Elevation gain: apply exponential smoothing and limit changes (defined change diff per sensor value).

src/main/java/de/dennisguse/opentracks/util/PressureSensorUtils.java

@@ -7,6 +7,8 @@ public class PressureSensorUtils {
     //Everything above is considered a meaningful change in elevation.
     private static float ELEVATION_CHANGE_DIFF_M = 5.0f;
 
+    private static float EXPONENTIAL_SMOOTHING = 0.3f;
+
     private PressureSensorUtils() {
     }
 
@@ -28,6 +30,23 @@ public class PressureSensorUtils {
         public float getElevationChange_m() {
             return elevationChange_m;
         }
+
+        public float getElevationGain_m() {
+            return elevationChange_m > 0 ? elevationChange_m : 0;
+        }
+
+        public float getElevationLoss_m() {
+            return elevationChange_m < 0 ? elevationChange_m : 0;
+        }
+    }
+
+    /**
+     * Applies exponential smoothing to sensor value before computation.
+     */
+    public static ElevationChange computeChangesWithSmoothing_m(float lastAcceptedSensorValue_hPa, float lastSeenSensorValue_hPa, float currentSensorValue_hPa) {
+        float nextSensorValue_hPa = EXPONENTIAL_SMOOTHING * currentSensorValue_hPa + (1 - EXPONENTIAL_SMOOTHING) * lastSeenSensorValue_hPa;
+
+        return computeChanges_m(lastAcceptedSensorValue_hPa, nextSensorValue_hPa);
     }
 
     /**
@@ -35,8 +54,8 @@ public class PressureSensorUtils {
      *
      * @return null if no meaningful elevation change occurred.
      */
-    public static ElevationChange computeChanges_m(float lastSensorValue_hPa, float currentSensorValue_hPa) {
-        float lastSensorValue_m = SensorManager.getAltitude(SensorManager.PRESSURE_STANDARD_ATMOSPHERE, lastSensorValue_hPa);
+    public static ElevationChange computeChanges_m(float lastAcceptedSensorValue_hPa, float currentSensorValue_hPa) {
+        float lastSensorValue_m = SensorManager.getAltitude(SensorManager.PRESSURE_STANDARD_ATMOSPHERE, lastAcceptedSensorValue_hPa);
         float currentSensorValue_m = SensorManager.getAltitude(SensorManager.PRESSURE_STANDARD_ATMOSPHERE, currentSensorValue_hPa);
 
         float elevationChange_m = currentSensorValue_m - lastSensorValue_m;
@@ -44,6 +63,21 @@ public class PressureSensorUtils {
             return null;
         }
 
-        return new ElevationChange(currentSensorValue_hPa, elevationChange_m);
+        // Limit elevation change by ELEVATION_CHANGE_DIFF and computes pressure value accordingly.
+        ElevationChange elevationChange = new ElevationChange(currentSensorValue_hPa, elevationChange_m);
+        if (elevationChange.getElevationGain_m() > 0) {
+            return new ElevationChange(getBarometricPressure(SensorManager.PRESSURE_STANDARD_ATMOSPHERE, lastSensorValue_m + ELEVATION_CHANGE_DIFF_M), ELEVATION_CHANGE_DIFF_M);
+        } else {
+            return new ElevationChange(getBarometricPressure(SensorManager.PRESSURE_STANDARD_ATMOSPHERE, lastSensorValue_m - ELEVATION_CHANGE_DIFF_M), -1 * ELEVATION_CHANGE_DIFF_M);
+        }
+    }
+
+    /*
+     * Barometeric pressure to elevation estimation; inverts of SensorManager.getAltitude(float, float)
+     * https://de.wikipedia.org/wiki/Barometrische_H%C3%B6henformel#Internationale_H%C3%B6henformel
+     * {\color{White} p(h)} = p_0 \cdot \left( 1 - \frac{0{,}0065 \frac{\mathrm K}{\mathrm m} \cdot h}{T_0\ } \right)^{5{,}255}
+     */
+    private static float getBarometricPressure(float p0, float altitude_m) {
+        return (float) (p0 * Math.pow(1.0 - 0.0065 * altitude_m / 288.15, 5.255f));
     }
 }