diff --git a/lidar_pipeline/index.py b/lidar_pipeline/index.py
index 579195a..5e54c0c 100644
--- a/lidar_pipeline/index.py
+++ b/lidar_pipeline/index.py
@@ -2474,7 +2474,7 @@ async function syncCustomPresets() {
 const BUILTIN_PRESETS = [
   { label: 'Défaut', on: ['positive_openness', 'aspect', 'slope'],
     opacity: { slope: 0, aspect: 0.5, positive_openness: 1 },
-    osm: { on: false } },
+    osm: { on: true, opacity: 0.3 } },
 ];
 
 // Niveaux de transparence d'un preset, lisibles dans la liste
diff --git a/lidar_pipeline/tests/test_index.py b/lidar_pipeline/tests/test_index.py
index 2305504..49cd1fc 100644
--- a/lidar_pipeline/tests/test_index.py
+++ b/lidar_pipeline/tests/test_index.py
@@ -287,7 +287,7 @@ def test_build_index_generates_html(tmp_path):
     assert 'id="presetSave"' in content
     assert "BUILTIN_PRESETS" in app_js and "applyPreset" in app_js
     assert "presetDetail" in app_js and "lidarPresets_v1" in app_js
-    assert "osm: { on: false }" in app_js      # presets portent le fond OSM
+    assert "osm: { on: true, opacity: 0.3 }" in app_js  # preset porte le fond OSM
     assert "p.osm" in app_js                   # l'application y inclut le fond
     # Partage de la vue : bouton « Partager » + lien contenant vue, couches
     # et transparences (hash #zoom,lat,lng&L=...&O=...) — le destinataire