From ee04c7de961902b14c7100368ed323ec94225a0f Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <tridge@samba.org>
Date: Wed, 23 Jul 2014 12:18:37 +1000
Subject: [PATCH] AP_Terrain: hold home location as a special location

this ensures home altitude is always available
---
 libraries/AP_Terrain/AP_Terrain.cpp | 15 +++++++++++++++
 libraries/AP_Terrain/AP_Terrain.h   |  4 ++++
 2 files changed, 19 insertions(+)

diff --git a/libraries/AP_Terrain/AP_Terrain.cpp b/libraries/AP_Terrain/AP_Terrain.cpp
index 551263a499..50d2a7718a 100644
--- a/libraries/AP_Terrain/AP_Terrain.cpp
+++ b/libraries/AP_Terrain/AP_Terrain.cpp
@@ -201,6 +201,13 @@ bool AP_Terrain::height_amsl(const Location &loc, float &height)
         return false;
     }
 
+    // quick access for home altitude
+    if (loc.lat == home_loc.lat &&
+        loc.lng == home_loc.lng) {
+        height = home_height;
+        return true;
+    }
+
     struct grid_info info;
 
     calculate_grid_info(loc, info);
@@ -237,6 +244,14 @@ bool AP_Terrain::height_amsl(const Location &loc, float &height)
     float avg  = (1.0f-info.frac_y) * avg1 + info.frac_y * avg2;
 
     height = avg;
+
+    if (loc.lat == ahrs.get_home().lat &&
+        loc.lng == ahrs.get_home().lng) {
+        // remember home altitude as a special case
+        home_height = height;
+        home_loc = loc;
+    }
+
     return true;
 }
 
diff --git a/libraries/AP_Terrain/AP_Terrain.h b/libraries/AP_Terrain/AP_Terrain.h
index 0af9c555cd..402822ca7c 100644
--- a/libraries/AP_Terrain/AP_Terrain.h
+++ b/libraries/AP_Terrain/AP_Terrain.h
@@ -269,6 +269,10 @@ private:
 
     // have we created the terrain directory?
     bool directory_created;
+
+    // cache the home altitude, as it is needed so often
+    float home_height;
+    Location home_loc;
 };
 #endif // HAVE_AP_TERRAIN
 #endif // __AP_TERRAIN_H__