Added temp compensated altitude calculations

Adafruit_BMP085.cpp

@@ -19,6 +19,7 @@
 #endif
 
 #include <Wire.h>
+#include <Math.h>
 #include <limits.h>
 
 #include "Adafruit_BMP085.h"
@@ -322,14 +323,29 @@ void Adafruit_BMP085::getTemperature(float *temp)
 
 /**************************************************************************/
 /*!
-    Converts pressure in hPa to altitude in meters
+    Calculates the altitude (in meters) from the specified atmospheric
+    pressure (in hPa), sea-level pressure (in hPa), and temperature (in <20>C)
+
+    @param  seaLevel      Sea-level pressure in hPa
+    @param  atmospheric   Atmospheric pressure in hPa
+    @param  temp          Temperature in degrees Celsius
 */
 /**************************************************************************/
-float Adafruit_BMP085::pressureToAltitude(float pressure_hPa)
+float Adafruit_BMP085::pressureToAltitude(float seaLevel, float atmospheric, float temp)
 {
-  return (pressure_hPa
+  /* Hyposometric formula:                      */
-      / (float) pow(1.0F - (SENSORS_PRESSURE_SEALEVELHPA / 44330.0f), 5.255f))
+  /*                                            */
-      / 10;
+  /*     ((P0/P)^(1/5.257) - 1) * (T + 273.15)  */
+  /* h = -------------------------------------  */
+  /*                   0.0065                   */
+  /*                                            */
+  /* where: h   = height (in meters)            */
+  /*        P0  = sea-level pressure (in hPa)   */
+  /*        P   = atmospheric pressure (in hPa) */
+  /*        T   = temperature (in <20>C)           */
+
+  return (((float)pow((seaLevel/atmospheric), 0.190223F) - 1.0F)
+         * (temp + 273.15F)) / 0.0065F;
 }
 
 /**************************************************************************/

Adafruit_BMP085.h

@@ -96,7 +96,7 @@ class Adafruit_BMP085 : public Adafruit_Sensor
     bool  begin(bmp085_mode_t mode = BMP085_MODE_ULTRAHIGHRES);
     void  getTemperature(float *temp);
     void  getPressure(float *pressure);
-    float pressureToAltitude(float pressure_hPa);
+    float pressureToAltitude(float seaLevel, float atmospheric, float temp);
     void  getEvent(sensors_event_t*);
     void  getSensor(sensor_t*);
 

examples/sensorapi/sensorapi.pde

@@ -23,6 +23,7 @@
     
    History
    =======
+   2013/JUN/17  - Updated altitude calculations (KTOWN)
    2013/FEB/13  - First version (KTOWN)
 */
    
@@ -79,7 +80,7 @@ void setup(void)
 */
 /**************************************************************************/
 void loop(void) 
-{  
+{
   /* Get a new sensor event */ 
   sensors_event_t event;
   bmp.getEvent(&event);
@@ -87,12 +88,37 @@ void loop(void)
   /* Display the results (barometric pressure is measure in hPa) */
   if (event.pressure)
   {
+    /* Display atmospheric pressue in hPa */
     Serial.print(event.pressure); Serial.print(" hPa");
-    Serial.print(" ("); Serial.print(bmp.pressureToAltitude(event.pressure)); Serial.println(" m)");
+    
+    /* Calculating altitude with reasonable accuracy requires pressure    *
+     * sea level pressure for your position at the moment the data is     *
+     * converted, as well as the ambient temperature in degress           *
+     * celcius.  If you don't have these values, a 'generic' value of     *
+     * 1013.25 hPa can be used (defined as SENSORS_PRESSURE_SEALEVELHPA   *
+     * in sensors.h), but this isn't ideal and will give variable         *
+     * results from one day to the next.                                  *
+     *                                                                    *
+     * You can usually find the current SLP value by looking at weather   *
+     * websites or from environmental information centers near any major  *
+     * airport.                                                           *
+     *                                                                    *
+     * For example, for Paris, France you can check the current mean      *
+     * pressure and sea level at: http://bit.ly/16Au8ol                   */
+     
+    float seaLevelPressure = SENSORS_PRESSURE_SEALEVELHPA;
+    float temperature;
+    bmp.getTemperature(&temperature);
+    
+    Serial.print(" ("); 
+    Serial.print(bmp.pressureToAltitude(seaLevelPressure, 
+                                        event.pressure,
+                                        temperature)); 
+    Serial.println(" m)");
   }
   else
   {
     Serial.println("Sensor error");
   }
   delay(250);
-}
+}