Similarly, if I bump into an old acquaintance at a conference, we both tend to say "If only I'd known you were here, we could have had dinner together last night!"

I do enjoy the serendipity of events like FOSDEM - randomly seeing a mate and expressing the joy of spontaneity. But I also like arranging to meet up in advance.

At the moment, my strategy is sending a blast on social media saying "I'm visiting [this city] next week, anyone fancy a beer and a natter?" I've met friends all over Europe, Australia, and New Zealand that way. It mostly works. But I can't help feeling it is inefficient and prone to missing connections.

I even wrote my own code to auto-post FourSquare checkins to my other social media sites.

Here are my ideal scenarios. Imagine something built in to Signal / WhatsApp / Whatever app you already use.

Plan In Advance

I tell my app that I'm going to Barcelona from 14th - 19th February and am happy to meet any of my friends.

✨Background Magic✨

My friend Alice has also planned a trip to Barcelona around those dates. She gets a ping saying that one of her friends is going to be in the same city. Does she want to know more?

So far, so Dopplr.

My friend Bob lives just outside of Barcelona. He's set his "willing to travel" settings to be about 30 minutes, so also receives a ping.

I don't know that either of them have seen the notification until they decide they want to meet.

Spontaneous Fun

I step off the train in Manchester, England England. Perhaps the app notices I'm away from home, or maybe I press the "Anyone Around?" button.

On a map I can see friends who have shared their rough location. I decide to message Chuck to see if he's free for a chat.

Dave notices my location is now within his preferred travel distance. He gives me a ring.

A bit like how FourSquare used to be - but with less precision.

Downsides

The above is very much the "happy path". It doesn't look at any of the knotty problems or grapple with the UI that would be needed to make this work. But we know the technology for sharing location is viable - so what are the social issues that make this so difficult?

Social Awkwardness

"Oh, fuck, Edgar's location says he's in town. Can we pretend to be out of the country?"

Alternatively, "Huh, I know at least a dozen people who live in Skegness. Why aren't any of them responding to me?"

Social pressure and awkwardness are hard problems. No one wants to use the app that makes you feel like a friendless loser.

Privacy

Do you want your friends knowing your every movement? I'm sure some people do, but most probably don't. It's possible to sketch out some vague controls:

• Only send a notification if I push this button.
• Don't send alerts if I am within this radius of my home / work.
• Fuzz my location to the city / state / country level.

Danger

Is it a risk to let people know vaguely where you are? Is meeting up with (semi-) strangers from the Internet a smart life choice? Is having an app stalk you across the globe giving too much data to advertisers?

Does that creep from work abuse the system to keep popping up whenever you're out with friends?

Technology

I said the technology exists for this, and that was sort of true. Every device has GPS & an Internet connection. Storing a log of friends and sending them a message is a solved problem.

But is it solved in a decentralised and privacy preserving way?

No one wants to give all this power to one company. Google will build it and kill it. Facebook will sell your secrets to dropshippers. A funky start-up will be acquhired by Apple & restricted to iOS devices.

My location is fuzzed to an acceptable degree of imprecision and then sent… where? To all my friends directly? To a central server? Can k-anonymity help?

Is this a separate app? Everyone seemed to leave FourSquare after they buggered around with it. Perhaps it is just a feature in existing apps?

What's Already There?

Messaging apps like Signal, Telegram, and WhatsApp allow you to share your location with one or more friends.

To me, it feels a bit weird to manually send a dropped pin to some / all of my contact. It also doesn't let you share "tomorrow I will be in…"

Using "Stories" is the common way to share an update with all contacts - but none of them let you automatically share your location in a story.

FourSquare's Swarm app allows you to check in to a "neighbourhood". But there's no obvious way of saying "London" or "Manchester" - and I'm not sure how close to an area you need to be to get an alert that your friend is there.

What's Next?

I don't want to build this. Trying to get everyone I know to adopt a new app isn't going to happen. With the fragmentation of messaging and the lack of interoperability, this is likely to remain an unsolved problem for some time.

So here's my strategy.

• Get back in to using FourSquare. Most of my friends seemed to stop using it back in 2017 when it was split into Swarm. But a few are still on there.
• Manually post a story on Mastodon, BlueSky, Facebook, WhatsApp, Signal, and Telegram saying "Visiting Hamburg next week. Anyone want a beer?"
• Hope that something better comes along.

So, is there a way to recreate that early Web 2.0 experience with open data and ActivityPub? Let's find out!

This quest is divided into two parts.

1. Get nearby "Points of Interest" (POI) from OpenStreetMap.
2. Share a location on the Fediverse.

OpenStreetMap API

OpenStreetMap is the Wikipedia of maps. It is a freely available resource which anyone can edit (if they're skilled enough).

It also comes with a pretty decent API for querying things. For example, nw["amenity"]({{bbox}}); finds all "amenities" near a specific location.

As you can see, it has highlighted some useful areas - a pharmacy and a pub. But it has ignored other useful locations - the train station and the park. It has also included some things that we may not want - bike parking and a taxi rank.

What API call is needed to get useful locations of of OverPass?

It's possible to specify the type of thing to find using nw["amenity"="restaurant"]; - but adding every single type of thing would quickly end up with a very large query containing hundreds of types.

It is also possible to exclude specific types of places. This retrieves all amenities except for fast food joints:

nw["amenity"]({{bbox}});
-
nw["amenity"="fast_food"]({{bbox}});

Again, that would be complex.

Perhaps one solution is just to return everything and let the user decide if they want to check in to a telephone kiosk or a fire hydrant? That's a bit user-hostile.

