Time Traveling with OHM

Today I've been playing with OpenHistoricalMap and exploring how it can be used to create a map that visualizes how a country's borders have changed over time.

History is rarely static, but few nations illustrate the fluidity of borders quite like Poland. Over the last two centuries, the Polish state has expanded, contracted, and even physically shifted hundreds of miles across the European plain.

My Poland Border Map visualizes three pivotal snapshots of Poland over time: the semi-autonomous Congress Poland of 1815, the restored Second Republic of 1939, and the modern "shifted" borders of 1945.

The map allows you to toggle between three distinct periods of Polish history. For example, by switching eras, you can see the dramatic "westward slide" that occurred after World War II - when Poland lost its eastern territories (Kresy) but gained a massive industrial coastline and former German lands in the west.

What is OpenHistoricalMap?

Think of OpenHistoricalMap as the "Wikipedia of Geography." It is a community-driven effort to map the world as it looked at every point in history. Unlike a standard map that only shows the world right now, OHM includes a Time Slider. You can slide this bar back to 1939, 1815, or even 1025, and the map transforms to show the geopolitical reality of that specific moment.

How I Built the Poland Border Map: From Timeline to GeoJSON

Creating a map like this requires precise historical boundaries. Rather than drawing them by hand, I extracted them directly from the OHM database. If you want to build your own historical visualization, here is the workflow I used:

1. Targeting the Time

First, I went to the main OHM site and moved the Timeline Slider to the specific date I needed (e.g., August 31, 1939). This filters the global database to show only the boundaries active on that day.

2. Querying the Features

Once zoomed in on Poland, I used the Query Features tool (the mouse-arrow icon in the sidebar). Then when clicking on the Polish border on the map, the OHM sidebar is populated with a list of historical entities.

3. Identifying the Relation

In mapping, a country isn't just a line; it’s a Relation - a collection of ways and nodes that form a polygon. I identified the specific Relation ID (the Polish border) for the era I wanted.

4. Extracting via Overpass Turbo

With the Relation ID in hand, I moved over to the OHM Overpass Turbo instance. This is a powerful data mining tool that allows you to "talk" to the OHM database. I used a simple query to grab the geometry of that specific ID, eg:

overpassql

[out:json][timeout:25];

relation(2692576); // The ID for the specific historical border

out geom;

Running this query returned the raw geographic data, which I then exported as a GeoJSON file to power my MapLibre map of Poland's border at three points in history.