diff --git a/assets/js/propagation_map_hook.js b/assets/js/propagation_map_hook.js
index d570eea3..2ab1a3d9 100644
--- a/assets/js/propagation_map_hook.js
+++ b/assets/js/propagation_map_hook.js
@@ -264,11 +264,26 @@ export const PropagationMap = {
       }
     })
 
-    // Click on map to request factor detail from server
+    // Click on map to request viewshed + factor detail from server
     this.map.on("click", (e) => {
+      this.rangeCircles.clearLayers()
+
+      // Show a temporary marker while computing
+      L.circleMarker([e.latlng.lat, e.latlng.lng], {
+        radius: 6,
+        color: "#fff",
+        weight: 2,
+        fillColor: "#888",
+        fillOpacity: 0.8,
+        interactive: false
+      }).addTo(this.rangeCircles)
+
+      // Always request terrain viewshed
+      this.pushEvent("compute_viewshed", { lat: e.latlng.lat, lon: e.latlng.lng })
+
+      // Also request propagation detail if grid data exists
       const basic = this.lookupPoint(e.latlng.lat, e.latlng.lng)
       if (basic) {
-        this.showRangeCircles(basic)
         this.pushEvent("point_detail", { lat: e.latlng.lat, lon: e.latlng.lng })
       }
     })
@@ -283,6 +298,37 @@ export const PropagationMap = {
       }
     })
 
+    this.handleEvent("viewshed_result", ({ origin, points }) => {
+      this.rangeCircles.clearLayers()
+
+      if (points && points.length > 0) {
+        const latlngs = points.map(p => [p.lat, p.lon])
+
+        L.polygon(latlngs, {
+          color: "#222",
+          weight: 2,
+          opacity: 0.8,
+          fillColor: "#000",
+          fillOpacity: 0.12,
+          interactive: false,
+          smoothFactor: 1
+        }).addTo(this.rangeCircles)
+
+        // Center marker colored by score tier if available
+        const basic = this.lookupPoint(origin.lat, origin.lon)
+        const markerColor = basic ? scoreTier(basic.score).color : "#888"
+
+        L.circleMarker([origin.lat, origin.lon], {
+          radius: 6,
+          color: "#fff",
+          weight: 2,
+          fillColor: markerColor,
+          fillOpacity: 1,
+          interactive: false
+        }).addTo(this.rangeCircles)
+      }
+    })
+
     this.map.on("popupclose", () => this.rangeCircles.clearLayers())
 
     // Close popup on double-click so zoom works through it
@@ -442,54 +488,6 @@ export const PropagationMap = {
     return { ...data, ...this.bandInfo }
   },
 
