feat: DEM-based elevation recalculation via edit drawer button

Adds a "Recalculate from terrain map (DEM)" button to the activity edit
drawer. On click it queries an Open-Elevation-compatible API to replace
GPS altitude with SRTM terrain data, applies 5m hysteresis, and updates
the activity's elevation stats and timeseries chart in place.

- bincio/extract/dem.py: lookup_elevations() (batched HTTP POST) +
  recalculate_elevation() (subsample → DEM → interpolate → hysteresis →
  patch activity JSON, timeseries JSON, index.json)
- POST /api/activity/{id}/recalculate-elevation on both serve and edit
  servers; serve endpoint is auth-gated and triggers merge + rebuild
- --dem-url flag (also DEM_URL env var) on bincio serve and bincio edit;
  logged at startup; missing URL returns a clear 503 with setup instructions
- /api/me response gains dem_configured bool
- EditDrawer: button with loading state, shows new ↑/↓ values on success

bincio/edit/cli.py

@@ -24,6 +24,8 @@ console = Console()
               help="Strava API client ID (enables Strava sync in the UI). Also reads STRAVA_CLIENT_ID env var.")
 @click.option("--strava-client-secret", default=None, envvar="STRAVA_CLIENT_SECRET",
               help="Strava API client secret. Also reads STRAVA_CLIENT_SECRET env var.")
+@click.option("--dem-url", default=None, envvar="DEM_URL",
+              help="Base URL of an Open-Elevation-compatible API (enables 'Recalculate elevation' button).")
 def edit(
     data_dir: Optional[str],
     port: int,
@@ -31,6 +33,7 @@ def edit(
     config_path: Optional[str],
     strava_client_id: Optional[str],
     strava_client_secret: Optional[str],
+    dem_url: Optional[str],
 ) -> None:
     """Start a local web UI for editing activity sidecar files.
 
@@ -69,11 +72,16 @@ def edit(
     srv.site_url = site_url
     srv.strava_client_id = strava_client_id or ""
     srv.strava_client_secret = strava_client_secret or ""
+    srv.dem_url = dem_url or ""
 
     if strava_client_id:
         console.print(f"Strava sync: [green]enabled[/green] (client {strava_client_id})")
     else:
         console.print("Strava sync: [yellow]disabled[/yellow] (pass --strava-client-id to enable)")
+    if dem_url:
+        console.print(f"DEM:         [green]enabled[/green] ({dem_url})")
+    else:
+        console.print("DEM:         [yellow]disabled[/yellow] (pass --dem-url to enable elevation recalculation)")
 
     uvicorn.run(srv.app, host="127.0.0.1", port=port, log_level="warning")
 

bincio/edit/server.py

@@ -20,6 +20,7 @@ data_dir: Path | None = None
 site_url: str = "http://localhost:4321"
 strava_client_id: str = ""
 strava_client_secret: str = ""
+dem_url: str = ""   # Open-Elevation-compatible API base URL; empty = feature disabled
 
 # In-memory CSRF state tokens for OAuth flows (token → True); cleared after use
 _oauth_states: set[str] = set()
@@ -427,6 +428,32 @@ async def save_activity(activity_id: str, payload: dict[str, Any]) -> JSONRespon
     return JSONResponse({"ok": True, "sidecar": str(sidecar_path)})
 
 
+@app.post("/api/activity/{activity_id}/recalculate-elevation")
+async def recalculate_elevation_endpoint(activity_id: str) -> JSONResponse:
+    """Replace GPS altitude with DEM terrain elevation and recompute gain/loss.
+
+    Requires --dem-url to be set when starting bincio edit.
+    """
+    if not dem_url:
+        raise HTTPException(
+            503,
+            "DEM URL not configured. "
+            "Pass --dem-url <api-url> to bincio edit (e.g. https://api.open-elevation.com).",
+        )
+    dd = _get_data_dir()
+    _check_id(activity_id)
+    try:
+        from bincio.extract.dem import recalculate_elevation
+        from bincio.render.merge import merge_one
+        result = recalculate_elevation(dd, activity_id, dem_url)
+        merge_one(dd, activity_id)
+        return JSONResponse(result)
+    except FileNotFoundError as e:
+        raise HTTPException(404, str(e))
+    except ValueError as e:
+        raise HTTPException(422, str(e))
+
+
 @app.post("/api/activity/{activity_id}/images")
 async def upload_image(activity_id: str, file: UploadFile = File(...)) -> JSONResponse:
     dd = _get_data_dir()

