diff --git a/examples/osgintersection/osgintersection.cpp b/examples/osgintersection/osgintersection.cpp
index db226fdbc..6fc5fcdc2 100644
--- a/examples/osgintersection/osgintersection.cpp
+++ b/examples/osgintersection/osgintersection.cpp
@@ -1,6 +1,7 @@
 #include <osg/ArgumentParser>
 #include <osg/ApplicationUsage>
 #include <osg/Timer>
+#include <osg/CoordinateSystemNode>
 #include <osg/Notify>
 #include <osg/io_utils>
 
@@ -9,6 +10,7 @@
 #include <osgUtil/IntersectionVisitor>
 
 #include <osgSim/LineOfSight>
+#include <osgSim/HeightAboveTerrain>
 
 #include <iostream>
 
@@ -31,9 +33,9 @@ int main(int argc, char **argv)
     arguments.getApplicationUsage()->setCommandLineUsage(arguments.getApplicationName());
     arguments.getApplicationUsage()->addCommandLineOption("-h or --help","Display this information");
     
-    osg::ref_ptr<osg::Node> root = osgDB::readNodeFiles(arguments);
+    osg::ref_ptr<osg::Node> scene = osgDB::readNodeFiles(arguments);
     
-    if (!root) 
+    if (!scene) 
     {
         std::cout<<"No model loaded, please specify a valid model on the command line."<<std::endl;
         return 0;
@@ -42,8 +44,10 @@ int main(int argc, char **argv)
     std::cout<<"Intersection "<<std::endl;
 
     
+    osg::CoordinateSystemNode* csn = dynamic_cast<osg::CoordinateSystemNode*>(scene.get());
+    osg::EllipsoidModel* em = csn ? csn->getEllipsoidModel() : 0;
     
-    osg::BoundingSphere bs = root->getBound();
+    osg::BoundingSphere bs = scene->getBound();
 
 
     bool useIntersectorGroup = true;
@@ -57,10 +61,13 @@ int main(int argc, char **argv)
         osg::Vec3d end = bs.center();// - osg::Vec3d(0.0, bs.radius(),0.0);
         osg::Vec3d deltaRow( 0.0, 0.0, bs.radius()*0.01);
         osg::Vec3d deltaColumn( bs.radius()*0.01, 0.0, 0.0);
-        unsigned int numRows = 50;
-        unsigned int numColumns = 50;
+        unsigned int numRows = 20;
+        unsigned int numColumns = 20;
 
         osgSim::LineOfSight los;
+        
+        osgSim::HeightAboveTerrain hat;
+        hat.setDatabaseCacheReadCallback(los.getDatabaseCacheReadCallback());
 
         for(unsigned int r=0; r<numRows; ++r)
         {
@@ -69,17 +76,20 @@ int main(int argc, char **argv)
                 osg::Vec3d s = start + deltaColumn * double(c) + deltaRow * double(r);
                 osg::Vec3d e = end + deltaColumn * double(c) + deltaRow * double(r);
                 los.addLOS(s,e);
+                hat.addPoint(s);
             }
         }
 
 
-        los.computeIntersections(root.get());
+        std::cout<<"Computing LineOfSight"<<std::endl;
+
+        los.computeIntersections(scene.get());
         
         osg::Timer_t endTick = osg::Timer::instance()->tick();
 
         std::cout<<"Completed in "<<osg::Timer::instance()->delta_s(startTick,endTick)<<std::endl;
 
-#if 0
+#if 1
         for(unsigned int i=0; i<los.getNumLOS(); i++)
         {
             const osgSim::LineOfSight::Intersections& intersections = los.getIntersections(i);
@@ -95,8 +105,15 @@ int main(int argc, char **argv)
         // now do a second traversal to test performance of cache.
         startTick = osg::Timer::instance()->tick();
 
-        los.computeIntersections(root.get());
+        std::cout<<"Computing HeightAboveTerrain"<<std::endl;
+
+        hat.computeIntersections(scene.get());
         
