If you've traveled to Korea, you've hit this wall: Google Maps can't give you walking or transit directions here.
Map-data export is restricted, so locals use Naver Map or KakaoMap instead. The usual workaround for visitors is
painful — copy a place's Korean name from Google, paste it into Naver, repeat for every stop.

So I built K-Map Router: paste a Google Maps link → it opens that place (and the route) in
Naver or Kakao. Free, no sign-up, nothing stored. Code is here:
github.com/piyaklabs/k-map-router.

This post isn't a "look how hard I worked" story — honestly, I shipped it fast with heavy AI pair-programming (Claude Code).
It's a field guide to the stuff that's genuinely hard to find documented: how Korean map deep links and coordinates
actually work. If you ever build something in this space, I hope this saves you a few days.

The architecture, in one breath

A single Cloudflare Worker serves both the React SPA (static assets) and the POST /api/resolve endpoint — same origin,
free tier, zero runtime dependencies. Coordinate resolution is server-side (the browser → Google is blocked by CORS) and
is nothing but fetch + regex + a little decoding. No DB, stateless.

Hard part #1: coordinates live in different formats per URL

You can't just regex one pattern. A Google Maps URL (after following redirects) hides the coordinates in one of several
shapes, and you have to try them in priority order:

  // 1) place pin — most authoritative
  //    ...!3d{lat}!4d{lng}
  // 2) directions waypoint — ⚠️ REVERSED: !1d{lng}!2d{lat}
  //    multiple pairs => last = destination, first = origin
  // 3) viewport center — /@{lat},{lng},17z
  // 4) ?query= / &destination= / &daddr=
  // 5) ?ll= / &sll=

The one that bit me hardest: /dir/ directions URLs store !1d{longitude}!2d{latitude} — longitude first. Read it as
(lat, lng) and you'll happily return a point that's in the ocean. And when there are multiple pairs, the last pair is the
destination, the first is the start point (which is how the tool can preserve A→B routes).

Hard part #2: mobile "Copy link" hides coordinates in a protobuf

Links shared from the Google Maps mobile app (the ones with ?g_st=...) are special. They resolve to something like:

  .../maps?saddr=Seoul+Station&daddr=Gyeongbokgung&geocode=FWoPPQId...;FWFrPQIdEYSRBy...

There are no plaintext coordinates anywhere — they're base64url-encoded in the geocode= param, as a tiny protobuf. Each
;-separated entry encodes one endpoint:

  // 0x15 = field 2, fixed32 (little-endian) = lat * 1e6
  // 0x1D = field 3, fixed32 (little-endian) = lng * 1e6
  function decodeGeocodeEntry(entry) {
    const bin = atob(entry.replace(/-/g, "+").replace(/_/g, "/"));
    let lat = null, lng = null;
    for (let i = 0; i + 4 < bin.length && (lat === null || lng === null); i++) {
      const tag = bin.charCodeAt(i);
      if (tag !== 0x15 && tag !== 0x1d) continue;
      let v = 0;
      for (let j = 3; j >= 0; j--) v = v * 256 + bin.charCodeAt(i + 1 + j); // LE
      if (v > 0x7fffffff) v -= 0x100000000;
      if (tag === 0x15) lat = v / 1e6; else lng = v / 1e6;
      i += 4;
    }
    return lat !== null && lng !== null ? { lat, lng } : null;
  }

Verified against 경복궁 (Gyeongbokgung): FWFrPQIdEYSRBy... → 37.579617, 126.977041. ✅

Hard part #3: the deep link specs (and their gotchas)

Naver (primary — best transit + English):

  nmap://route/public?dlat={lat}&dlng={lng}&dname={enc}&appname={APPNAME}

• appname is required (silently fails without it).
• dname is optional — omit it and Naver shows the real address. Don't send a literal Destination placeholder.
• Modes are different action paths: route/walk, route/car, route/public.

Kakao (secondary):

  kakaomap://route?ep={lat},{lng}&by=publictransit

• Modes: by=foot | car | publictransit.

Android: custom schemes are flaky from Chrome. Use an intent:// URL with a built-in store fallback:

  intent://route/public?...#Intent;scheme=nmap;package=com.nhn.android.nmap;S.browser_fallback_url=...;end

Hard part #4: Kakao's web URL uses a coordinate system from another dimension

For desktop fallback, Kakao's legacy link/to API can't take a start point. But its redirect target can — if you feed it
Kakao's internal WCongnamul coordinates:

  https://map.kakao.com/?map_type=TYPE_MAP&target=traffic&rt={sx},{sy},{ex},{ey}&rt1={from}&rt2={to}

WCongnamul turned out to be EPSG:5181 (a GRS80 Transverse Mercator) scaled ×2.5. I implemented the projection by hand and
it matched Kakao's own conversion to the integer for every test point. (Also: never put a comma in the rt1/rt2 label — it
breaks the parser and silently drops the destination.)

Hard part #5: the iOS clipboard "paste" button that wouldn't paste

The "Paste from clipboard" button worked everywhere except iOS. Two reasons, both subtle:

1. Google Maps "Copy link" puts the URL on the clipboard as text/uri-list only — no text/plain. So navigator.clipboard.readText() returns an empty string.
2. iOS WebKit expires the user activation after your first await. So a readText() → fall back to read() chain always fails on the second call with NotAllowedError.

The fix is to make exactly one clipboard call inside the gesture, then read the type off the already-resolved
ClipboardItem (those getType calls reuse the granted permission):

  const items = await navigator.clipboard.read();   // one call, in the gesture
  for (const item of items) {
    for (const type of ["text/uri-list", "text/plain", "text/html"]) {
      if (!item.types.includes(type)) continue;
      const text = (await (await item.getType(type)).text()).trim();
      // uri-list: first non-comment line is the URL
      if (text) return text;
    }
  }

The iOS "Paste" permission bubble itself is unavoidable — it's OS-enforced for any programmatic clipboard read.

Bonus: respect the mode the user already picked

Google encodes the travel mode in the link (travelmode=driving, dirflg=d, or !3e0). Reading it means a shared driving
route opens directly in driving directions in Naver/Kakao — "plan in Google Maps, navigate in Korea," unchanged.

Try it / take it

• Live: kmap.piyaklabs.com
• Code (MIT-ish, stateless, zero deps): github.com/piyaklabs/k-map-router

If you're building anything that bridges Google Maps and Korean map apps, steal the deep-link and coordinate logic — that's
exactly why it's public. Questions welcome. 🐣