https://shkspr.mobi/blog/2025/04/reverse-geocoding-is-hard/

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2025-04-25

Reverse Geocoding is Hard

@edent   [a]

geolocation geotagging OpenBenches * 7 comments * 850 words *
Viewed ~17,337 times.

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My wife and I run OpenBenches - a crowd-sourced database of nearly
40,000 memorial benches. Every bench is geo-tagged with a latitude
and longitude. But how do you go from a string of digits to something
human readable?

How do I turn -33.755780,150.603769 into "42 Wallaby Way, Sydney,
Australia"?

Luckily, that's a (somewhat) solved problem. Services like OpenCage,
StadiaMaps, OpenStreetMap, and Geocode.Earth all provide APIs which
transform co-ordinates into addresses. Done! Let's go home.

Except... Not everywhere has an address. Some benches are in parks.
They typically don't have a street number, but might have an
interesting feature nearby to help with location. For example a
statue or prominent landmark.

And... Not every address is relevant. Some benches are on streets. But
we probably don't want to imply that the bench is inside or belongs
to a specific nearby house.

Let's step back a bit. Why do we want to display a human-readable
address?

We have two use-cases.

"As a visitor to the site, I want to:"

 1. Read a (rough) textual representation of where the bench is.
 2. Click on a component of the address to see all benches within
    that area.

The first is easy to explain:

Map. Under it is an address of "Middlewich, Cheshire East, England,
United Kingdom".

The second is harder. Suppose a bench is in Wellington, New Zealand.
We want to create a URl like openbenches.org/location/New Zealand/
Wellington/. That way, users can click on the word "Wellington" and
find all the benches nearby. A user can also manually edit that URl
to increase or decrease precision.

Both of these are problems of precision.

Let's take a look at how one of the reverse geocoding services deals
with transforming 51.476845,-0.295296 into an address:

    Royal Botanic Gardens, Kew, Sandycombe Road, Kew, London Borough
    of Richmond upon Thames, London, Greater London, England, TW9
    2EN, United Kingdom

That is too much address!

Yes, it is technically accurate. But it contains far too much detail
for humans, the postcode is irrelevant, and the weird-subdivisions
are nothing that a local person would use.

Looking at the full API response, we can see:

[jso] JSON{
  "place_id": 258770727,
  "licence": "Data (c) OpenStreetMap contributors, ODbL 1.0. http://osm.org/copyright",
  "name": "Royal Botanic Gardens, Kew",
  "display_name": "Royal Botanic Gardens, Kew, Elizabeth Cottages, Kew, London Borough of Richmond upon Thames, London, Greater London, England, TW9 3NJ, United Kingdom",
  "address": {
    "leisure": "Royal Botanic Gardens, Kew",
    "road": "Elizabeth Cottages",
    "suburb": "Kew",
    "city_district": "London Borough of Richmond upon Thames",
    "ISO3166-2-lvl8": "GB-RIC",
    "city": "London",
    "state_district": "Greater London",
    "state": "England",
    "ISO3166-2-lvl4": "GB-ENG",
    "postcode": "TW9 3NJ",
    "country": "United Kingdom",
    "country_code": "gb"
  }
}

Aha! Perhaps I can build a better address using just those
components!

Except... Not every country has states. And not all states are used
when giving addresses. Not every location is in a city. Some places
have villages, prefectures, municipalities, and hamlets.

New York, New York is a valid address, but Berlin, Berlin is not!

There's an address formatter by OpenCage which is pretty sensible
about stripping off irrelevant details. But, to go back to my first
point, not every map location on OpenBenches is a street address and
- even if it is on a street - it probably shouldn't have a house
number.

Well, there's kind of a solution to that! Most mapping provider have
a POI function - we can find nearby things of interest and use them
as a location.

Here's a bench in Cook County, Illinois, USA. The POI address is:

[jso] JSON{
...
   "name": "Central Park",
   "coarse_location": "Des Plaines, IL, USA",
...
}

I assume there's only one Central Park in Des Plaines. Do people know
that "Il" is Illinois? Would "Cook County" be useful?

On the subject of localisation, not everywhere speaks English. Do I
want to display addresses like "Yuan Bao noZi noXiang , Guang Dao , Ri Ben "? How about
"Yuan Bao noZi noXiang , Hiroshima, Japan"?

We're an international site, but most benches are in Anglophone
countries.

Of course, just because something is physically near a POI, that
doesn't mean it is logically close to it.

Consider a bench situated at the edge of this park A map, with a
marker situated just across a river.

The nearest POI is "Gay's Creamery" - across the river. Is that what
you'd expect? Is there any way to easily say "if a point is inside an
amenity* then use that as the address?

I don't want the users of our site to have to select from a list of
POIs or addresses, this should be as automated as possible.

The Plan

For each bench:

 1. Use StadiaMaps to get the nearest POI.
 2. Get the data in English.
 3. Concatenate the name and coarse location.
 4. Save the "address".
 5. Wait for complaints?

Thoughts?

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7 thoughts on "Reverse Geocoding is Hard"

 1. 2025-04-25 12:42
    [64b70c86]

    [mastodon] NAB :marmite: said on mastodon.me.uk:

    @Edent If you can do the geolocation in realtime when I'm
    uploading a picture, show the results and ask "Is this location
    good enough?" with a Yes/No/Edit option, that would work,
    wouldn't it?

    Reply | Reply to original comment on mastodon.me.uk

 2. 2025-04-25 13:21
    [37df032a]

    [mastodon] Terence Eden said on mastodon.social:

    @NAB I could. But I'm wary of making people do too much hard
    work. And I don't want people to be able to accidentally or
    maliciously change the address.
    Perhaps a drop down is the answer?

    Reply | Reply to original comment on mastodon.social

 3. 2025-04-25 13:31
    [a51b0e3c]

    [crispsandw] Matt Round said on crispsandwi.ch:

    @Edent Hmm, wonder if you could compare nearest POI with bench
    address, only use POI if it has enough levels of address detail
    in common

    Reply | Reply to original comment on crispsandwi.ch

 4. 2025-04-25 18:38
    [ad25c12a]

    [our-qcodes] Merton says:

    I need to digest this. I'd be happy to let you use qCodes
    (remember me) [free of charge]. As you know I am not very
    technical so I'll have to study what you have done to see if
    qCodes can help you.
    Something like: OBN-XXXX.
    I'll post here when i have something. Just got the european
    patent, finally.

    Reply

 5. 2025-04-26 11:19
    [ad25c12a]

    Merton says:

    Further comment. I saw a great little clip. UK government wanted
    to encourage peple to walk more. So they wanted to build more
    walking paths. The guy in the clip, a real true marketing guru
    said: "You don't need more walking paths, you need more benches
    for people to take a break."
    Very clever idea.

    Reply

 6. 2025-04-27 16:13
    [18241c35]

    [news] news.ycombinator.com said on news.ycombinator.com:

    Reverse Geocoding Is Hard | Hacker News

    Reply | Reply to original comment on news.ycombinator.com

 7. 2025-04-28 04:16
    [94182dfe]

    [irrelevant] Rob says:

    Maybe get a distance to the POI from route planning software/
    service. This should allow you to filter out close-but-irrelevant
    results, like that other-side-of-the-river one.

    Reply

 8. [svg]

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