+        for(unsigned int i=0; i<hat.getNumPoints(); i++)
+        {
+             std::cout<<"  point = "<<hat.getPoint(i)<<" hat = "<<hat.getHeightAboveTerrain(i)<<std::endl;
+        }
+
         endTick = osg::Timer::instance()->tick();
 
         std::cout<<"Completed in "<<osg::Timer::instance()->delta_s(startTick,endTick)<<std::endl;
@@ -128,7 +145,7 @@ int main(int argc, char **argv)
 
         
         osgUtil::IntersectionVisitor intersectVisitor( intersectorGroup.get(), new MyReadCallback );
-        root->accept(intersectVisitor);
+        scene->accept(intersectVisitor);
 
         osg::Timer_t endTick = osg::Timer::instance()->tick();
 
@@ -181,7 +198,7 @@ int main(int argc, char **argv)
 
         osgUtil::IntersectionVisitor intersectVisitor( intersector.get(), new MyReadCallback );
 
-        root->accept(intersectVisitor);
+        scene->accept(intersectVisitor);
 
         osg::Timer_t endTick = osg::Timer::instance()->tick();
 
diff --git a/include/osgSim/HeightAboveTerrain b/include/osgSim/HeightAboveTerrain
index 13365f9f8..39b77b155 100644
--- a/include/osgSim/HeightAboveTerrain
+++ b/include/osgSim/HeightAboveTerrain
@@ -16,7 +16,8 @@
 
 #include <osgUtil/IntersectionVisitor>
 
-#include <osgSim/Export>
+// include so we can get access to the DatabaseCacheReadCallback
+#include <osgSim/LineOfSight>
 
 namespace osgSim {
 
@@ -25,7 +26,85 @@ class OSGSIM_EXPORT HeightAboveTerrain
 {
     public :
 
+    
+        HeightAboveTerrain();
+        
+    
+        /** Clear the internal HAT List so it contains no height above terrain tests.*/
+        void clear();
+        
+        /** Add a height above terrain test point in the CoordinateFrame.*/
+        unsigned int addPoint(const osg::Vec3d& point);
+
+        /** Get the number of height above terrain tests.*/
+        unsigned int getNumPoints() const { return _HATList.size(); }
+        
+        /** Set the source point of single height above terrain test.*/
+        void setPoint(unsigned int i, const osg::Vec3d& point) { _HATList[i]._point = point; }
+
+        /** Get the source point of single height above terrain test.*/
+        const osg::Vec3d& getPoint(unsigned int i) const { return _HATList[i]._point; }
+
+        /** Get the intersection height for a single height above terrain test.
+          * Note, you must call computeIntersections(..) before you can querry the HeightAboveTerrain. 
+          * If no intersections are found then height returned will be the height above mean sea level. */
+        double getHeightAboveTerrain(unsigned int i) const  { return _HATList[i]._hat; }
+
+        /** Set the lowest height that the should be tested for.
+          * Defaults to -1000, i.e. 1000m below mean sea level. */
+        void setLowestHeight(double lowestHeight) { _lowestHeight = lowestHeight; }
+
+        /** Get the lowest height that the should be tested for.*/
+        double getLowestHeight() const { return _lowestHeight; }
+        
+        /** Compute the HAT intersections with the specified scene graph.
+          * The results are all stored in the form of a single height above terrain value per HAT test.
+          * Note, if the topmost node is a CoordinateSystemNode then the input points are assumed to be geocentric,
+          * with the up vector defined by the EllipsoidModel attached to the CoordinateSystemNode.
+          * If the topmost node is not a CoordinateSystemNode then a local coordinates frame is assumed, with a local up vector. */
+        void computeIntersections(osg::Node* scene);
+
+        /** Compute the vertical distance between the specified scene graph and a single HAT point. .*/
+        static double computeHeightAboveTerrain(osg::Node* scene, const osg::Vec3d& point);
+        
+        
+        /** Clear the database cache.*/
+        void clearDatabaseCache() { if (_dcrc.valid()) _dcrc->clearDatabaseCache(); }
+
+        /** Set the ReadCallback that does the reading of external PagedLOD models, and caching of loaded subgraphs.
+          * Note, if you have mulitple LineOfSight or HeightAboveTerrain objects in use at one time then you should share a single
+          * DatabaseCacheReadCallback between all of them. */
+        void setDatabaseCacheReadCallback(DatabaseCacheReadCallback* dcrc);
+
+        /** Get the ReadCallback that does the reading of external PagedLOD models, and caching of loaded subgraphs.*/
+        DatabaseCacheReadCallback* getDatabaseCacheReadCallback() { return _dcrc.get(); }
+        
+        /** Get the IntersectionVistor that does the intersection traversal over the scene.
+          * Note, if you want to customized the traversal then you can use the IntersectionVisitor's method to alter its behavior. */
+        osgUtil::IntersectionVisitor& getIntersectionVisitor() { return _intersectionVisitor; }
+
     protected :
+    
+        struct HAT
+        {
+            HAT(const osg::Vec3d& point):
+                _point(point),
+                _hat(0.0) {}
+                
+            osg::Vec3d      _point;
+            double          _hat;
+        };
+        
+        typedef std::vector<HAT> HATList;
+        
+
+        double                                  _lowestHeight;
+        HATList                                 _HATList;
+
+        
+        osg::ref_ptr<DatabaseCacheReadCallback> _dcrc;
+        osgUtil::IntersectionVisitor            _intersectionVisitor;
+
 
