Height and altimetry — transcript

00:00:00.400 --> 00:00:05.416 Height and altimetry, and the four terms every drone pilot needs.

00:00:06.400 --> 00:00:12.747 Height is measured above the ground: AGL. Altitude is above mean sea level: AMSL.

00:00:12.747 --> 00:00:16.927 Elevation is the ground's own height above sea level.

00:00:17.277 --> 00:00:22.099 Altitude equals elevation plus height. Hold 400 feet AGL over hills,

00:00:22.099 --> 00:00:25.663 and your altitude rises and falls with the ground.

00:00:26.377 --> 00:00:33.144 Your 400 foot limit is AGL, but manned aircraft use AMSL. At an aerodrome 1,

00:00:33.144 --> 00:00:39.648 500 feet up, your drone is at 1,900. A Cessna at 2,000 is 100 feet above.

00:00:39.998 --> 00:00:44.877 Pressure falls as you climb, about one hectopascal every 30 feet.

00:00:44.877 --> 00:00:51.014 An altimeter turns that pressure into height. Standard sea-level pressure is 1013.

00:00:51.014 --> 00:00:52.197 2 hectopascals.

00:00:52.547 --> 00:00:57.759 Set QNH, the sea-level pressure, and it reads altitude: on the ground,

00:00:57.759 --> 00:01:03.779 the aerodrome's elevation. Set QFE, the pressure at the field, and it reads zero.

00:01:04.129 --> 00:01:06.729 If pressure falls and the setting isn't updated,

00:01:06.729 --> 00:01:09.276 the altimeter reads higher than you really are.

00:01:11.629 --> 00:01:16.667 Your drone measures the pressure when it's armed and calls that zero. That's QFE.

00:01:16.667 --> 00:01:20.537 Unless it has a lidar, its height is above the take-off point.

00:01:23.829 --> 00:01:27.805 Here's the trap. Take off from a ridge and fly out over a valley.

00:01:27.805 --> 00:01:31.901 The screen still says 400 feet, but you're well above 400 feet AGL.

00:01:36.129 --> 00:01:42.202 Recap. The limit is AGL. QNH gives altitude, QFE reads zero at the field,

00:01:42.202 --> 00:01:45.403 and your drone measures from take-off.

00:01:45.753 --> 00:01:49.462 Final check. You launch from a hilltop and fly over a valley.

00:01:49.462 --> 00:01:53.172 The screen shows 400 feet. How high are you above the ground?

00:01:57.153 --> 00:02:01.332 More than 400 feet. Your drone measures from take-off, not the ground below.