bincio/extract/dem.py

@@ -0,0 +1,221 @@
+"""DEM (Digital Elevation Model) lookup and elevation recalculation.
+
+Queries any Open-Elevation-compatible HTTP API to replace noisy GPS altitude
+with terrain altitude, then re-applies hysteresis-based accumulation.
+
+Compatible APIs:
+  - https://api.open-elevation.com          (free, SRTM, accepts large batches)
+  - https://api.opentopodata.org/v1/srtm30m  (more reliable, max 100 pts/req)
+
+Pass the base URL (without path) to bincio serve/edit via --dem-url.
+"""
+
+from __future__ import annotations
+
+import json
+import urllib.error
+import urllib.request
+from pathlib import Path
+from typing import Optional
+
+# Sample one GPS point per N seconds when building the DEM query.
+# SRTM30 resolution is ~30 m; at 30 km/h cycling that's ~3 s per tile —
+# sampling every 10 s is more than enough.
+_DEFAULT_SAMPLE_INTERVAL_S = 10
+
+# Maximum locations per API request.  Open-Elevation supports ~512 per POST.
+_DEFAULT_BATCH_SIZE = 512
+
+# Hysteresis threshold after DEM correction.  DEM data is already smooth
+# (30 m horizontal resolution) so 5 m is a generous dead-band.
+_DEM_HYSTERESIS_M = 5.0
+
+
+def lookup_elevations(
+    latlons: list[tuple[float, float]],
+    api_url: str,
+    batch_size: int = _DEFAULT_BATCH_SIZE,
+    timeout_s: int = 30,
+) -> list[Optional[float]]:
+    """Query a DEM API for terrain elevation at the given (lat, lon) pairs.
+
+    Uses the Open-Elevation API format::
+
+        POST {api_url}/api/v1/lookup
+        {"locations": [{"latitude": lat, "longitude": lon}, ...]}
+
+    Returns a list the same length as *latlons*.  Elements are ``None``
+    wherever the API returned no data (network error, ocean, etc.).
+    """
+    if not latlons:
+        return []
+
+    results: list[Optional[float]] = [None] * len(latlons)
+    url = f"{api_url.rstrip('/')}/api/v1/lookup"
+
+    for start in range(0, len(latlons), batch_size):
+        batch = latlons[start : start + batch_size]
+        payload = json.dumps(
+            {"locations": [{"latitude": lat, "longitude": lon} for lat, lon in batch]}
+        ).encode("utf-8")
+        req = urllib.request.Request(
+            url,
+            data=payload,
+            headers={"Content-Type": "application/json", "Accept": "application/json"},
+            method="POST",
+        )
+        try:
+            with urllib.request.urlopen(req, timeout=timeout_s) as resp:
+                data = json.loads(resp.read().decode("utf-8"))
+            for i, item in enumerate(data.get("results", [])):
+                elev = item.get("elevation")
+                if elev is not None:
+                    results[start + i] = float(elev)
+        except (urllib.error.URLError, json.JSONDecodeError, KeyError, ValueError):
+            pass  # leave this batch as None; caller checks overall coverage
+
+    return results
+
+
+def recalculate_elevation(
+    user_dir: Path,
+    activity_id: str,
+    dem_url: str,
+    sample_interval_s: int = _DEFAULT_SAMPLE_INTERVAL_S,
+) -> dict:
+    """Replace GPS elevation with DEM terrain elevation and recompute gain/loss.
+
+    Algorithm
+    ---------
+    1. Read the activity's 1 Hz timeseries for lat / lon / t arrays.
+    2. Subsample GPS points every *sample_interval_s* seconds.
+    3. Query the DEM API for those points (batched).
+    4. Linearly interpolate DEM elevation back to every GPS-valid second.
+    5. Apply :data:`_DEM_HYSTERESIS_M` hysteresis to compute gain / loss.
+    6. Write the updated ``elevation_m`` array to the timeseries JSON.
+    7. Patch ``elevation_gain_m`` / ``elevation_loss_m`` in the detail JSON.
+    8. Patch ``elevation_gain_m`` in ``index.json`` (summary entry for the feed).
+
+    Returns
+    -------
+    dict with keys ``elevation_gain_m``, ``elevation_loss_m``,
+    ``points_queried`` (DEM responses received).
+
+    Raises
+    ------
+    FileNotFoundError
+        Activity detail or timeseries file not found.
+    ValueError
+        Activity has no GPS coordinates, or the DEM API returned too few results.
+    """
+    acts_dir = user_dir / "activities"
+    json_path = acts_dir / f"{activity_id}.json"
