Auto-download missing SRTM tiles, treat water areas as 0m elevation

SRTM profiles no longer abort on missing tiles — water/ocean areas
return 0m elevation instead of failing the entire profile and falling
back to the rate-limited API. Elevation client and viewshed now pass
download: true to auto-fetch missing tiles from S3. NFS mount changed
to writable so downloaded tiles persist across pods.

k8s/deployment.yaml

@@ -105,10 +105,8 @@ spec:
           volumeMounts:
             - name: srtm-data
               mountPath: /srtm
-              readOnly: true
       volumes:
         - name: srtm-data
           nfs:
             server: 204.110.191.8
             path: /data/srtm
-            readOnly: true

lib/microwaveprop/terrain/elevation_client.ex

@@ -16,14 +16,8 @@ defmodule Microwaveprop.Terrain.ElevationClient do
         fetch_elevation_profile_api(lat1, lon1, lat2, lon2, n)
 
       tiles_dir ->
-        case Srtm.fetch_elevation_profile(lat1, lon1, lat2, lon2, tiles_dir, n, opts) do
-          {:ok, _profile} = ok ->
-            ok
-
-          {:error, reason} ->
-            Logger.warning("SRTM failed (#{inspect(reason)}), falling back to API")
-            fetch_elevation_profile_api(lat1, lon1, lat2, lon2, n)
-        end
+        srtm_opts = Keyword.merge([download: true], opts)
+        Srtm.fetch_elevation_profile(lat1, lon1, lat2, lon2, tiles_dir, n, srtm_opts)
     end
   end
 

lib/microwaveprop/terrain/srtm.ex

@@ -84,26 +84,19 @@ defmodule Microwaveprop.Terrain.Srtm do
     pts = sample_path(lat1, lon1, lat2, lon2, n)
     dist_km = haversine_km(lat1, lon1, lat2, lon2)
 
-    results =
-      Enum.reduce_while(pts, {:ok, []}, fn pt, {:ok, acc} ->
-        case lookup(pt.lat, pt.lon, tiles_dir, opts) do
-          {:ok, elev} ->
-            entry = %{
-              lat: pt.lat,
-              lon: pt.lon,
-              d: pt.d,
-              elev: elev,
-              dist_km: pt.d * dist_km
-            }
+    profile =
+      Enum.map(pts, fn pt ->
+        elev =
+          case lookup(pt.lat, pt.lon, tiles_dir, opts) do
+            {:ok, elev} -> elev
+            # Missing tile (water/ocean) or void value — treat as sea level
+            {:error, _} -> 0
+          end
 
-            {:cont, {:ok, acc ++ [entry]}}
-
-          {:error, reason} ->
-            {:halt, {:error, reason}}
-        end
+        %{lat: pt.lat, lon: pt.lon, d: pt.d, elev: elev, dist_km: pt.d * dist_km}
       end)
 
-    results
+    {:ok, profile}
   end
 
   defp read_elevation(fd, lat, lon) do

lib/microwaveprop/terrain/viewshed.ex

@@ -151,25 +151,21 @@ defmodule Microwaveprop.Terrain.Viewshed do
     terrain_check_km = min(radio_horizon_km(ant_height_m) * 2.0, max_range_km)
     {end_lat, end_lon} = destination_point(origin_lat, origin_lon, bearing, terrain_check_km)
 
-    case Srtm.fetch_elevation_profile(origin_lat, origin_lon, end_lat, end_lon, tiles_dir) do
-      {:ok, profile} ->
-        analysis =
-          TerrainAnalysis.analyse(profile, terrain_check_km, freq_ghz, ant_ht_a: ant_height_m, ant_ht_b: ant_height_m)
+    {:ok, profile} =
+      Srtm.fetch_elevation_profile(origin_lat, origin_lon, end_lat, end_lon, tiles_dir, 64, download: true)
 
-        reach_km = effective_reach_km(analysis, max_range_km, score)
-        {reach_lat, reach_lon} = destination_point(origin_lat, origin_lon, bearing, reach_km)
+    analysis =
+      TerrainAnalysis.analyse(profile, terrain_check_km, freq_ghz, ant_ht_a: ant_height_m, ant_ht_b: ant_height_m)
 
-        %{
-          bearing: bearing,
-          reach_km: reach_km,
-          lat: reach_lat,
-          lon: reach_lon
-        }
+    reach_km = effective_reach_km(analysis, max_range_km, score)
+    {reach_lat, reach_lon} = destination_point(origin_lat, origin_lon, bearing, reach_km)
 
-      {:error, _} ->
-        {end_lat2, end_lon2} = destination_point(origin_lat, origin_lon, bearing, max_range_km)
-        %{bearing: bearing, reach_km: max_range_km, lat: end_lat2, lon: end_lon2}
-    end
+    %{
+      bearing: bearing,
+      reach_km: reach_km,
+      lat: reach_lat,
+      lon: reach_lon
+    }
   end
 
   # Radio horizon distance in km for a given antenna height (K=4/3 atmosphere)

test/microwaveprop/terrain/elevation_client_test.exs

@@ -148,21 +148,14 @@ defmodule Microwaveprop.Terrain.ElevationClientSrtmTest do
     end
 
     @tag :srtm
-    test "falls back to API when SRTM tiles missing" do
+    test "returns zero elevation when SRTM tiles missing (no API fallback)" do
       Application.put_env(:microwaveprop, :srtm_tiles_dir, "/nonexistent/tiles")
 
-      Req.Test.stub(ElevationClient, fn conn ->
-        Req.Test.json(conn, %{"elevation" => [200.0, 250.0, 180.0]})
-      end)
+      assert {:ok, profile} =
+               ElevationClient.fetch_elevation_profile(32.9, -97.0, 30.3, -97.7, 2)
 
-      {result, _log} =
-        with_log(fn ->
-          ElevationClient.fetch_elevation_profile(32.9, -97.0, 30.3, -97.7, 2)
-        end)
-
-      assert {:ok, profile} = result
       assert length(profile) == 3
-      assert hd(profile).elev == 200.0
+      assert Enum.all?(profile, fn p -> p.elev == 0 end)
     end
   end
 end

test/microwaveprop/terrain/srtm_test.exs

@@ -190,9 +190,11 @@ defmodule Microwaveprop.Terrain.SrtmTest do
       assert last.dist_km > 0
     end
 
-    test "returns error when tiles dir does not exist" do
-      assert {:error, _} =
+    test "returns zero elevation when tiles dir does not exist" do
+      assert {:ok, profile} =
                Srtm.fetch_elevation_profile(32.78, -96.8, 32.5, -96.5, "/nonexistent", 4)
+
+      assert Enum.all?(profile, fn p -> p.elev == 0 end)
     end
   end
 end