diff --git a/libraries/AC_Fence/AC_Fence.cpp b/libraries/AC_Fence/AC_Fence.cpp
index cdbf55e51f..f078bd5eb0 100644
--- a/libraries/AC_Fence/AC_Fence.cpp
+++ b/libraries/AC_Fence/AC_Fence.cpp
@@ -524,7 +524,7 @@ bool AC_Fence::check_fence_alt_max()
 
     float alt;
     AP::ahrs().get_relative_position_D_home(alt);
-    _curr_alt = -alt; // translate Down to Up
+    const float _curr_alt = -alt; // translate Down to Up
 
     // check if we are over the altitude fence
     if (_curr_alt >= _alt_max) {
@@ -573,7 +573,7 @@ bool AC_Fence::check_fence_alt_min()
 
     float alt;
     AP::ahrs().get_relative_position_D_home(alt);
-    _curr_alt = -alt; // translate Down to Up
+    const float _curr_alt = -alt; // translate Down to Up
 
     // check if we are under the altitude fence
     if (_curr_alt <= _alt_min) {
@@ -625,7 +625,7 @@ bool AC_Fence::auto_enable_fence_floor()
 
     float alt;
     AP::ahrs().get_relative_position_D_home(alt);
-    _curr_alt = -alt; // translate Down to Up
+    const float _curr_alt = -alt; // translate Down to Up
 
     // check if we are over the altitude fence
     if (!floor_enabled() && _curr_alt >= _alt_min + _margin) {
diff --git a/libraries/AC_Fence/AC_Fence.h b/libraries/AC_Fence/AC_Fence.h
index 3165d7b73d..8ed6817d69 100644
--- a/libraries/AC_Fence/AC_Fence.h
+++ b/libraries/AC_Fence/AC_Fence.h
@@ -251,8 +251,6 @@ private:
     // other internal variables
     uint8_t         _auto_enable_mask = AC_FENCE_ALL_FENCES;  // fences that can be auto-enabled or auto-disabled
     float           _home_distance;         // distance from home in meters (provided by main code)
-    float           _curr_alt;
-
 
     // breach information
     uint8_t         _breached_fences;       // bitmask holding the fence type that was breached (i.e. AC_FENCE_TYPE_ALT_MIN, AC_FENCE_TYPE_CIRCLE)