AC_WPNav: add get_terrain_source and rename existing method

this allows RTL to determine which source of terrain data will be used which is required when building the return path

libraries/AC_WPNav/AC_WPNav.cpp

@@ -95,6 +95,23 @@ AC_WPNav::AC_WPNav(const AP_InertialNav& inav, const AP_AHRS_View& ahrs, AC_PosC
     _wp_radius_cm = MAX(_wp_radius_cm, WPNAV_WP_RADIUS_MIN);
 }
 
+// get expected source of terrain data if alt-above-terrain command is executed (used by Copter's ModeRTL)
+AC_WPNav::TerrainSource AC_WPNav::get_terrain_source() const
+{
+    // use range finder if connected
+    if (_rangefinder_available && _rangefinder_use) {
+        return AC_WPNav::TerrainSource::TERRAIN_FROM_RANGEFINDER;
+    }
+#if AP_TERRAIN_AVAILABLE
+    if ((_terrain != nullptr) && _terrain->enabled()) {
+        return AC_WPNav::TerrainSource::TERRAIN_FROM_TERRAINDATABASE;
+    } else {
+        return AC_WPNav::TerrainSource::TERRAIN_UNAVAILABLE;
+    }
+#else
+    return AC_WPNav::TerrainSource::TERRAIN_UNAVAILABLE;
+#endif
+}
 
 ///
 /// waypoint navigation
@@ -941,17 +958,18 @@ void AC_WPNav::calc_spline_pos_vel(float spline_time, Vector3f& position, Vector
 // get terrain's altitude (in cm above the ekf origin) at the current position (+ve means terrain below vehicle is above ekf origin's altitude)
 bool AC_WPNav::get_terrain_offset(float& offset_cm)
 {
-    // use range finder if connected
-    if (_rangefinder_available && _rangefinder_use) {
+    // calculate offset based on source (rangefinder or terrain database)
+    switch (get_terrain_source()) {
+    case AC_WPNav::TerrainSource::TERRAIN_UNAVAILABLE:
+        return false;
+    case AC_WPNav::TerrainSource::TERRAIN_FROM_RANGEFINDER:
         if (_rangefinder_healthy) {
             offset_cm = _inav.get_altitude() - _rangefinder_alt_cm;
             return true;
         }
         return false;
-    }
-
+    case AC_WPNav::TerrainSource::TERRAIN_FROM_TERRAINDATABASE:
 #if AP_TERRAIN_AVAILABLE
-    // use terrain database
         float terr_alt = 0.0f;
         if (_terrain != nullptr && _terrain->height_above_terrain(terr_alt, true)) {
             offset_cm = _inav.get_altitude() - (terr_alt * 100.0f);
@@ -959,6 +977,10 @@ bool AC_WPNav::get_terrain_offset(float& offset_cm)
         }
 #endif
         return false;
+    }
+
+    // we should never get here but just in case
+    return false;
 }
 
 // convert location to vector from ekf origin.  terrain_alt is set to true if resulting vector's z-axis should be treated as alt-above-terrain

libraries/AC_WPNav/AC_WPNav.h

@@ -55,7 +55,15 @@ public:
     void set_rangefinder_alt(bool use, bool healthy, float alt_cm) { _rangefinder_available = use; _rangefinder_healthy = healthy; _rangefinder_alt_cm = alt_cm; }
 
     // return true if range finder may be used for terrain following
-    bool rangefinder_used() const { return _rangefinder_use && _rangefinder_healthy; }
+    bool rangefinder_used_and_healthy() const { return _rangefinder_use && _rangefinder_healthy; }
+
+    // get expected source of terrain data if alt-above-terrain command is executed (used by Copter's ModeRTL)
+    enum class TerrainSource {
+        TERRAIN_UNAVAILABLE,
+        TERRAIN_FROM_RANGEFINDER,
+        TERRAIN_FROM_TERRAINDATABASE,
+    };
+    AC_WPNav::TerrainSource get_terrain_source() const;
 
     ///
     /// waypoint controller