 };
 
diff --git a/include/osgSim/LineOfSight b/include/osgSim/LineOfSight
index 65c781368..e82247edc 100644
--- a/include/osgSim/LineOfSight
+++ b/include/osgSim/LineOfSight
@@ -52,7 +52,7 @@ class OSGSIM_EXPORT LineOfSight
         LineOfSight();
 
         /** Clear the internal LOS List so it contains no line of sight tests.*/
-        void clearLOS();
+        void clear();
     
         /** Add a line of sight test, consisting of start and end point. Returns the index number of the newly adding LOS test.*/
         unsigned int addLOS(const osg::Vec3d& start, const osg::Vec3d& end);
@@ -78,7 +78,7 @@ class OSGSIM_EXPORT LineOfSight
         const Intersections& getIntersections(unsigned int i) const  { return _LOSList[i]._intersections; }
 
         /** Compute the LOS intersections with the specified scene graph.
-          * The results are all stored in the form of IntersectionList, one per LOS test.*/
+          * The results are all stored in the form of Intersections list, one per LOS test.*/
         void computeIntersections(osg::Node* scene);
 
         /** Compute the intersection between the specified scene graph and a single LOS start,end pair. Returns an IntersectionList, of all the points intersected.*/
diff --git a/src/osgSim/HeightAboveTerrain.cpp b/src/osgSim/HeightAboveTerrain.cpp
index a7b468a7f..1a28935ea 100644
--- a/src/osgSim/HeightAboveTerrain.cpp
+++ b/src/osgSim/HeightAboveTerrain.cpp
@@ -11,6 +11,111 @@
  * OpenSceneGraph Public License for more details.
 */
 
