Merge pull request sparkfun#47 from tedder/ted/ref_pressure_explanation

add some comments about "refPressure"

Author: nseidle

src/SparkFunBME280.cpp

@@ -390,7 +390,13 @@ float BME280::readFloatPressure( void )
 
 }
 
-//Sets the internal variable _referencePressure so the 
+// Sets the internal variable _referencePressure so the altitude is calculated properly.
+// This is also known as "sea level pressure" and is in Pascals. The value is probably
+// within 10% of 101325. This varies based on the weather:
+// https://en.wikipedia.org/wiki/Atmospheric_pressure#Mean_sea-level_pressure
+//
+// if you are concerned about accuracy or precision, make sure to pull the
+// "sea level pressure"value from a trusted source like NOAA.
 void BME280::setReferencePressure(float refPressure)
 {
 	_referencePressure = refPressure;
@@ -406,7 +412,13 @@ float BME280::readFloatAltitudeMeters( void )
 {
 	float heightOutput = 0;
 
-	//heightOutput = ((float)-45846.2)*(pow(((float)readFloatPressure()/(float)_referencePressure), 0.190263) - (float)1);
+  // Getting height from a pressure reading is called the "international barometric height formula".
+  // The magic value of 44330.77 was adjusted in issue #30.
+  // There's also some discussion of it here: https://www.sparkfun.com/tutorials/253
+  // This calculation is NOT designed to work on non-Earthlike planets such as Mars or Venus;
+  // see NRLMSISE-00. That's why it is the "international" formula, not "interplanetary".
+  // Sparkfun is not liable for incorrect altitude calculations from this
+  // code on those planets. Interplanetary selfies are welcome, however.
 	heightOutput = ((float)-44330.77)*(pow(((float)readFloatPressure()/(float)_referencePressure), 0.190263) - (float)1); //Corrected, see issue 30
 	return heightOutput;