Instead, this query returns everything which has a name nw["name"]({{bbox}});

That cuts out any unnamed things - like park benches and car-sharing spots. But it does add named roads and train lines.

It is possible to use filters to exclude results from OverPass. The best that I can come up with is: nw["name"][!"highway"][!"railway"][!"waterway"][!"power"]({{bbox}});

That gets everything which has a name, but isn't a highway or railway or waterway or powerline. It isn't perfect - but it will do!

This is the query which will retrieve the 25 nearest things within 100 metres of a specific latitude and longitude. It includes the name and any other tags, the location, and the OSM ID.

overpass-api.de/api/interpreter?data=[out:json];nw["name"][!"highway"][!"railway"][!"waterway"][!"power"](around:100,51.5202,-0.1040);out center qt 25;

ActivityPub

There's good news and bad news here. Firstly, ActivityStreams (which are subscribed to in ActivityPub) supports the concept of "Place" and "Location".

Once the user has a latitude and longitude, the can share it - along with a message, photo, or anything else.

Something like:

{
    "@context": "https://www.w3.org/ns/activitystreams",
    "type": "Note",
    "content": "Here in NYC! <a href=\"https://www.openstreetmap.org/way/958999496\">John Lennon's Imagine Mosaic</a>.",
    "attachment": [
        {
            "type": "Image",
            "mediaType": "image\/jpeg",
            "url": "https:\/\/fastly.4sqi.net\/img\/general\/590x786\/56367_9pxuZJD7d1hgPdaMFcFq1pipvTTMynBJsYcpHH-b8mU.jpg",
            "name": "A photo of a mosaic which says 'Imagine'."
        }
    ],
    "location": {
        "name": "John Lennon's Imagine",
        "type": "Place",
        "longitude": 40.77563,
        "latitude": -73.97474
    }
}

For example, here's a PixelFed post with an attached location - and this is the JSON representation. That status can be reposted into other social networks.

It is worth noting that Mastodon doesn't (natively) support location - if you view my repost of that PixelFed post you'll see there's no location metadata attached. That's OK! It just means that the status needs to include human-readable data.

Similarly, Mastodon doesn't support the arrive vocabulary. So this will be limited to a message with a location attached.

Other ActivityPub services do support location.

Putting it all together

Well… that's a job for next week. Probably!

• Building a web site which gets the user's location is easy.
• Getting the data from OverPass should be straightforward.
• Creating an ActivityPub server which can post geotagged notes into the Fediverse might be a little beyond my skillset! Some testing with Darius Kazemi's AP Glitch suggests this should work.

If you'd like to help, please leave a comment.

So, how do we go from http://4sq.com/fgIWov to

1 Expand the URL

Get your Bit.ly API Key.

http://api.bitly.com/v3/expand
   ?shortUrl=http://4sq.com/fgIWov
   &login=YOUR_BIT_LY_USERNAME
   &apiKey=YOUR_BIT_LY_API_KEY
   &format=txt

You can, if you prefer, get the info back in JSON or XML. See the Bit.ly API Expand Documentation.

2 The Foursquare URL

Bit.ly gives us the URL which includes a checkin ID and a signature.

http://foursquare.com/edent/checkin/4d7e5b4f9df3f04d7400c394
   ?s=afH3jJg3L9HpLqVIwiqp-YpNL5k

3 The Foursquare API Call

From this, we construct an API call to Foursquare. It looks like this

https://api.foursquare.com/v2/checkins/4d7e5b4f9df3f04d7400c394
   ?signature=afH3jJg3L9HpLqVIwiqp-YpNL5k
   &oauth_token=YOUR_FOURSQUARE_OAUTH_TOKEN

NOTE the Foursquare URL says ?s= whereas the API call requires ?signature= That caused me more frustration than it should...

Getting your OAuth token is a little cumbersome - follow the steps at the Foursquare Developers site.

4 The JSON Response

I'll cut the cruft out of here, so you only see what you need to display images. The response gives us the link to the full sized image - as well as several different sizes should you wish to display thumbnails.

{
  "meta":{
     "code":200
  },
  "response":{
     "checkin":{
        },
   ...
   "photos":{
      "count":1,
      "items":[
         {
            "id":"4d7e5b4f9df3f04d7400c394",
            "createdAt":1300184596,
            "url":"http://playfoursquare.s3.amazonaws.com/pix/A.jpg",
            "sizes":{
               "count":4,
               "items":[
                  {
                     "url":"http://playfoursquare.s3.amazonaws.com/pix/B.jpg",
                     "width":720,"height":540
                  },
                  {
                     "url":"http://playfoursquare.s3.amazonaws.com/derived_pix/C.jpg",
                     "width":300,"height":300
                  },
                  {
                     "url":"http://playfoursquare.s3.amazonaws.com/derived_pix/D.jpg",
                     "width":100,"height":100
                  },
                  {
                     "url":"http://playfoursquare.s3.amazonaws.com/derived_pix/E.jpg",
                     "width":36,
                     "height":36
                  }
               ]
            },

5 Gotchas

Again, nothing in life is easy. Not all foursquare checkins have an image associated with them. In which case, we can do one of three things.

1. Display no image
2. Display the user's avatar - if it's public
3. Display an image of the venue - or one of the icons associated with it.

6 Embed.ly

Dabr uses the awesome Embedly service. Rather than having hundreds of different regular expressions and API calls to get embedded images, Embed.ly gives us a single end point to call. So, rather than rewrite huge chunks of Dabr's code, I've submitted this to the clever-clogs behind Embed.ly - that way, everyone can benefit.

Any questions, comments, or clarifications - please let me know in the comments box.