County boundary processing rules per country – seeking documentation

Question

i Project-GC team,

I'm building a desktop app for geocaching challenge-checking that needs to assign caches to counties / regions in the same way Project-GC does. Thanks to Pleu's earlier answer (https://project-gc.com/?qa=30819) we already know that PGC uses **osm-boundaries.com, database `osm20210412`, admin_level 6** as the base, and that **UK has "many special rules and unions"** on top.

I downloaded the same source data and compared the raw feature count with what Project-GC shows in its statistics. Most countries differ – sometimes by a few features, sometimes drastically:

| Country | osm-boundaries (level 6 unless noted) | PGC shows | Δ |
|---------|----:|----:|----:|
| AT | 93 | ~94 | -1 |
| BE (level 8) | 581 | 565 | +16 |
| CH (level 8) | 2234 | ~2113 | +121 |
| CZ (level 7) | ? | 77 | ? |
| DE | 405 | 401 | +4 *(found: 1× church boundary + 3× historical Landkreise)* |
| FR | 131 | 96 | +35 *(overseas territories?)* |
| GB | 226 | 102 | +124 *(special rules per Pleu)* |
| HU (level 7) | 174 | 175 | -1 |
| IT | 102 | 107 | -5 |
| LU (level 8) | 102 | 102 | 0 ✅ |
| NL (level 8) | 354 | 342 | +12 |
| PL | 387 | 380 | +7 |
| SK (level 7) | ? | 79 | ? |

To match Project-GC exactly, I'd need to know per country:

1. **Which admin_level is used?** (We assume level 6 for most, but BE/CH/LU/NL=8, HU=7, etc. – is this correct?)
2. **What additional filtering is applied?** Examples for DE we found:
   - Excluded: `Evangelische Landeskirche in Baden`, `Landkreis Bühl (historisch)`, `Landkreis Kehl`, `Landkreis Lahr` (the latter three were dissolved in 1973)
   Are there similar exclusion lists for other countries?
3. **For UK specifically:** What "unions" are performed? E.g., is `Bristol` formed by merging multiple OSM boundaries? Is there a list of these unions?
4. **For FR:** Are overseas departments (DOM-TOM) included or excluded? Anything else?
5. **Is there any data file or documentation** that lists the exact set of regions used per country?

I'm happy to share my work back to the community. Goal is a stable, project-gc-compatible county lookup for offline use.

Thanks for any pointers!

Answer 1

This will not be possible as this data differs between different countries and is updated when needed.

1) Depends on the data available, different for different countries.

2) Depends on the data available, different for different countreis.

3) No. The regions were counstructed to match the regions available on geocaching.com, they are a montrosity and I hate them.

4) I don't understand this question. But my guess is: if they are countries on Geocaching.com then no, if they aren't then yes.

5) No. This was based on manual research over a period of ~three months and have since been updated multiple times. What you're asking for do not exist and is not possible to create or keep updated. Even if you manage to match it exactly today it might differ tomorrow or next week and it will be guaranteed to not match next year.

Comments

Thanks for the honest answer, Pleu – that actually confirms we took the right approach: we reverse-engineered it empirically by comparing the raw OSM boundary data country by country until the names matched exactly what PGC shows. Took a while, but we got there for all 13 countries.

One follow-up question: since the data can change at any time – is there any place where those changes are announced? E.g. a changelog, a forum thread, or any other notification channel? We'd like to know when a county gets renamed or restructured on PGC's side so we can update our data accordingly.

Thanks again!

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Updates are generally announced here: https://project-gc.com/forum/list?6