diff --git a/include/osgSim/ElevationSlice b/include/osgSim/ElevationSlice
index ba02fa478..d433e6a7c 100644
--- a/include/osgSim/ElevationSlice
+++ b/include/osgSim/ElevationSlice
@@ -21,7 +21,14 @@
 
 namespace osgSim {
 
-/** Helper class for setting up and acquiring height above terrain intersections with terrain.*/
+/** Helper class for setting up and acquiring height above terrain intersections with terrain.
+  * By default assigns a osgSim::DatabaseCacheReadCallback that enables automatic loading
+  * of external PagedLOD tiles to ensure that the highest level of detail is used in intersections.
+  * This automatic loading of tiles is done by the intersection traversal that is done within
+  * the computeIntersections(..) method, so can result in long intersection times when external
+  * tiles have to be loaded.
+  * The external loading of tiles can be disabled by removing the read callback, this is done by
+  * calling the setDatabaseCacheReadCallback(DatabaseCacheReadCallback*) method with a value of 0.*/
 class OSGSIM_EXPORT ElevationSlice
 {
     public :
diff --git a/include/osgSim/HeightAboveTerrain b/include/osgSim/HeightAboveTerrain
index e94a6767a..b7f1d22c2 100644
--- a/include/osgSim/HeightAboveTerrain
+++ b/include/osgSim/HeightAboveTerrain
@@ -21,7 +21,14 @@
 
 namespace osgSim {
 
-/** Helper class for setting up and acquiring height above terrain intersections with terrain.*/
+/** Helper class for setting up and acquiring height above terrain intersections with terrain.
+  * By default assigns a osgSim::DatabaseCacheReadCallback that enables automatic loading
+  * of external PagedLOD tiles to ensure that the highest level of detail is used in intersections.
+  * This automatic loading of tiles is done by the intersection traversal that is done within
+  * the computeIntersections(..) method, so can result in long intersection times when external
+  * tiles have to be loaded.
+  * The external loading of tiles can be disabled by removing the read callback, this is done by
+  * calling the setDatabaseCacheReadCallback(DatabaseCacheReadCallback*) method with a value of 0.*/
 class OSGSIM_EXPORT HeightAboveTerrain
 {
     public :
diff --git a/include/osgSim/LineOfSight b/include/osgSim/LineOfSight
index 0964b9e29..5f4f5e319 100644
--- a/include/osgSim/LineOfSight
+++ b/include/osgSim/LineOfSight
@@ -44,7 +44,13 @@ class OSGSIM_EXPORT DatabaseCacheReadCallback : public osgUtil::IntersectionVisi
 };
 
 /** Helper class for setting up and acquiring line of sight intersections with terrain.
-  * Supports automatic paging in of PagedLOD tiles. */
+  * By default assigns a osgSim::DatabaseCacheReadCallback that enables automatic loading 
+  * of external PagedLOD tiles to ensure that the highest level of detail is used in intersections.
+  * This automatic loading of tiles is done by the intersection traversal that is done within
+  * the computeIntersections(..) method, so can result in long intersection times when external
+  * tiles have to be loaded.
+  * The external loading of tiles can be disabled by removing the read callback, this is done by
+  * calling the setDatabaseCacheReadCallback(DatabaseCacheReadCallback*) method with a value of 0.*/
 class OSGSIM_EXPORT LineOfSight
 {
     public :