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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. 

## Types of Altitude 

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] 

• 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 

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