Added .readAltitude function

Adafruit_BME280.cpp

@@ -309,3 +309,25 @@ float Adafruit_BME280::readHumidity(void) {
   float h = (v_x1_u32r>>12);
   return  h / 1024.0;
 }
+
+/**************************************************************************/
+/*!
+    Calculates the altitude (in meters) from the specified atmospheric
+    pressure (in hPa), and sea-level pressure (in hPa).
+
+    @param  seaLevel      Sea-level pressure in hPa
+    @param  atmospheric   Atmospheric pressure in hPa
+*/
+/**************************************************************************/
+float Adafruit_BME280::readAltitude(float seaLevel)
+{
+  // Equation taken from BMP180 datasheet (page 16):
+  //  http://www.adafruit.com/datasheets/BST-BMP180-DS000-09.pdf
+
+  // Note that using the equation from wikipedia can give bad results
+  // at high altitude.  See this thread for more information:
+  //  http://forums.adafruit.com/viewtopic.php?f=22&t=58064
+
+  float atmospheric = readPressure() / 100.0F;
+  return 44330.0 * (1.0 - pow(atmospheric / seaLevel, 0.1903));
+}

Adafruit_BME280.h

@@ -140,9 +140,7 @@ class Adafruit_BME280
     float readTemperature(void);
     float readPressure(void);
     float readHumidity(void);
-
+    float readAltitude(float seaLevel);
-    //  float pressureToAltitude(float seaLevel, float atmospheric, float temp);
-    // float seaLevelForAltitude(float altitude, float atmospheric, float temp);
 
   private:
 

examples/bme280test/bme280test.ino

@@ -25,6 +25,8 @@
 #define BME_MOSI 11
 #define BME_CS 10
 
+#define SEALEVELPRESSURE_HPA (1013.25)
+
 Adafruit_BME280 bme; // I2C
 //Adafruit_BME280 bme(BME_CS); // hardware SPI
 //Adafruit_BME280 bme(BME_CS, BME_MOSI, BME_MISO,  BME_SCK);
@@ -48,6 +50,10 @@ void loop() {
     Serial.print(bme.readPressure() / 100.0F);
     Serial.println(" hPa");
 
+    Serial.print("Approx. Altitude = ");
+    Serial.print(bme.readAltitude(SEALEVELPRESSURE_HPA));
+    Serial.println(" m");
+
     Serial.print("Humidity = ");
     Serial.print(bme.readHumidity());
     Serial.println(" %");