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# Version 1 May 2006

NAV 11

Altitude is the vertical distance above mean sea level (AMSL). Watch out – aviation charts and your altimeter are calibrated in feet, but topographical maps usually show contours and spot heights in metres!

Elevation is the vertical distance of a particular reference point on or fixed to the earth’s surface, measured from mean sea level.

Height refers to the vertical extent of an object above ground level (AGL).

Example: The height of a tower is 350ft AGL. Ground level is 500ft above mean sea level. The elevation of the top of the tower is therefore 350 + 500 = 850ft AMSL. A balloon flying 700ft above the ground at the same point would therefore be at a height of 700ft AGL. Its altitude would be 700 + 500 = 1200ft AMSL.

# Altimetry

An altimeter is simply a very sensitive barometer, as it measures changes in air pressure. As altitude increases, air pressure decreases, due to the fact that there is less weight of air above. The main scale of an altimeter is graduated in feet (or metres) to enable you to read the height of the altimeter above a given point, for example above sea level. A secondary scale (subscale) shows air pressure.

Air pressure is measured in hectopascals (HPa), previously called millibars (mb). As a guide: 1HPa less = 30ft higher (approx.)

At any given location and height the air pressure is not constant, but is gradually changing as the weather systems move through. So an altimeter may read zero feet at sea level one day, while on the next day (if the atmospheric pressure has dropped) it will indicate a height above sea level even though the instrument has not been moved.

In order to override this local daily variation, the subscale pressure setting is adjusted before each flight so that the altimeter again reads the correct height for the current air pressure.

The pressure setting which causes the altimeter to display the altitude in feet AMSL (above mean sea level) is called QNH. A balloon altimeter is normally set to local QNH or if this is not available the area QNH (these are obtained from aviation forecasts). An alternative is to set the altimeter to read the elevation of your launch field just before you take off, which is effectively the same as setting local QNH.

An alternative pressure setting, called QFE, causes the altimeter to display the height above a given location, such as an aerodrome or launch field. This setting is not generally used by balloons. (‘QNH’ and ‘QFE’ are just convenient 3 letter codes.)