AP_Baro: expose get_altitude_difference()

this is useful for pressure altitude calculations

libraries/AP_Baro/AP_Baro.cpp

@@ -129,13 +129,33 @@ void AP_Baro::update_calibration()
     _ground_temperature.set(get_temperature());
 }
 
+// return altitude difference in meters between current pressure and a
+// given base_pressure in Pascal
+float AP_Baro::get_altitude_difference(float base_pressure, float pressure)
+{
+    float ret;
+#if HAL_CPU_CLASS <= HAL_CPU_CLASS_16
+    // on slower CPUs use a less exact, but faster, calculation
+    float scaling = base_pressure / pressure;
+    float temp    = _ground_temperature + 273.15f;
+    ret = logf(scaling) * temp * 29.271267f;
+#else
+    // on faster CPUs use a more exact calculation
+    float scaling = pressure / base_pressure;
+    float temp    = _ground_temperature + 273.15f;
+
+    // This is an exact calculation that is within +-2.5m of the standard atmosphere tables
+    // in the troposphere (up to 11,000 m amsl).
+	ret = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
+#endif
+    return ret;
+}
+
 // return current altitude estimate relative to time that calibrate()
 // was called. Returns altitude in meters
 // note that this relies on read() being called regularly to get new data
 float AP_Baro::get_altitude(void)
 {
-    float scaling, temp;
-
     if (_ground_pressure == 0) {
         // called before initialisation
         return 0;
@@ -146,21 +166,7 @@ float AP_Baro::get_altitude(void)
         return _altitude + _alt_offset;
     }
 
-
+    _altitude = get_altitude_difference(_ground_pressure, get_pressure());
-#if HAL_CPU_CLASS <= HAL_CPU_CLASS_16
-    // on slower CPUs use a less exact, but faster, calculation
-    scaling                                 = (float)_ground_pressure / (float)get_pressure();
-    temp                                    = ((float)_ground_temperature) + 273.15f;
-    _altitude = logf(scaling) * temp * 29.271267f;
-#else
-    // on faster CPUs use a more exact calculation
-    scaling                                 = (float)get_pressure() / (float)_ground_pressure;
-    temp                                    = ((float)_ground_temperature) + 273.15f;
-
-    // This is an exact calculation that is within +-2.5m of the standard atmosphere tables
-    // in the troposphere (up to 11,000 m amsl).
-	_altitude = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
-#endif
 
     _last_altitude_t = _last_update;
 

libraries/AP_Baro/AP_Baro.h

@@ -41,6 +41,10 @@ public:
     // of the last calibrate() call
     float        get_altitude(void);
 
+    // get altitude difference in meters relative given a base
+    // pressure in Pascal
+    float        get_altitude_difference(float base_pressure, float pressure);
+
     // get scale factor required to convert equivalent to true airspeed
     float        get_EAS2TAS(void);
 
@@ -67,7 +71,7 @@ public:
     }
 
     // get last time sample was taken (in ms)
-    uint32_t        get_last_update() { return _last_update; };
+    uint32_t        get_last_update() const { return _last_update; };
 
     static const struct AP_Param::GroupInfo        var_info[];