feat(rover): snap top candidates to SRTM hilltops + add hillshade overlay

assets/js/rover_map_hook.ts

@@ -18,6 +18,7 @@ interface RoverMapHook extends ViewHook {
   map: L.Map
   osmLayer: L.TileLayer
   topoLayer: L.TileLayer | null
+  hillshadeLayer: L.TileLayer
   layersControl: L.Control.Layers
   stations: Station[]
   stationMarkers: L.LayerGroup
@@ -108,12 +109,17 @@ export const RoverMap: Partial<RoverMapHook> = {
     this.topoLayer = L.tileLayer("https://{s}.tile.opentopomap.org/{z}/{x}/{y}.png", {
       maxZoom: 17
     })
+    this.hillshadeLayer = L.tileLayer(
+      "https://services.arcgisonline.com/ArcGIS/rest/services/Elevation/World_Hillshade/MapServer/tile/{z}/{y}/{x}",
+      { maxZoom: 16, opacity: 0.45 }
+    )
 
     this.osmLayer.addTo(map)
+    this.hillshadeLayer.addTo(map)
 
     this.layersControl = L.control.layers(
       { "OSM": this.osmLayer, "Topo": this.topoLayer },
-      undefined,
+      { "Hillshade": this.hillshadeLayer },
       { position: "topright" }
     ).addTo(map)
 

lib/microwaveprop/rover/compute.ex

@@ -12,6 +12,7 @@ defmodule Microwaveprop.Rover.Compute do
   alias Microwaveprop.Rover.Aggregator
   alias Microwaveprop.Rover.DriveTime
   alias Microwaveprop.Rover.Elevation
+  alias Microwaveprop.Rover.Hilltop
   alias Microwaveprop.Rover.LinkMargin
   alias Microwaveprop.Rover.PathTerrain
   alias Microwaveprop.Rover.Prominence
@@ -73,6 +74,7 @@ defmodule Microwaveprop.Rover.Compute do
     clearance_lookup = Keyword.get(deps, :clearance_lookup, &PathTerrain.clearance_map/2)
     prominence_lookup = Keyword.get(deps, :prominence_lookup, &Prominence.prominence_map/1)
     road_lookup = Keyword.get(deps, :road_lookup, &RoadProximity.road_distances/2)
+    hilltop_snap = Keyword.get(deps, :hilltop_snap, &Hilltop.snap/1)
 
     %{
       home: home,
@@ -131,7 +133,7 @@ defmodule Microwaveprop.Rover.Compute do
       cells
       |> Enum.sort_by(& &1.score, :desc)
       |> Enum.take(@top_n)
-      |> Enum.map(&candidate_payload(&1, home))
+      |> Enum.map(&candidate_payload(&1, home, hilltop_snap))
 
     warnings = build_warnings(raw_cells, in_radius, elev_map, cells)
 
@@ -242,24 +244,31 @@ defmodule Microwaveprop.Rover.Compute do
     db |> max(-@prominence_cap_db) |> min(@prominence_cap_db)
   end
 
-  defp candidate_payload(cell, home) do
-    drive_min = DriveTime.drive_min(cell.distance_km)
-    bearing = DriveTime.bearing_compass({home.lat, home.lon}, {cell.lat, cell.lon})
-    grid = Maidenhead.from_latlon(cell.lat, cell.lon, 10)
+  defp candidate_payload(cell, home, hilltop_snap) do
+    {lat, lon, elev_m} =
+      case hilltop_snap.({cell.lat, cell.lon}) do
+        {hl_lat, hl_lon, hl_elev} -> {hl_lat, hl_lon, hl_elev}
+        _ -> {cell.lat, cell.lon, cell.elev_m}
+      end
+
+    distance_km = DriveTime.haversine_km({home.lat, home.lon}, {lat, lon})
+    drive_min = DriveTime.drive_min(distance_km)
+    bearing = DriveTime.bearing_compass({home.lat, home.lon}, {lat, lon})
+    grid = Maidenhead.from_latlon(lat, lon, 10)
 
