Where is the boundary between postal codes?

0 0 0 15 January 2026

#30DayMapChallenge Day 3. Polygons.

🌎 Polygons can be misleading. They seem natural because since childhood we look at administrative maps, where every piece of land belongs to a country, province, or municipality… That’s why we like to represent many features this way. But not all data is suited for it.

➡️ An example?
A map of postal code areas. At first glance, everything looks fine—but it’s not.

In Poland, postal codes are managed by Poczta Polska, which maintains the “Register of Postal Address Numbers.” It’s a document over 1,700 pages long, where every town or address is assigned a specific code.

So, a postal code is assigned to individual address points.

So, the boundary between address points runs… anywhere between two points with different postal codes. By convention, it’s drawn in the middle, but that’s just a convention. There’s no such thing as an official boundary between postal codes.

❓ Where does this cause difficulties?

Based on the coverage areas (polygons), it is impossible to accurately answer the often-asked question: What percentage of postal code X’s area is covered by our delivery zone? According to the map below, even the reed beds along the Vistula are assigned a postal code.
The correctly phrased question is: What percentage of address points within a given postal code is served by our delivery zone?

Using Usemaps for such analyses, you’ll always get the correct answer 😎

Unfortunately, we don’t know the answer to why Bydgoszcz has several hundred postal codes while Toruń has just one…