Originally, VPLive was a test program while I developed the barometer formulas for the VP. There are a number of different versions of the various pressure formulas. I probably ought to take the multiple versions out
The ASOS altimeter equation is the one I found in the ASOS system documentation. I don't think this is available directly online anymore, but there is a copy of the relevant part here: http://wahiduddin.net/calc/refs/ASOS_Pressure.htm
. The MADIS equation was found on the MADIS website: http://madis.noaa.gov/madis_api.html
in software associated with the MADIS system. Since the data sent through CWOP ends up at MADIS, I chose to use that formula since I assume MADIS reverses the altimeter calculation to derive the other pressure values it calculates from the altimeter it receives.
MADIS and ASOS should be pretty close to each other. In a well calibrated system, the VP console display may not exactly match airport SLP or Altimeter. The console displays SLP, so we should not expect it to match airport altimeter values. And the console's SLP calculation differes from the NWS, so the console may not match the airport's SLP exactly.
What I do to calibrate my VP console is to look at recent recorded data at a specific time for both my station's altimeter as sent to CWOP, and for a nearby, reliable NWS site. Mesowest http://www.met.utah.edu/mesowest/
is a good source for that data. So, if at 1pm yesterday, my station reported altimeter of 29.92, and the airport reported 29.85 at about the same time, then I note that my barometer is reading .07 inHg too high. I then go to my console, display pressure in inHg, push the buttons that put the console into adjust barometer mode, and then lower the pressure by .07 inHg. It's not really important what the console happens to be showing at the time. All that's important is that I lower it by .07 inHg. Doing it this way, I get my pressure right on.