+ int update( double lon, double lat, double visibility_meters );
+ int update( double lon, double lat, double visibility_meters, sgdVec3 abs_pos_vector, SGBucket p_current, SGBucket p_previous, Point3D center );
+ void setCurrentTile( double longitude, double latitude );
+ int updateCurrentElevAtPos(sgdVec3 abs_pos_vector, Point3D center );