-  showRangeCircles(detail) {
-    this.rangeCircles.clearLayers()
-    const center = [detail.lat, detail.lon]
-    const score = detail.score
-    const tier = scoreTier(score)
-
-    let rangeKm
-    if (score >= 80) rangeKm = detail.exceptional_range_km
-    else if (score >= 65) rangeKm = detail.extended_range_km
-    else if (score >= 50) rangeKm = detail.typical_range_km
-    else if (score >= 33) rangeKm = Math.round(detail.typical_range_km * 0.6)
-    else rangeKm = Math.round(detail.typical_range_km * 0.3)
-
-    const typicalKm = detail.typical_range_km
-
-    L.circle(center, {
-      radius: rangeKm * 1000,
-      color: "#222",
-      weight: 3,
-      opacity: 0.9,
-      fillColor: "#000",
-      fillOpacity: 0.2,
-      dashArray: "8,6",
-      interactive: false
-    }).addTo(this.rangeCircles)
-
-    if (typicalKm < rangeKm) {
-      L.circle(center, {
-        radius: typicalKm * 1000,
-        color: "#222",
-        weight: 3,
-        opacity: 0.9,
-        fillColor: "#000",
-        fillOpacity: 0.25,
-        interactive: false
-      }).addTo(this.rangeCircles)
-    }
-
-    L.circleMarker(center, {
-      radius: 6,
-      color: "#fff",
-      weight: 2,
-      fillColor: tier.color,
-      fillOpacity: 1,
-      interactive: false
-    }).addTo(this.rangeCircles)
-  },
-
   sendBounds() {
     topbar.show()
     const b = this.map.getBounds()
diff --git a/lib/microwaveprop/terrain/viewshed.ex b/lib/microwaveprop/terrain/viewshed.ex
new file mode 100644
index 00000000..ba848805
--- /dev/null
+++ b/lib/microwaveprop/terrain/viewshed.ex
@@ -0,0 +1,133 @@
+defmodule Microwaveprop.Terrain.Viewshed do
+  @moduledoc false
+
+  alias Microwaveprop.Terrain.Srtm
+  alias Microwaveprop.Terrain.TerrainAnalysis
+
+  @earth_radius_km 6371.0
+  @default_angular_step 2
+  @default_max_range_km 50
+
+  @doc "Compute destination lat/lon given origin, bearing (degrees), and distance (km)."
+  def destination_point(lat, lon, bearing_deg, dist_km) do
+    lat_rad = deg_to_rad(lat)
+    lon_rad = deg_to_rad(lon)
+    brg_rad = deg_to_rad(bearing_deg)
+    d = dist_km / @earth_radius_km
+
+    lat2 =
+      :math.asin(
+        :math.sin(lat_rad) * :math.cos(d) +
+          :math.cos(lat_rad) * :math.sin(d) * :math.cos(brg_rad)
+      )
+
+    lon2 =
+      lon_rad +
+        :math.atan2(
+          :math.sin(brg_rad) * :math.sin(d) * :math.cos(lat_rad),
+          :math.cos(d) - :math.sin(lat_rad) * :math.sin(lat2)
+        )
+
+    {rad_to_deg(lat2), rad_to_deg(lon2)}
+  end
+
+  @doc """
+  Find the max clear distance along a ray from TerrainAnalysis points.
+  Skips endpoints (first/last), returns the dist_km of the last clear
+  interior point before the first obstruction.
+  """
+  def find_reach_km(points, max_range_km) do
+    interior = Enum.slice(points, 1..-2//1)
+
+    case find_first_obstructed_index(interior) do
+      nil ->
+        max_range_km
+
+      0 ->
+        0.0
+
+      idx ->
+        Enum.at(interior, idx - 1).dist_km
+    end
+  end
+
+  @doc """
+  Compute a terrain viewshed from a point. Returns a map with :origin
+  and :boundary (list of %{bearing, reach_km, lat, lon}).
+
+  Options:
+    - :freq_ghz — frequency for Fresnel zone calc (default 10.0)
+    - :max_range_km — max ray distance (default 50)
+    - :ant_height_m — antenna height at both ends (default 2.4)
+    - :angular_step — degrees between rays (default 2)
+    - :tiles_dir — SRTM tiles directory (default from config)
+  """
+  def compute(lat, lon, opts \\ []) do
+    freq_ghz = Keyword.get(opts, :freq_ghz, 10.0)
+    max_range_km = Keyword.get(opts, :max_range_km, @default_max_range_km)
+    ant_height_m = Keyword.get(opts, :ant_height_m, 2.4)
+    angular_step = Keyword.get(opts, :angular_step, @default_angular_step)
+    tiles_dir = Keyword.get(opts, :tiles_dir, srtm_tiles_dir())
+
+    bearings = Enum.to_list(0..359//angular_step)
+
+    boundary =
+      bearings
+      |> Task.async_stream(
+        fn bearing ->
+          compute_ray(lat, lon, bearing, freq_ghz, max_range_km, ant_height_m, tiles_dir)
+        end,
+        max_concurrency: System.schedulers_online(),
+        timeout: 30_000,
+        on_timeout: :kill_task
+      )
+      |> Enum.map(fn
+        {:ok, result} -> result
+        {:exit, _} -> nil
+      end)
+      |> Enum.reject(&is_nil/1)
+
+    %{origin: %{lat: lat, lon: lon}, boundary: boundary}
+  end
+
+  @doc """
+  Analyse a single ray's profile. Public for testing.
+  Returns %{reach_km: float, verdict: string}.
+  """
+  def analyse_ray(profile, dist_km, freq_ghz, ant_ht_a_m, ant_ht_b_m) do
+    analysis = TerrainAnalysis.analyse(profile, dist_km, freq_ghz, ant_ht_a_m, ant_ht_b_m)
+    reach_km = find_reach_km(analysis.points, dist_km)
+    %{reach_km: reach_km, verdict: analysis.verdict}
+  end
+
+  defp compute_ray(origin_lat, origin_lon, bearing, freq_ghz, max_range_km, ant_height_m, tiles_dir) do
+    {end_lat, end_lon} = destination_point(origin_lat, origin_lon, bearing, max_range_km)
+
+    case Srtm.fetch_elevation_profile(origin_lat, origin_lon, end_lat, end_lon, tiles_dir) do
+      {:ok, profile} ->
+        result = analyse_ray(profile, max_range_km, freq_ghz, ant_height_m, ant_height_m)
+        {reach_lat, reach_lon} = destination_point(origin_lat, origin_lon, bearing, result.reach_km)
+
+        %{
+          bearing: bearing,
+          reach_km: result.reach_km,
+          lat: reach_lat,
+          lon: reach_lon
+        }
+
+      {:error, _} ->
+        %{bearing: bearing, reach_km: max_range_km, lat: end_lat, lon: end_lon}
+    end
+  end
+
+  defp srtm_tiles_dir do
+    Application.get_env(:microwaveprop, :srtm_tiles_dir, Path.expand("~/srtm/tiles"))
+  end
+
+  defp find_first_obstructed_index(interior) do
+    Enum.find_index(interior, & &1.obstructed)
+  end
+
+  defp deg_to_rad(deg), do: deg * :math.pi() / 180
+  defp rad_to_deg(rad), do: rad * 180 / :math.pi()
+end
diff --git a/lib/microwaveprop_web/live/map_live.ex b/lib/microwaveprop_web/live/map_live.ex
index 5ae72626..e0496cb4 100644
--- a/lib/microwaveprop_web/live/map_live.ex
+++ b/lib/microwaveprop_web/live/map_live.ex
@@ -4,6 +4,7 @@ defmodule MicrowavepropWeb.MapLive do
 