+    ts_path   = acts_dir / f"{activity_id}.timeseries.json"
+
+    if not json_path.exists():
+        raise FileNotFoundError(f"Activity not found: {activity_id}")
+    if not ts_path.exists():
+        raise ValueError("Activity has no timeseries data")
+
+    ts      = json.loads(ts_path.read_text(encoding="utf-8"))
+    lat_arr: list[Optional[float]] = ts.get("lat") or []
+    lon_arr: list[Optional[float]] = ts.get("lon") or []
+    t_arr:   list[int]             = ts.get("t") or []
+
+    if not lat_arr or not lon_arr:
+        raise ValueError(
+            "Activity has no GPS coordinates "
+            "(privacy=no_gps or data recorded without GPS)"
+        )
+
+    n = len(t_arr)
+
+    # ── 1. Subsample GPS-valid indices ────────────────────────────────────────
+    gps_idx = [
+        i for i in range(n)
+        if lat_arr[i] is not None and lon_arr[i] is not None
+    ]
+    if len(gps_idx) < 2:
+        raise ValueError("Too few GPS points for DEM lookup")
+
+    sampled_idx = gps_idx[::sample_interval_s]
+    if gps_idx[-1] not in sampled_idx:
+        sampled_idx.append(gps_idx[-1])  # always include the last point
+
+    # ── 2. Query DEM ──────────────────────────────────────────────────────────
+    latlons = [(float(lat_arr[i]), float(lon_arr[i])) for i in sampled_idx]  # type: ignore[arg-type]
+    dem_elev = lookup_elevations(latlons, dem_url)
+
+    # Build (t, elevation) pairs for valid DEM responses only
+    valid_pairs: list[tuple[int, float]] = [
+        (t_arr[sampled_idx[k]], dem_elev[k])
+        for k in range(len(sampled_idx))
+        if dem_elev[k] is not None
+    ]
+    n_queried = len(valid_pairs)
+    if n_queried < 2:
+        raise ValueError(
+            f"DEM API returned too few results "
+            f"({n_queried} of {len(sampled_idx)} points). "
+            f"Check the DEM URL: {dem_url}"
+        )
+
+    # ── 3. Linear interpolation → full 1 Hz series ───────────────────────────
+    new_ele: list[Optional[float]] = [None] * n
+    j = 0
+    for i in gps_idx:
+        t = t_arr[i]
+        # Advance j so valid_pairs[j] is the left anchor for t
+        while j + 1 < len(valid_pairs) - 1 and valid_pairs[j + 1][0] <= t:
+            j += 1
+        t0, e0 = valid_pairs[j]
+        if j + 1 < len(valid_pairs):
+            t1, e1 = valid_pairs[j + 1]
+            frac = max(0.0, min(1.0, (t - t0) / (t1 - t0))) if t1 > t0 else 0.0
+            new_ele[i] = round(e0 + frac * (e1 - e0), 1)
+        else:
+            new_ele[i] = round(e0, 1)
+
+    # ── 4. Hysteresis accumulation ────────────────────────────────────────────
+    valid_eles = [e for e in new_ele if e is not None]
+    gain = loss = 0.0
+    committed = valid_eles[0]
+    for e in valid_eles[1:]:
+        diff = e - committed
+        if abs(diff) >= _DEM_HYSTERESIS_M:
+            if diff > 0:
+                gain += diff
+            else:
+                loss += diff
+            committed = e
+
+    gain_r = round(gain, 1)
+    loss_r = round(abs(loss), 1)
+
+    # ── 5. Write timeseries ───────────────────────────────────────────────────
+    ts["elevation_m"] = new_ele
+    ts_path.write_text(json.dumps(ts, indent=2, ensure_ascii=False), encoding="utf-8")
+
+    # ── 6. Patch activity detail JSON ─────────────────────────────────────────
+    detail = json.loads(json_path.read_text(encoding="utf-8"))
+    detail["elevation_gain_m"] = gain_r
+    detail["elevation_loss_m"] = loss_r
+    json_path.write_text(json.dumps(detail, indent=2, ensure_ascii=False), encoding="utf-8")
+
+    # ── 7. Patch index.json summary (so merge_all picks up the new value) ─────
+    index_path = user_dir / "index.json"
+    if index_path.exists():
+        index = json.loads(index_path.read_text(encoding="utf-8"))
+        for s in index.get("activities", []):
+            if s.get("id") == activity_id:
+                s["elevation_gain_m"] = gain_r
+                break
+        index_path.write_text(
+            json.dumps(index, indent=2, ensure_ascii=False), encoding="utf-8"
+        )
+
+    return {
+        "elevation_gain_m": gain_r,
+        "elevation_loss_m": loss_r,
+        "points_queried": n_queried,
+    }

