From 72440e2af3c31e0c701bd299d3428b41ade9f4e5 Mon Sep 17 00:00:00 2001
From: Peter Barker <pbarker@barker.dropbear.id.au>
Date: Tue, 6 Nov 2018 12:04:00 +1100
Subject: [PATCH] Copter: allow VFR_HUD.alt to be relative-alt if DEV_OPTIONS
 has bit 2 set

---
 ArduCopter/GCS_Mavlink.cpp | 10 ++++++++++
 ArduCopter/GCS_Mavlink.h   |  2 ++
 ArduCopter/Parameters.cpp  |  2 +-
 ArduCopter/defines.h       |  1 +
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/ArduCopter/GCS_Mavlink.cpp b/ArduCopter/GCS_Mavlink.cpp
index 52e1e257a9..e5e8a6d173 100644
--- a/ArduCopter/GCS_Mavlink.cpp
+++ b/ArduCopter/GCS_Mavlink.cpp
@@ -1553,3 +1553,13 @@ bool GCS_MAVLINK_Copter::set_mode(const uint8_t mode)
 #endif
     return copter.set_mode((control_mode_t)mode, MODE_REASON_GCS_COMMAND);
 }
+
+float GCS_MAVLINK_Copter::vfr_hud_alt() const
+{
+    if (copter.g2.dev_options.get() & DevOptionVFR_HUDRelativeAlt) {
+        // compatability option for older mavlink-aware devices that
+        // assume Copter returns a relative altitude in VFR_HUD.alt
+        return copter.current_loc.alt / 100.0f;
+    }
+    return GCS_MAVLINK::vfr_hud_alt();
+}
diff --git a/ArduCopter/GCS_Mavlink.h b/ArduCopter/GCS_Mavlink.h
index 0133637386..eeee96f84e 100644
--- a/ArduCopter/GCS_Mavlink.h
+++ b/ArduCopter/GCS_Mavlink.h
@@ -58,6 +58,8 @@ private:
     MAV_STATE system_status() const override;
 
     int16_t vfr_hud_throttle() const override;
+    float vfr_hud_alt() const override;
 
     void send_pid_tuning();
+
 };
diff --git a/ArduCopter/Parameters.cpp b/ArduCopter/Parameters.cpp
index 7a0b383412..b57ccda04f 100644
--- a/ArduCopter/Parameters.cpp
+++ b/ArduCopter/Parameters.cpp
@@ -814,7 +814,7 @@ const AP_Param::GroupInfo ParametersG2::var_info[] = {
     // @Param: DEV_OPTIONS
     // @DisplayName: Development options
     // @Description: Bitmask of developer options. The meanings of the bit fields in this parameter may vary at any time. Developers should check the source code for current meaning
-    // @Bitmask: 0:ADSBMavlinkProcessing
+    // @Bitmask: 0:ADSBMavlinkProcessing,1:DevOptionVFR_HUDRelativeAlt
     // @User: Advanced
     AP_GROUPINFO("DEV_OPTIONS", 7, ParametersG2, dev_options, 0),
 
diff --git a/ArduCopter/defines.h b/ArduCopter/defines.h
index 82b1a39ca3..61563e5dce 100644
--- a/ArduCopter/defines.h
+++ b/ArduCopter/defines.h
@@ -232,6 +232,7 @@ enum PayloadPlaceStateType {
 // bit options for DEV_OPTIONS parameter
 enum DevOptions {
     DevOptionADSBMAVLink = 1,
+    DevOptionVFR_HUDRelativeAlt = 2,
 };
 
 //  Logging parameters