diff --git a/libraries/AP_Terrain/AP_Terrain.h b/libraries/AP_Terrain/AP_Terrain.h
index a62edfeb22..82a0f44e1e 100644
--- a/libraries/AP_Terrain/AP_Terrain.h
+++ b/libraries/AP_Terrain/AP_Terrain.h
@@ -177,7 +177,7 @@ public:
     /*
       get some statistics for TERRAIN_REPORT
      */
-    void get_statistics(uint16_t &pending, uint16_t &loaded);
+    void get_statistics(uint16_t &pending, uint16_t &loaded) const;
 
 private:
     // allocate the terrain subsystem data
@@ -308,7 +308,7 @@ private:
     /*
       get some statistics for TERRAIN_REPORT
      */
-    uint8_t bitcount64(uint64_t b);
+    uint8_t bitcount64(uint64_t b) const;
 
     /*
       disk IO functions
diff --git a/libraries/AP_Terrain/TerrainGCS.cpp b/libraries/AP_Terrain/TerrainGCS.cpp
index bc77579f13..e57a68cec6 100644
--- a/libraries/AP_Terrain/TerrainGCS.cpp
+++ b/libraries/AP_Terrain/TerrainGCS.cpp
@@ -149,7 +149,7 @@ void AP_Terrain::send_request(mavlink_channel_t chan)
 /*
   count bits in a uint64_t
 */
-uint8_t AP_Terrain::bitcount64(uint64_t b)
+uint8_t AP_Terrain::bitcount64(uint64_t b) const
 {
     return __builtin_popcount((unsigned)(b&0xFFFFFFFF)) + __builtin_popcount((unsigned)(b>>32));
 }
@@ -157,7 +157,7 @@ uint8_t AP_Terrain::bitcount64(uint64_t b)
 /*
   get some statistics for TERRAIN_REPORT
 */
-void AP_Terrain::get_statistics(uint16_t &pending, uint16_t &loaded)
+void AP_Terrain::get_statistics(uint16_t &pending, uint16_t &loaded) const
 {
     pending = 0;
     loaded = 0;