bincio/serve/cli.py

@@ -21,10 +21,11 @@ console = Console()
 @click.option("--max-users", default=None, type=int, help="Override max users for this instance (0 = unlimited; updates the DB setting)")
 @click.option("--public-url", default=None, envvar="PUBLIC_URL", help="Public base URL (e.g. https://yourdomain.com). Required for Strava OAuth to work behind a reverse proxy.")
 @click.option("--webroot", default=None, type=click.Path(), help="Nginx webroot (e.g. /var/www/bincio). When set, uploads trigger a full Astro build + rsync so new activity pages are immediately accessible without a git push.")
+@click.option("--dem-url", default=None, envvar="DEM_URL", help="Base URL of an Open-Elevation-compatible API (enables 'Recalculate elevation' button in the edit drawer).")
 def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
           strava_client_id: Optional[str], strava_client_secret: Optional[str],
           max_users: Optional[int], public_url: Optional[str],
-          webroot: Optional[str]) -> None:
+          webroot: Optional[str], dem_url: Optional[str]) -> None:
     """Start the bincio multi-user application server.
 
     Handles auth, user management, and write operations.
@@ -58,6 +59,8 @@ def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
         srv.public_url = public_url
     if webroot and site_dir:
         srv.webroot = Path(webroot).expanduser().resolve()
+    if dem_url:
+        srv.dem_url = dem_url
 
     db = open_db(dd)
     current_limit = get_setting(db, "max_users")
@@ -74,6 +77,8 @@ def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
         console.print(f"  Users: [yellow]max {current_limit}[/yellow]")
     else:
         console.print(f"  Users: [dim]unlimited[/dim]")
+    if dem_url:
+        console.print(f"  DEM:   [cyan]{dem_url}[/cyan]")
     console.print()
 
     log_config = uvicorn.config.LOGGING_CONFIG.copy()

bincio/serve/server.py

@@ -158,6 +158,7 @@ webroot: Path | None = None    # nginx webroot — when set, trigger full rebuil
 strava_client_id: str = ""
 strava_client_secret: str = ""
 public_url: str = ""   # e.g. "https://yourdomain.com" — used for OAuth redirect URIs
+dem_url: str = ""      # Open-Elevation-compatible API base URL; empty = feature disabled
 _db = None  # sqlite3.Connection, opened lazily
 
 
@@ -404,6 +405,7 @@ async def me(bincio_session: Optional[str] = Cookie(default=None)) -> JSONRespon
         "display_name": user.display_name,
         "is_admin": user.is_admin,
         "store_originals_default": store_orig != "false",
+        "dem_configured": bool(dem_url),
     })
 
