With this patch, you can fly under bridges, then turn around and land on

them lengthwise.

src/Scenery/hitlist.cxx

@@ -511,7 +511,8 @@ static void ssgGetCurrentBSphere( ssgEntity *entity, sgVec3 center, float *radiu
 // ======================
 // Determine scenery altitude via ssg.
 // returned results are in meters
-bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
+bool fgCurrentElev( sgdVec3 abs_view_pos, double max_alt_m,
+                    sgdVec3 scenery_center,
                     FGHitList *hit_list,
                     double *terrain_elev, double *radius, double *normal)
 {
@@ -539,7 +540,8 @@ bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
                      &alt, &sea_level_r);
         // cout << "hit " << i << " lon = " << geoc.lon() << " lat = "
         //      << lat_geod << " alt = " << alt << endl;
-        if ( alt > result && alt < 10000 ) {
+        if ( alt > result && alt < max_alt_m ) {
+            // cout << "  it's a keeper" << endl;
             result = alt;
             this_hit = i;
         }
@@ -571,7 +573,8 @@ bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
 // ======================
 // Determine scenery altitude via ssg.
 // returned results are in meters
-bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
+bool fgCurrentElev( sgdVec3 abs_view_pos, double max_alt_m,
+                    sgdVec3 scenery_center,
                     ssgTransform *terra_transform,
                     FGHitList *hit_list,
                     double *terrain_elev, double *radius, double *normal)
@@ -604,7 +607,7 @@ bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
         double lat_geod, alt, sea_level_r;
         sgGeocToGeod(geoc.lat(), geoc.radius(), &lat_geod,
                      &alt, &sea_level_r);
-        if ( alt > result && alt < 20000 ) {
+        if ( alt > result && alt < max_alt_m ) {
             result = alt;
             this_hit = i;
         }
@@ -624,7 +627,7 @@ bool fgCurrentElev( sgdVec3 abs_view_pos, sgdVec3 scenery_center,
         return true;
     } else {
         SG_LOG( SG_TERRAIN, SG_DEBUG, "DOING FULL TERRAIN INTERSECTION" );
-        return fgCurrentElev( abs_view_pos, scenery_center, hit_list,
+        return fgCurrentElev( abs_view_pos, max_alt_m, scenery_center, hit_list,
                               terrain_elev,radius,normal);
     }
 }

src/Scenery/hitlist.hxx

@@ -85,6 +85,7 @@ public:
 // center, find the current terrain intersection elevation for the
 // point specified.
 bool fgCurrentElev( sgdVec3 abs_view_pos,
+                    double max_alt_m,
                     sgdVec3 scenery_center,
                     ssgTransform *terra_transform,
                     FGHitList *hit_list,
@@ -93,6 +94,7 @@ bool fgCurrentElev( sgdVec3 abs_view_pos,
                     double *normal );
 
 bool fgCurrentElev( sgdVec3 abs_view_pos,
+                    double max_alt_m,
                     sgdVec3 scenery_center,
                     FGHitList *hit_list,
                     double *terrain_elev,

src/Scenery/tilemgr.cxx

@@ -367,6 +367,15 @@ int FGTileMgr::update( SGLocation *location, double visibility_meters,
 {
     longitude = location->getLongitude_deg();
     latitude = location->getLatitude_deg();
+    // add 2.0m to the max altitude to give a little leeway to the
+    // ground reaction code.
+    altitude_m = location->getAltitudeASL_ft() * SG_FEET_TO_METER + 2.0;
+
+    // if current altitude is apparently not initialized, set max
+    // altitude to something big.
+    if ( altitude_m < -1000 ) {
+        altitude_m = 10000;
+    }
     // SG_LOG( SG_TERRAIN, SG_DEBUG, "FGTileMgr::update() for "
     //         << longitude << " " << latatitude );
 
@@ -411,7 +420,7 @@ int FGTileMgr::update( SGLocation *location, double visibility_meters,
     // no reason to update this if we haven't moved...
     if ( longitude != last_longitude || latitude != last_latitude ) {
         // update current elevation... 
-        if ( updateCurrentElevAtPos( abs_pos_vector,
+        if ( updateCurrentElevAtPos( abs_pos_vector, altitude_m,
                                      location->get_tile_center() ) )
         {
             last_longitude = longitude;
@@ -432,8 +441,10 @@ void FGTileMgr::refresh_view_timestamps() {
 }
 
 
-int FGTileMgr::updateCurrentElevAtPos(sgdVec3 abs_pos_vector, Point3D center) {
-
+int FGTileMgr::updateCurrentElevAtPos( sgdVec3 abs_pos_vector,
+                                       double max_alt_m,
+                                       Point3D center)
+{
     sgdVec3 sc;
 
     sgdSetVec3( sc, center[0], center[1], center[2]);
@@ -449,6 +460,7 @@ int FGTileMgr::updateCurrentElevAtPos(sgdVec3 abs_pos_vector, Point3D center) {
         // scenery center has been properly defined so any hit should
         // be valid (and not just luck)
         hit = fgCurrentElev(abs_pos_vector,
+                            max_alt_m,
                             sc,
 		// uncomment next paramater to fly under
 		// bridges and a slightly faster algorithm

src/Scenery/tilemgr.hxx

@@ -100,6 +100,7 @@ private:
     // current longitude latitude
     double longitude;
     double latitude;
+    double altitude_m;
     double last_longitude;
     double last_latitude;
 
@@ -166,7 +167,8 @@ public:
     int update( SGLocation *location, double visibility_meters,
                 sgdVec3 abs_pos_vector );
 
-    int updateCurrentElevAtPos(sgdVec3 abs_pos_vector, Point3D center );
+    int updateCurrentElevAtPos( sgdVec3 abs_pos_vector, double altitude_m,
+                                Point3D center );
 
     // Determine scenery altitude.  Normally this just happens when we
     // render the scene, but we'd also like to be able to do this