Since the altitude scale is adjusta

Since the altitude scale is adjustable, you can set the altimeter to read true altitude at some
specified height. Takeoff and landing are the most critical phases of flight; therefore, airport
elevation is the most desirable altitude for a true reading of the altimeter. Altimeter setting is the
value to which the scale of the pressure altimeter is set so the altimeter indicates true altitude at
field elevation.
In order to ensure that your altimeter reading is compatible with altimeter readings of other
aircraft in your vicinity, keep your altimeter setting current. Adjust it frequently in flight to the
altimeter setting reported by the nearest tower or weather reporting station. Figure 15 shows the
trouble you can encounter if you are lax in adjusting your altimeter in flight. Note that as you fly
from high pressure to low pressure, you are lower than your altimeter indicates.