2. By applying a correction factor to the indicated  altitude according to the reported “altimeter setting,”  [Figure 11-3]  3.  By using a flight computer  1. By setting the barometric scale of the altimeter to  29.92 "Hg and reading the indicated altitude,  11-3  Figure 11-3.  Field elevation versus pressure. The aircraft is located  on a field that happens to be at sea level. Set the altimeter to the  current altimeter setting (29.7). The difference of 205 feet is added  to the elevation or a PA of 205 feet.  The pressure altitude can be determined by any of the three  following methods:  The SDP is a theoretical level at which the pressure of the  atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. As  atmospheric pressure changes, the SDP may be below, at, or  above sea level. Pressure altitude is important as a basis for  determining aircraft performance, as well as for assigning  flight levels to aircraft operating at above 18,000 feet.  Pressure altitude is the height above the standard datum  plane (SDP). The aircraft altimeter is essentially a sensitive  barometer calibrated to indicate altitude in the standard  atmosphere. If the altimeter is set for 29.92 "Hg SDP, the  altitude indicated is the pressure altitude—the altitude in the  standard atmosphere corresponding to the sensed pressure.  Since all aircraft performance is compared and evaluated  using the standard atmosphere, all aircraft instruments  are calibrated for the standard atmosphere. Thus, certain  corrections must apply to the instrumentation, as well as the  aircraft performance, if the actual operating conditions do  not fit the standard atmosphere. In order to account properly  for the nonstandard atmosphere, certain related terms must  be defined.  The more appropriate term for correlating aerodynamic  performance in the nonstandard atmosphere is density  altitude—the altitude in the standard atmosphere  corresponding to a particular value of air density.  Density altitude is pressure altitude corrected for nonstandard  temperature. As the density of the air increases (lower  density altitude), aircraft performance increases. Conversely,  Altitude (ft)  Pressure  ( " Hg)  Temperature  (°C)  (°F)  0  1,000  2,000  3,000  4,000  5,000  6,000  7,000  8,000  9,000  10,000  11,000  12,000  13,000  14,000  15,000  16,000  17,000  18,000  19,000  20,000  29.92  28.86  27.82  26.82  25.84  24.89  23.98  23.09  22.22  21.38  20.57  19.79  19.02  18.29  17.57  16.88  16.21  15.56  14.94  14.33  13.74  15.0  13.0  11.0  9.1  7.1  5.1  3.1  1.1  −0.9 −2.8 −4.8 −6.8 −8.8 −10.8 −12.7 −14.7 −16.7 −18.7 −20.7 −22.6 −24.6 59.0  55.4  51.9  48.3  44.7  41.2  37.6  34.0  30.5  26.9  23.3  19.8  16.2  12.6  9.1  5.5  1.9  −1.6 −5.2 −8.8 −12.3 Altimeter  setting  Altitude  correction  28.0  28.1  28.2  28.3  28.4  28.5  28.6  28.7  28.8  28.9  29.0  29.1  29.2  29.3  29.4  29.5  29.6  29.7  29.8  29.9  29.92  30.0  30.1  30.2  30.3  30.4  30.5  30.6  30.7  30.8  30.9  31.0  1,824  1,727  1,630  1,533  1,436  1,340  1,244  1,148  1,053  957  863  768  673  579  485  392  298  205  112  20  0  −73 −165 −257 −348 −440 −531 −622 −712 −803 −893 −983 Subtract  Add To field  elevation  From field  elevation  To get  pressure altitude  Alternate Method for Determining  Pressure Altitude  Method for Determining  Pressure Altitude  Field elevation is sea level  Density Altitude  Pressure Altitude  Figure 11-2.  Properties of standard atmosphere. 