     %{
       grid: grid,
-      lat: cell.lat,
-      lon: cell.lon,
-      elev_m: cell.elev_m,
+      lat: lat,
+      lon: lon,
+      elev_m: elev_m,
       prominence_m: cell.prominence_m,
       road_km: cell.road_km,
       drive_min: drive_min,
       score: cell.score,
       tier_color: cell.tier_color,
-      distance_km: cell.distance_km,
+      distance_km: distance_km,
       bearing_compass: bearing,
-      name: "#{grid} — #{round(cell.distance_km)} km #{bearing} of home"
+      name: "#{grid} — #{round(distance_km)} km #{bearing} of home"
     }
   end
 

lib/microwaveprop/rover/hilltop.ex

@@ -0,0 +1,55 @@
+defmodule Microwaveprop.Rover.Hilltop do
+  @moduledoc """
+  Snaps a propagation-grid cell center to the highest SRTM point inside
+  its footprint. The propagation grid is on a 0.125° lattice (~14 km),
+  but a rover wants to park on the actual hilltop inside that cell, not
+  at the nominal cell center. Sample SRTM at sub-cell resolution and
+  return the highest point.
+  """
+
+  alias Microwaveprop.Rover.Elevation
+
+  @default_half_deg 0.0625
+  @default_step_deg 0.01
+
+  @type latlon :: {float(), float()}
+
+  @doc """
+  Returns `{best_lat, best_lon, best_elev}` for the highest SRTM point
+  inside `±half_deg` of `cell_center`, or `nil` if SRTM coverage is
+  missing across the whole footprint.
+  """
+  @spec snap(latlon(), keyword()) :: {float(), float(), integer()} | nil
+  def snap({lat, lon}, opts \\ []) do
+    half = Keyword.get(opts, :half_deg, @default_half_deg)
+    step = Keyword.get(opts, :step_deg, @default_step_deg)
+    elev_lookup = Keyword.get(opts, :elev_lookup, &Elevation.lookup_many/1)
+
+    points = sample_grid(lat, lon, half, step)
+    elev_map = elev_lookup.(points)
+
+    points
+    |> Enum.flat_map(fn pt ->
+      case Map.get(elev_map, pt) do
+        nil -> []
+        elev -> [{pt, elev}]
+      end
+    end)
+    |> case do
+      [] ->
+        nil
+
+      list ->
+        {{best_lat, best_lon}, best_elev} = Enum.max_by(list, fn {_, e} -> e end)
+        {best_lat, best_lon, best_elev}
+    end
+  end
+
+  defp sample_grid(lat, lon, half, step) do
+    n = max(trunc(half / step), 1)
+
+    for i <- -n..n, j <- -n..n do
+      {Float.round(lat + i * step, 4), Float.round(lon + j * step, 4)}
+    end
+  end
+end

test/microwaveprop/rover/hilltop_test.exs

@@ -0,0 +1,41 @@
+defmodule Microwaveprop.Rover.HilltopTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Rover.Hilltop
+
+  test "snap/2 returns the highest sampled SRTM point inside the cell" do
+    cell_center = {33.0, -96.0}
+
+    # Stub elev lookup: highest elevation NE of cell center.
+    elev_lookup = fn points ->
+      Map.new(points, fn {lat, lon} ->
+        elev = round((lat - 33.0) * 1000 + (lon + 96.0) * 1000)
+        {{lat, lon}, elev}
+      end)
+    end
+
+    {lat, lon, elev} = Hilltop.snap(cell_center, elev_lookup: elev_lookup)
+
+    # Best point should be the corner with the largest +lat, +lon offsets
+    # (NE corner of the sample grid).
+    assert lat > 33.0
+    assert lon > -96.0
+    assert is_integer(elev)
+    assert elev > 0
+  end
+
+  test "snap/2 returns nil when SRTM lookup yields no values" do
+    elev_lookup = fn points -> Map.new(points, fn p -> {p, nil} end) end
+
+    assert Hilltop.snap({33.0, -96.0}, elev_lookup: elev_lookup) == nil
+  end
+
+  test "snap/2 stays within the requested half-degree radius" do
+    elev_lookup = fn points -> Map.new(points, fn p -> {p, 100} end) end
+
+    {lat, lon, _} = Hilltop.snap({33.0, -96.0}, elev_lookup: elev_lookup, half_deg: 0.0625)
+
+    assert_in_delta lat, 33.0, 0.0625 + 1.0e-6
+    assert_in_delta lon, -96.0, 0.0625 + 1.0e-6
+  end
+end