The  standard  atmosphere  is  theoretical.  It  was derived by averaging the readings taken over a period of  years.  The  list  of  altitudes  and  their  corresponding values of temperature and pressure given in figure 2-7 were  determined  by  these  averages. INDICATED ALTITUDE.—  The term  indicated altitude  means  the  value  of  altitude  that  is  displayed on  the  pressure  altimeter. CALIBRATED ALTITUDE.—  Calibrated  altitude is   indicated   altitude   corrected   for   installation/ positional  error. PRESSURE  ALTITUDE.—   The height of the aircraft  above  the  standard  datum  plane  is  called pressure  altitude. DENSITY  ALTITUDE.—  Density is mass per unit  volume.  The  density  of  the  air  varies  with temperature  and  with  height.  Warm  air  expands,  and is less dense than cold air. Normally, the higher the pressure  altitude,  the  less  dense  the  air  becomes.  The density of the air can be expressed in terms of the standard atmosphere. Density altitude is the pressure altitude corrected for temperature. This calculation converts   the   density   of   the   air   to   the   standard atmospheric  altitude  having  the  same  density. Density altitude is used in performance data and true airspeed  calculations. TRUE ALTITUDE.—  True altitude is the actual vertical distance above mean sea level, measured in feet. It can be determined by two methods: (1) Set the local altimeter setting on the barometric scale of the pressure  altimeter  to  obtain  the  indicated  true  altitude. The  indicated  true  altitude  can  then  be  resolve  to  the true altitude by use of a DR computer. (2) Measure altitude over water with an absolute altimeter. ABSOLUTE  ALTITUDE.—  The  height  above the terrain is called absolute altitude. It is computed by  subtracting  terrain  elevation  from  true  altitude,  or it can be read directly from an absolute altimeter. Pressure Altimeter As  stated  earlier,  every  naval  aircraft  has  a pressure  altimeter.  The  altitude  indicated  is  indicated altitude,  not  absolute  altitude. PRINCIPLES  OF  OPERATION.—  The pres- sure altimeter is an aneroid barometer calibrated to indicate  feet  of  altitude  instead  of  pressure.  The pointers are connected by a mechanical linkage to a set  of  aneroid  cells.  These  aneroid  cells  expand  or contract  with  changes  in  barometric  pressure.  The cells assume a particular thickness at a given pressure level,  and  thereby  position  the  altitude  pointers accordingly. On  the  face  of  the  indicator  is  a barometric   scale   that   indicates   the   barometric pressure (in.Hg) of the point or plane from which the instrument  is  measuring  altitude.  If  you  turn  the barometric  pressure  set  knob  on  the  altimeter,  it manually  changes  the  setting  on  the  scale.  It  also results in simultaneous movement of the pointers to the  corresponding  altitude  reading. Like  all  measurements,  an  altitude  reading  is meaningless if the reference point is unknown. The pressure  altimeter  face  supplies  both  values.  The position of the pointers indicate the altitude in feet, and  the  barometric  scale  indicates  the  pressure  of  the reference  plane. TYPES   OF   PRESSURE   ALTIMETERS.— There are two different types of altimeters that you will be concerned with. They are the counter-pointer altimeter and the counter-drum-pointer altimeter. Counter-Pointer Altimeter.—Ths altimeter has a  two-digit  counter  display  unit  located  in  the 9 o’clock position of the dial. The counter indicates altitude  in  1,000-foot  increments  from  0  to  80,000 feet  (fig.  2-8).  A  single  conventional  pointer  indicates hundreds  of  feet  on  the  fixed  circular  scale.  The pointer makes 1 revolution per 1,000 feet of altitude, and as it passes through the 900- to 1,000-foot area of the  dial,  the  1,000-foot  counter  is  actuated.  The  shaft of  the  1,000-foot  counter  actuates  the  10,000-foot counter  at  each  10,000  feet  of  altitude  change.  To determine  the  indicated  altitude,  you  read  the Figure  2-8.-Counter-pointer  altimeter. 2-8