   alias Microwaveprop.Propagation
   alias Microwaveprop.Propagation.BandConfig
+  alias Microwaveprop.Terrain.Viewshed
 
   @default_band 10_000
   @initial_bounds %{
@@ -31,7 +32,8 @@ defmodule MicrowavepropWeb.MapLive do
        initial_scores_json: Jason.encode!(initial_scores),
        valid_time: valid_time,
        bounds: @initial_bounds,
-       grid_visible: false
+       grid_visible: false,
+       antenna_height_ft: 8
      )}
   end
 
@@ -49,6 +51,16 @@ defmodule MicrowavepropWeb.MapLive do
     {:noreply, socket}
   end
 
+  def handle_event("set_antenna_height", %{"height_ft" => value}, socket) do
+    height =
+      case Integer.parse(value) do
+        {h, _} when h >= 0 and h <= 200 -> h
+        _ -> 8
+      end
+
+    {:noreply, assign(socket, :antenna_height_ft, height)}
+  end
+
   def handle_event("toggle_grid", _params, socket) do
     visible = !socket.assigns.grid_visible
 
@@ -76,6 +88,24 @@ defmodule MicrowavepropWeb.MapLive do
     {:noreply, socket}
   end
 
+  def handle_event("compute_viewshed", %{"lat" => lat, "lon" => lon}, socket) do
+    band_config = BandConfig.get(socket.assigns.selected_band)
+    freq_ghz = socket.assigns.selected_band / 1_000
+    ant_height_m = socket.assigns.antenna_height_ft * 0.3048
+    max_range_km = min(band_config.typical_range_km, 100)
+
+    socket =
+      start_async(socket, :viewshed, fn ->
+        Viewshed.compute(lat, lon,
+          freq_ghz: freq_ghz,
+          ant_height_m: ant_height_m,
+          max_range_km: max_range_km
+        )
+      end)
+
+    {:noreply, socket}
+  end
+
   @impl true
   def handle_info({:propagation_updated, valid_time}, socket) do
     scores = Propagation.latest_scores(socket.assigns.selected_band, socket.assigns.bounds)
@@ -88,6 +118,23 @@ defmodule MicrowavepropWeb.MapLive do
     {:noreply, socket}
   end
 
