Types of Altitude When the glider altitude is changed, the changing pressure  within the altimeter case expands or contracts the aneroid  barometer that, through linkage, rotates the pointers. 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. If the glider is flown from a pressure level  of 28.75 "Hg to a pressure level of 29.75 "Hg, the altimeter  would show a decrease of approximately 1,000 feet in altitude.  The other method of moving the pointers does not rely on  changing air pressure, but on the mechanical construction  of the altimeter. When the knob on the altimeter is rotated,  the altimeter setting pressure scale moves simultaneously  with the altimeter pointers. This may be confusing because  the numerical values of pressure indicated in the window  increase while the altimeter indicates an increase in altitude,  and decrease while the altimeter indicates a decrease in  altitude. This is contrary to the reaction on the pointers when  air pressure changes and is based solely on the mechanical  makeup of the altimeter. To further explain this point, assume  that the correct altimeter setting is 29.50, or a .50 difference.  This would cause a 500 foot error in altitude. In this case,  if the altimeter setting is adjusted from 30.00 to 29.50, the  numerical value decreases and the altimeter indicates a  decrease of 500 feet in altitude. Before this correction was  made, the glider was actually flying at an altitude of 500  feet lower than was shown on the altimeter. It is important  to calibrate the altimeter for all VFR flights because if they  are off by more than a comfortable margin, calibration or  repair is advisable. Knowing the glider’s altitude is vitally important to the pilot  for several reasons. The pilot must be sure that the glider is  flying high enough to clear the highest terrain or obstruction  along the intended route. To keep above mountain peaks, the  pilot must be aware of the glider’s altitude and elevation of  the surrounding terrain at all times. Knowledge of the altitude  is necessary to calculate TAS. Altitude is vertical distance above some point or level used  as a reference. There may be as many kinds of altitude as  there are reference levels from which altitude is measured,  and each may be used for specific reasons. The following are the four types of altitude that affect glider  pilots.  [Figure 4-19] Figure 4-19.  Types of altitude. 4-10 •  Indicated altitude—altitude read directly from the  altimeter (uncorrected) after it is set to the current  altimeter setting (QNH) in the Kollsman window.  Indicated altitude can be used for maintaining terrain/ obstacle clearance and estimating distance to glide  over the terrain without benefit of lift. •  True altitude—true vertical distance of the glider  above sea level (known as MSL). This altitude is  measured above a standard datum due to the shape  of the earth (often expressed in this manner: 10,900  feet MSL, 5,280 feet MSL, or 940 feet MSL). Airport,  terrain, and obstacle elevations found on aeronautical  charts are expressed as MSL (true altitudes). With  Absolute altitude Above ground level (AGL) Standard datum plane Sea level Ture altitude Pressure altitude Terrain