The term Pb in our altitude equation is the pressure at the bottom of the atmospheric zone we are in. Since we are focusing on the lowest zone in the atmosphere, the bottom is zero altitude, aka sea level. So we have:
The sea level pressure, SLP, is just what it says. If you went down to the ocean and measured the pressure right at the water, that would be sea level pressure. The problem is this:
Ugh! So how do we deal with SLP so we can compute altitude? There are two general approaches.
This is super easy. If you know the current value for sea level pressure, SLP, then just plug it in. Then your pressure sensor measures your local pressure, P, and you can compute altitude, H, using:
This is what airplane pilots do. Before take off, they listen over the radio to a local report that provides SLP at the airport's location. They then turn a dial on the airplane's altimeter to set that value. Then off they go!
A pilot turns the knob (arrow) until the reading in the Kollsman window (square) matches the reported local SLP.
OK, but what if you are standing out in the middle of the mountains? Maybe you could use a radio or smartphone to get a value for SLP. But it won't be for your specific location. Also, being in the mountains is typically an off grid affair. So even getting SLP from the outside world is often not possible. In these situations you rely on knowing your current altitude and using this next approach.
In this case, you are standing at a known altitude, H. The pressure sensor itself gives the current local pressure, P. We can then rearrange our altitude equation to back compute what sea level pressure, SLP, would be:
You specify H, the pressure sensor provides P, and from that we compute SLP. We store that value for SLP and use it from then on in our main altitude equation - the same one used in previous section.
But how do you know your current altitude H? If you knew it, why would you even need an altimeter? With this approach you typically use certain geographical features which have known altitudes. Here are some examples:
If you were standing at the outflow to Azure Lake, then your actual altitude is going to be very close to 4055 feet, same as the lake's altitude.
If you are on the trail at Park Creek Pass, then your altitude is between 6040 and 6080 feet - the closest contours. Using something like 6070 feet would be good.
When you reach one of these locations, you stop for a bit, have a snack, and calibrate your altimeter. Then off you go!