Fix #7 by adding readSealevelPressure function.

Author: tdicola

Adafruit_BMP085.cpp

@@ -195,6 +195,10 @@ int32_t Adafruit_BMP085::readPressure(void) {
   return p;
 }
 
+int32_t Adafruit_BMP085::readSealevelPressure(float altitude_meters) {
+  float pressure = readPressure();
+  return (int32_t)(pressure / pow(1.0-altitude_meters/44330, 5.255));
+}
 
 float Adafruit_BMP085::readTemperature(void) {
   int32_t UT, B5;     // following ds convention

Adafruit_BMP085.h

@@ -58,6 +58,7 @@ class Adafruit_BMP085 {
   boolean begin(uint8_t mode = BMP085_ULTRAHIGHRES);  // by default go highres
   float readTemperature(void);
   int32_t readPressure(void);
+  int32_t readSealevelPressure(float altitude_meters = 0);
   float readAltitude(float sealevelPressure = 101325); // std atmosphere
   uint16_t readRawTemperature(void);
   uint32_t readRawPressure(void);

examples/BMP085test/BMP085test.ino

@@ -49,6 +49,10 @@ void loop() {
     Serial.print(bmp.readAltitude());
     Serial.println(" meters");
 
+    Serial.print("Pressure at sealevel (calculated) = ");
+    Serial.print(bmp.readSealevelPressure());
+    Serial.println(" Pa");
+
   // you can get a more precise measurement of altitude
   // if you know the current sea level pressure which will
   // vary with weather and such. If it is 1015 millibars