+#include <osg/CoordinateSystemNode>
 #include <osgSim/HeightAboveTerrain>
 
+#include <osg/Notify>
+
 using namespace osgSim;
+
+HeightAboveTerrain::HeightAboveTerrain()
+{
+    _lowestHeight = -1000.0;
+    
+    setDatabaseCacheReadCallback(new DatabaseCacheReadCallback);
+}
+
+void HeightAboveTerrain::clear()
+{
+    _HATList.clear();
+}
+
+unsigned int HeightAboveTerrain::addPoint(const osg::Vec3d& point)
+{
+    unsigned int index = _HATList.size();
+    _HATList.push_back(HAT(point));
+    return index;
+}
+
+void HeightAboveTerrain::computeIntersections(osg::Node* scene)
+{
+    osg::CoordinateSystemNode* csn = dynamic_cast<osg::CoordinateSystemNode*>(scene);
+    osg::EllipsoidModel* em = csn ? csn->getEllipsoidModel() : 0;
+
+    osg::ref_ptr<osgUtil::IntersectorGroup> intersectorGroup = new osgUtil::IntersectorGroup();
+
+    for(HATList::iterator itr = _HATList.begin();
+        itr != _HATList.end();
+        ++itr)
+    {
+        if (em)
+        {
+        
+            osg::Vec3d start = itr->_point;
+            osg::Vec3d upVector = em->computeLocalUpVector(start.x(), start.y(), start.z());
+
+            double latitude, longitude, height;
+            em->convertXYZToLatLongHeight(start.x(), start.y(), start.z(), latitude, longitude, height);
+            osg::Vec3d end = start - upVector * (height - _lowestHeight);            
+            
+            itr->_hat = height;
+
+            osg::notify(osg::NOTICE)<<"lat = "<<latitude<<" longitude = "<<longitude<<" height = "<<height<<std::endl;
+
+            osg::ref_ptr<osgUtil::LineSegmentIntersector> intersector = new osgUtil::LineSegmentIntersector(start, end);
+            intersectorGroup->addIntersector( intersector.get() );
+        }
+        else
+        {
+            osg::Vec3d start = itr->_point;
+            osg::Vec3d upVector (0.0, 0.0, 1.0);
+            
+            double height = start.z();
+            osg::Vec3d end = start - upVector * (height - _lowestHeight);            
+
+            itr->_hat = height;
+
+            osg::ref_ptr<osgUtil::LineSegmentIntersector> intersector = new osgUtil::LineSegmentIntersector( start, end);
+            intersectorGroup->addIntersector( intersector.get() );
+        }
+    }
+    
+    _intersectionVisitor.reset();
+    _intersectionVisitor.setIntersector( intersectorGroup.get() );
+    
+    scene->accept(_intersectionVisitor);
+    
+    unsigned int index = 0;
+    osgUtil::IntersectorGroup::Intersectors& intersectors = intersectorGroup->getIntersectors();
+    for(osgUtil::IntersectorGroup::Intersectors::iterator intersector_itr = intersectors.begin();
+        intersector_itr != intersectors.end();
+        ++intersector_itr, ++index)
+    {
+        osgUtil::LineSegmentIntersector* lsi = dynamic_cast<osgUtil::LineSegmentIntersector*>(intersector_itr->get());
+        if (lsi)
+        {
+            osgUtil::LineSegmentIntersector::Intersections& intersections = lsi->getIntersections();
+            if (!intersections.empty())
+            {
+                const osgUtil::LineSegmentIntersector::Intersection& intersection = *intersections.begin();
+                osg::Vec3d intersectionPoint = intersection.localIntersectionPoint * (*intersection.matrix);
+                _HATList[index]._hat = (_HATList[index]._point - intersectionPoint).length();
+            }
+        }
+    }
+    
+}
+
+double HeightAboveTerrain::computeHeightAboveTerrain(osg::Node* scene, const osg::Vec3d& point)
+{
+    HeightAboveTerrain hat;
+    unsigned int index = hat.addPoint(point);
+    hat.computeIntersections(scene);
+    return hat.getHeightAboveTerrain(index);
+}
+
+void HeightAboveTerrain::setDatabaseCacheReadCallback(DatabaseCacheReadCallback* dcrc)
+{
+    _dcrc = dcrc;
+    _intersectionVisitor.setReadCallback(dcrc);
+}
diff --git a/src/osgSim/LineOfSight.cpp b/src/osgSim/LineOfSight.cpp
index f26259c9a..c8b8892ae 100644
--- a/src/osgSim/LineOfSight.cpp
+++ b/src/osgSim/LineOfSight.cpp
@@ -92,7 +92,7 @@ LineOfSight::LineOfSight()
     setDatabaseCacheReadCallback(new DatabaseCacheReadCallback);
 }
 
-void LineOfSight::clearLOS()
+void LineOfSight::clear()
 {
     _LOSList.clear();
 }