Add logging to SRTM elevation fallback chain

lib/microwaveprop/terrain/elevation_client.ex

@@ -3,6 +3,8 @@ defmodule Microwaveprop.Terrain.ElevationClient do
 
   alias Microwaveprop.Terrain.Srtm
 
+  require Logger
+
   @batch_size 100
 
   @spec fetch_elevation_profile(float(), float(), float(), float(), pos_integer()) ::
@@ -10,12 +12,18 @@ defmodule Microwaveprop.Terrain.ElevationClient do
   def fetch_elevation_profile(lat1, lon1, lat2, lon2, n \\ 64) do
     case srtm_tiles_dir() do
       nil ->
+        Logger.debug("SRTM tiles_dir not configured, using API")
         fetch_elevation_profile_api(lat1, lon1, lat2, lon2, n)
 
       tiles_dir ->
         case Srtm.fetch_elevation_profile(lat1, lon1, lat2, lon2, tiles_dir, n) do
-          {:ok, _profile} = ok -> ok
-          {:error, _} -> fetch_elevation_profile_api(lat1, lon1, lat2, lon2, n)
+          {:ok, _profile} = ok ->
+            Logger.info("Elevation profile served from SRTM tiles")
+            ok
+
+          {:error, reason} ->
+            Logger.warning("SRTM failed (#{inspect(reason)}), falling back to API")
+            fetch_elevation_profile_api(lat1, lon1, lat2, lon2, n)
         end
     end
   end

lib/microwaveprop/terrain/srtm.ex

@@ -1,6 +1,8 @@
 defmodule Microwaveprop.Terrain.Srtm do
   @moduledoc false
 
+  require Logger
+
   @samples 3601
   @void -32_768
   @base_url "https://elevation-tiles-prod.s3.amazonaws.com/skadi"
@@ -26,19 +28,25 @@ defmodule Microwaveprop.Terrain.Srtm do
     url = "#{@base_url}/#{lat_dir}/#{filename}.gz"
     path = Path.join(tiles_dir, filename)
 
+    Logger.info("Downloading SRTM tile #{filename} from S3")
+
     case Req.get(url, req_options()) do
       {:ok, %{status: 200, body: body}} ->
         decompressed = :zlib.gunzip(body)
         File.write!(path, decompressed)
+        Logger.info("SRTM tile #{filename} downloaded (#{byte_size(decompressed)} bytes)")
         {:ok, path}
 
       {:ok, %{status: 404}} ->
+        Logger.warning("SRTM tile #{filename} not available on S3 (404)")
         {:error, :not_available}
 
       {:ok, %{status: status}} ->
+        Logger.error("SRTM tile #{filename} download failed: HTTP #{status}")
         {:error, "SRTM download HTTP #{status}"}
 
       {:error, reason} ->
+        Logger.error("SRTM tile #{filename} download error: #{inspect(reason)}")
         {:error, "SRTM download error: #{inspect(reason)}"}
     end
   end