+  @impl true
+  def handle_async(:viewshed, {:ok, result}, socket) do
+    points = Enum.map(result.boundary, fn p -> %{lat: p.lat, lon: p.lon} end)
+
+    socket =
+      push_event(socket, "viewshed_result", %{
+        origin: result.origin,
+        points: points
+      })
+
+    {:noreply, socket}
+  end
+
+  def handle_async(:viewshed, {:exit, _reason}, socket) do
+    {:noreply, socket}
+  end
+
   defp band_info(band_mhz) do
     config = BandConfig.get(band_mhz)
 
@@ -143,10 +190,7 @@ defmodule MicrowavepropWeb.MapLive do
           <%!-- Band selector --%>
           <div class="dropdown">
             <div tabindex="0" role="button" class="btn btn-sm w-full justify-between">
-              <span class="flex items-center gap-1.5">
-                <.icon name="hero-signal" class="size-4" />
-                {selected_label(@bands, @selected_band)}
-              </span>
+              {selected_label(@bands, @selected_band)}
               <.icon name="hero-chevron-down" class="size-3" />
             </div>
             <ul
@@ -179,6 +223,21 @@ defmodule MicrowavepropWeb.MapLive do
             <span class="text-sm">Grid squares</span>
           </label>
 
+          <%!-- Antenna height --%>
+          <form phx-change="set_antenna_height" class="flex items-center gap-2 px-1">
+            <label class="text-sm whitespace-nowrap">Antenna</label>
+            <input
+              type="number"
+              name="height_ft"
+              min="0"
+              max="200"
+              step="1"
+              value={@antenna_height_ft}
+              class="input input-xs w-16 text-right"
+            />
+            <span class="text-xs opacity-70">ft</span>
+          </form>
+
           <%!-- Latest data --%>
           <div :if={@valid_time} class="text-xs opacity-70 px-1">
             Data: {Calendar.strftime(@valid_time, "%b %d, %H:%M UTC")} ({time_ago(@valid_time)})
diff --git a/test/microwaveprop/terrain/viewshed_test.exs b/test/microwaveprop/terrain/viewshed_test.exs
new file mode 100644
index 00000000..8c495efc
--- /dev/null
+++ b/test/microwaveprop/terrain/viewshed_test.exs
@@ -0,0 +1,110 @@
+defmodule Microwaveprop.Terrain.ViewshedTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Terrain.Viewshed
+
+  describe "destination_point/4" do
+    test "north bearing increases latitude, holds longitude" do
+      {lat, lon} = Viewshed.destination_point(32.0, -97.0, 0, 100.0)
+      assert_in_delta lat, 32.899, 0.01
+      assert_in_delta lon, -97.0, 0.01
+    end
+
+    test "east bearing increases longitude, holds latitude" do
+      {lat, lon} = Viewshed.destination_point(32.0, -97.0, 90, 100.0)
+      assert_in_delta lat, 32.0, 0.01
+      assert lon > -97.0
+    end
+
+    test "south bearing decreases latitude" do
+      {lat, lon} = Viewshed.destination_point(32.0, -97.0, 180, 50.0)
+      assert lat < 32.0
+      assert_in_delta lon, -97.0, 0.01
+    end
+
+    test "zero distance returns origin" do
+      {lat, lon} = Viewshed.destination_point(32.0, -97.0, 45, 0.0)
+      assert_in_delta lat, 32.0, 0.001
+      assert_in_delta lon, -97.0, 0.001
+    end
+  end
+
+  describe "find_reach_km/2" do
+    test "returns max range when no points are obstructed" do
+      points = [
+        %{obstructed: false, dist_km: 0.0},
+        %{obstructed: false, dist_km: 10.0},
+        %{obstructed: false, dist_km: 20.0},
+        %{obstructed: false, dist_km: 30.0},
+        %{obstructed: false, dist_km: 50.0}
+      ]
+
+      assert Viewshed.find_reach_km(points, 50.0) == 50.0
+    end
+
+    test "returns distance of point before first obstruction" do
+      points = [
