings for osm-server.auth-method, osm-server.password, osm-server.username, and osm-server.url. Once this is done, you can set up an alias in your shell profile such as:

josm-ohm='java -Djosm.home=/Users/myhomedir/.josm-ohm -jar /Users/myhomedir/JavaApps/josm-tested.jar

(need windows details)

Naming Conventions

For now, use OSM naming conventions, but keep in mind that local languages change over time. For example, use Dutch names in New Amsterdam rather than modern English names.

Tagging Conventions

OpenHistoricalMap tagging is mostly the same as OpenStreetMap tagging. In fact, some tags of marginal use in OSM (such as crop=* are much more useful in OHM, where what crop is planted at a particular point in time is both known and important.

(add link to more detailed page on tags and conventions when it exists)

There are some particularly important tags, described below.


source=* is used differently in OSM and OHM. The modern OSM model is to place source=* on changesets rather than on entities in the database. OHM adheres to the earlier model. If sourcing is simple then it's fine to just put a specification of the origin of the data. If sourcing becomes complex (it often does), some projects use the convention of providing a link to the OHM wiki page for the project where much more extensive detail is possible. See the Open_Historical_Map/Projects/New_York_Capital_District project page for an example of this approach.

Examples of direct links to maps

  • Ribeiro's_World_Map:
  • 1853 Mitchell Map of Asia:


Since everything in modern OSM is under the ODbL, this tag is not used there. OHM has a different concept of licensing. Entities in OHM should have the license=* tag specifying the terms and conditions associated with the entity. We prefer license=CC0 (effectively a public domain license) whenever possible, but it may not always be feasible.

You do not need to put a license=* tag on everything. Select a higher level object (a relation or a way) whenever possible.

IMPORTANT: Note that the difference in licensing approach means you cannot casually copy data from Open Street Map. So don't.

Start & End Dates

start_date=* and end_date=* are a key part of OHM tagging. The time slider feature of the main website depends on them in order to determine what to display at any point in time. They are not currently required, but but make a huge difference to the visualizations and so every effort should be made determine them. The time slider feature correctly recognizes start_date and end_date when ascribed to relations, so relations can be used to assemble pieces of features that change over time in the obvious way.

OHM is moving away from start_date=* tagging towards EDTF, a profile of ISO 8601. Currently, what a mapper should do is place an ISO 8601-1 extended format date (no time) in the start_date=* and end_date=* fields, e.g. 2007, 2010-02, 1982-03-16. A more detailed EDTF format date may be placed in start_date:edtf=* and end_date:edtf=*. currently only the date in start_date=* and end_date=* are directly supported. The EDTF tags are available to facilitate recording accurate information when a mapper has it.

Note on date formats

OHM usage is shifting in the direction of Much of the material on the OpenStreetMap start_date is adhoc and not in conformance with current standards.

If you do not know the actual day, you can just add the year, e.g., start_date=1925.


  • 1901-01-20
  • 1902-12
  • 1878
  • 1900/1925 (between 1900(-01-01) and 1925(-12-31))
  • 1900-06/1901-04-23
  • ../1910-01-20 (before a specific date - possibly when a photo was taken)


attribution=* may be used to provide attribution at any time that a license requires it, and even if the license does not. OHM is attempting to tag attribution at the object level (POIs, ways, relations) rather than at the changeset level.


The wikipedia=* is used to link to a relevant Wikipedia page on the OHM object. Wikipedia is a great source of historical information and a great source of ideas for what could be mapped in OHM. Be sure to link to a related article wherever possible. And, be sure to use the language prefix when using this tag, e.g. wikipedia=en:Carcassonne.


The wikidata=* will link your OHM data to a particular wikidata Q code. e.g. wikidata=Q6582.

Copyrights and Licenses

OHM is a bit different from OSM. Because of the differences, copying of data from OSM to OHM should NOT be done unless it can be determined that permissions are in place (e.g., if the author of the material can be identified and contacted to gain permission.)

OHM provides for mixed licenses using the license=*. Use of the CC0 license is preferred when sources allow. More information on CC0 may be found here.

Note on OSM derived data

Ss of June 2019, there is OSM-derived data in OHM and the markings on the OHM website are incorrect. The OHM community is working to address this.


OHM is in a state of transition. Since it started in 2012-13, it has been a fairly informal organization. It has become clear that a more formal structure is needed and OHM will be seeking to affiliate with a larger organization in the Open Source/Open Data community. In the meantime, folks involved in the informal organization are listening, and may be found in all of the communities listed in the next section. You find they are willing and eager to talk about where things are heading.


The various OHM communities are great places to get help and discuss the issues that come up in your projects.

Mailing list

Open Historical Map has a traditional electronic mailing list which may be found here.



OSM Slack

OSM US hosts a Slack workspace which is open to mappers worldwide, not just mappers in the US. There are two OHM channels in it, openhistoricalmap for general discussion and opehistoricalmap-tech for wonky discussions of internal details.

Sign up for OSM-US Slack & join the OHM discussion on #openhistoricalmap.


OpenHistoricalMap has a channel on the OSM World Discord server.

Sign up here and then join the OHM conversation on the #openhistoricalmap channel.


#ohm on


Facebook Group