 
@@ -1265,6 +1267,39 @@ async def post_activity(
     return JSONResponse({"ok": True})
 
 
+@app.post("/api/activity/{activity_id}/recalculate-elevation")
+async def recalculate_elevation_endpoint(
+    activity_id: str,
+    bincio_session: Optional[str] = Cookie(default=None),
+) -> JSONResponse:
+    """Replace GPS altitude with DEM terrain elevation and recompute gain/loss.
+
+    Requires --dem-url to be set when starting bincio serve.
+    """
+    user = _require_user(bincio_session)
+    _check_id(activity_id)
+    if not dem_url:
+        raise HTTPException(
+            503,
+            "DEM URL not configured. "
+            "Pass --dem-url <api-url> to bincio serve (e.g. https://api.open-elevation.com).",
+        )
+    dd = _get_data_dir() / user.handle
+    if not (dd / "activities" / f"{activity_id}.json").exists():
+        raise HTTPException(404, "Activity not found")
+    try:
+        from bincio.extract.dem import recalculate_elevation
+        from bincio.render.merge import merge_one
+        result = recalculate_elevation(dd, activity_id, dem_url)
+        merge_one(dd, activity_id)
+        _trigger_rebuild(user.handle)
+        return JSONResponse(result)
+    except FileNotFoundError as e:
+        raise HTTPException(404, str(e))
+    except ValueError as e:
+        raise HTTPException(422, str(e))
+
+
 @app.delete("/api/activity/{activity_id}", response_model=GenericResponse)
 async def delete_activity(
     activity_id: str,

site/src/components/EditDrawer.svelte

@@ -24,6 +24,11 @@
   let confirmDelete = false;
   let deleting = false;
 
+  // Elevation recalculation from DEM
+  let recalculating = false;
+  let recalcStatus = '';
+  let recalcOk = false;
+
   // Form state
   let title = '';
   let sport: Sport = 'cycling';
@@ -119,6 +124,26 @@
       : [...hideStats, key];
   }
 
+  async function recalculateElevation() {
+    recalculating = true;
+    recalcStatus = '';
+    recalcOk = false;
+    try {
+      const res = await fetch(`${api}/recalculate-elevation`, { method: 'POST' });
+      const d = await res.json();
+      if (!res.ok) throw new Error(d.detail ?? await res.text());
+      recalcOk = true;
+      const gain = d.elevation_gain_m != null ? `↑ ${Math.round(d.elevation_gain_m)} m` : '';
+      const loss = d.elevation_loss_m != null ? `↓ ${Math.round(d.elevation_loss_m)} m` : '';
+      recalcStatus = [gain, loss].filter(Boolean).join('  ');
+    } catch (e: any) {
+      recalcStatus = e.message;
+      recalcOk = false;
+    } finally {
+      recalculating = false;
+    }
+  }
+
   async function deleteActivity() {
     if (!confirmDelete) { confirmDelete = true; return; }
     deleting = true;
@@ -264,6 +289,24 @@
         </div>
       </div>
 
+      <!-- Elevation recalculation -->
+      <div class="mb-4">
+        <p class="text-xs text-zinc-500 mb-2">Elevation</p>
+        <button
+          type="button"
+          class="w-full flex items-center justify-center gap-2 px-3 py-2 rounded-lg border border-zinc-700 text-xs text-zinc-400 hover:border-zinc-500 hover:text-zinc-200 transition-colors disabled:opacity-40"
+          disabled={recalculating}
+          on:click={recalculateElevation}
+        >
+          {recalculating ? 'Querying terrain data…' : '⛰ Recalculate from terrain map (DEM)'}
+        </button>
+        {#if recalcStatus}
+          <p class="text-xs mt-1.5 text-center" class:text-green-400={recalcOk} class:text-red-400={!recalcOk}>
+            {recalcStatus}
+          </p>
+        {/if}
+      </div>
+
       <!-- Flags -->
       <div class="flex gap-3 mb-2">
         <button