+        %{obstructed: false, dist_km: 0.0},
+        %{obstructed: false, dist_km: 10.0},
+        %{obstructed: false, dist_km: 20.0},
+        %{obstructed: true, dist_km: 30.0},
+        %{obstructed: false, dist_km: 40.0},
+        %{obstructed: false, dist_km: 50.0}
+      ]
+
+      assert Viewshed.find_reach_km(points, 50.0) == 20.0
+    end
+
+    test "returns first interior point distance when obstruction is at second point" do
+      points = [
+        %{obstructed: false, dist_km: 0.0},
+        %{obstructed: true, dist_km: 5.0},
+        %{obstructed: false, dist_km: 10.0}
+      ]
+
+      # First point is endpoint (excluded), second is first interior and is obstructed
+      # No clear interior point before it, return minimum
+      assert Viewshed.find_reach_km(points, 10.0) == 0.0
+    end
+
+    test "ignores endpoint obstruction flags" do
+      # Endpoints (first/last) are never counted as obstructed by TerrainAnalysis
+      # but just to be safe, find_reach_km skips them
+      points = [
+        %{obstructed: true, dist_km: 0.0},
+        %{obstructed: false, dist_km: 25.0},
+        %{obstructed: true, dist_km: 50.0}
+      ]
+
+      assert Viewshed.find_reach_km(points, 50.0) == 50.0
+    end
+  end
+
+  describe "analyse_ray/5" do
+    test "returns full range for flat terrain with antenna heights" do
+      profile =
+        for i <- 0..10 do
+          f = i / 10
+          %{lat: 32.9 + f * 0.09, lon: -97.0, d: f, elev: 200.0, dist_km: f * 10.0}
+        end
+
+      result = Viewshed.analyse_ray(profile, 10.0, 10.0, 2.4, 2.4)
+      assert result.reach_km == 10.0
+    end
+
+    test "detects obstruction and returns reduced reach" do
+      # Use 10km total so earth bulge is negligible (~0.7m) and
+      # the 500m peak at index 5 is the only obstruction.
+      profile =
+        for i <- 0..10 do
+          f = i / 10
+          elev = if i == 5, do: 500.0, else: 200.0
+          %{lat: 32.9 + f * 0.009, lon: -97.0, d: f, elev: elev, dist_km: f * 10.0}
+        end
+
+      result = Viewshed.analyse_ray(profile, 10.0, 10.0, 2.4, 2.4)
+      assert result.reach_km < 10.0
+      assert result.reach_km > 0.0
+    end
+  end
+end
diff --git a/test/microwaveprop_web/live/map_live_test.exs b/test/microwaveprop_web/live/map_live_test.exs
index 6280dca5..9e17c964 100644
--- a/test/microwaveprop_web/live/map_live_test.exs
+++ b/test/microwaveprop_web/live/map_live_test.exs
@@ -58,6 +58,33 @@ defmodule MicrowavepropWeb.MapLiveTest do
     end
   end
 
+  describe "antenna height" do
+    test "renders antenna height input with default 8 ft", %{conn: conn} do
+      {:ok, _lv, html} = live(conn, ~p"/map")
+      assert html =~ ~s(name="height_ft")
+      assert html =~ ~s(value="8")
+      assert html =~ "ft"
+    end
+
+    test "updates antenna height on change", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/map")
+
+      html = render_change(lv, "set_antenna_height", %{"height_ft" => "20"})
+      assert html =~ ~s(value="20")
+    end
+  end
+
+  describe "compute_viewshed" do
+    test "accepts compute_viewshed event without crashing", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/map")
+      # Event should be accepted; async computation may fail (no SRTM in test)
+      # but the LiveView should not crash
+      render_click(lv, "compute_viewshed", %{"lat" => 32.9, "lon" => -97.0})
+      # If we get here without crash, the event handler exists
+      assert render(lv) =~ "propagation-map"
+    end
+  end
+
   describe "toggle_grid" do
     test "toggles grid visibility", %{conn: conn} do
       {:ok, lv, html} = live(conn, ~p"/map")