30.0 29.9 29.8 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 10,000-foot increments 1,000-foot increments 100-foot increments Displayed at altitudes below 10,000 feet I 0 0 F E T I 4 5 6 7 9 2 0 8 3 BARO IN Hg MB 3 1 1 2 9 9 2 27 26 25 24 2 9 9 2 1 8 1 0 7 0 3 0 0 3 2 9 9 2 1 8 1 0 7 0 3 0 0 3 2727 30 26 25 29 28$^{24}$$^{22}$ 23$^{21}$ BARO Pointer Dial Baro set knob Spiral gear 1,000 feet drum Barometric counters Mechanism body Calibration arm Static port Aneroid capsules Bimetal compensator 10-19 Figure 10-33.  drum-type counter can be driven by the altimeter’s aneroid for numerical display of altitude. Drums can also be used  for the altimeter’s setting indications.  Figure 10-32.  A sensitive altimeter with three pointers and a cross- hatched area displayed during operation below 10,000 feet.  True digital instrument displays can show altitude in  numerous ways. Use of a numerical display rather than a  reproduction of the clock-type dial is most common. Often a  digital numeric display of altitude is given on the electronic  primary flight display near the artificial horizon depiction.  A linear vertical scale may also be presented to put this hard  numerical value in perspective. An example of this type of  display of altitude information is shown in  Figure 10-34 . Accurate measurement of altitude is important for numerous  reasons. The importance is magnified in instrument  flight rules (IFR) conditions. For example, avoidance of  tall obstacles and rising terrain relies on precise altitude  indication, as does flying at a prescribed altitude assigned by  air traffic control (ATC) to avoid colliding with other aircraft.  Measuring altitude with a pressure measuring device is  fraught with complications. Steps are taken to refine pressure  altitude indication to compensate for factors that may cause  an inaccurate display.  A major factor that affects pressure altitude measurements  is the naturally occurring pressure variations throughout the  atmosphere due to weather conditions. Different air masses  develop and move over the earth’s surface, each with inherent  pressure characteristics. These air masses cause the weather  we experience, especially at the boundary areas between air  masses known as fronts. Accordingly, at sea level, even if  numerical altitude in feet. The motion of the rotating digits  or drum-type counter during rapid climb or descent makes  it difficult or impossible to read the numbers. Reference  can then be directed to the classic clock-style indication.  Figure 10-33  illustrates the inner workings behind this type  of mechanical digital display of pressure altitude.