When determining whether to add or subtract the amount of  altimeter error, remember that when the actual pressure is lower  than what is set in the altimeter window, the actual altitude  of the aircraft is lower than what is indicated on the altimeter.  The following is another method of computing the altitude  deviation. Start by subtracting the current altimeter setting  from 29.94 "Hg. Always remember to place the original setting  as the top number. Then subtract the current altimeter setting.  A decrease in pressure causes the altimeter to indicate an  increase in altitude, and an increase in pressure causes the  altimeter to indicate a decrease in altitude. Accordingly, if  the aircraft is sitting on the ground with a pressure level of  29.98 "Hg and the pressure level changes to 29.68 "Hg, the  altimeter would show an increase of approximately 300 feet  in altitude. This pressure change is most noticeable when the  aircraft is left parked over night. As the pressure falls, the  altimeter interprets this as a climb. The altimeter indicates  an altitude above the actual field elevation. If the barometric  pressure setting is reset to the current altimeter setting of 29.68  "Hg, then the field elevation is again indicated on the altimeter.  This pressure change is not as easily noticed in flight since  aircraft fly at specific altitudes. The aircraft steadily decreases  true altitude while the altimeter is held constant through pilot  action as discussed in the previous section.  Knowing the aircraft’s altitude is vitally important to a  pilot. The pilot must be sure that the aircraft is flying high  enough to clear the highest terrain or obstruction along the  intended route. It is especially important to have accurate  altitude information when visibility is restricted. To clear  obstructions, the pilot must constantly be aware of the altitude  of the aircraft and the elevation of the surrounding terrain. To  reduce the possibility of a midair collision, it is essential to  maintain altitude in accordance with air traffic rules.  (Since 1 inch of pressure is equal to approximately 1,000 feet  of altitude, 0.25  ×  1,000 feet = 250 feet.) Always subtract  the number from the indicated altitude.  Now, try a lower pressure setting. Adjust from altimeter  setting 29.94 to 30.56 "Hg.  (Since 1 inch of pressure is equal to approximately 1,000 feet  of altitude, 0.62  ×  1,000 feet = 620 feet.) Always subtract  the number from the indicated altitude.  The pilot will be 620 feet high.  Notice the difference is a negative number. Starting with the  current indicated altitude of 2,600 feet, subtracting a negative  number is the same as adding the two numbers. By utilizing  this method, a pilot will better understand the importance of  using the current altimeter setting (miscalculation of where  and in what direction an error lies can affect safety; if altitude  is lower than indicated altitude, an aircraft could be in danger  of colliding with an obstacle).  There are two means by which the altimeter pointers can  be moved. The first is a change in air pressure, while the  other is an adjustment to the barometric scale. When the  aircraft climbs or descends, changing pressure within the  altimeter case expands or contracts the aneroid barometer.  This movement is transmitted through mechanical linkage  to rotate the pointers.  Altitude in itself is a relevant term only when it is specifically  stated to which type of altitude a pilot is referring. Normally  when the term “altitude” is used, it is referring to altitude  above sea level since this is the altitude which is used to  depict obstacles and airspace, as well as to separate air traffic.  Altitude is vertical distance above some point or level used as  a reference. There are as many kinds of altitude as there are  reference levels from which altitude is measured, and each  may be used for specific reasons. Pilots are mainly concerned  with five types of altitudes:  Altimeter Operation  Types of Altitude  1.  Indicated altitude—read directly from the altimeter  (uncorrected) when it is set to the current altimeter  setting.  2.  True altitude—the vertical distance of the aircraft  above sea level—the actual altitude. It is often  expressed as feet above mean sea level (MSL). Airport,  terrain, and obstacle elevations on aeronautical charts  are true altitudes.  8-6  Mineral Wells altimeter setting  29.94  Abilene altimeter setting  29.69  29.94 – 29.69 = Difference  0.25  Mineral Wells altimeter setting  29.94  Altimeter setting  30.56  29.94 – 30.56 = Difference  –0.62  2,600 – (–620) = 3,220  2,600 – 250 = 2,350 