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Altitude

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Can someone tell me how the H determines it altitude. I used to know this. Yesterday I noticed that prior to take off the alt showed -6. It had never done that before. Then I noticed that I was in an area that was lower then I usually take off from. Like a gully. Is that what caused it. I recalibrated the Accelerometer but saw no change.
 

NorWiscPilot

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Can someone tell me how the H determines it altitude. I used to know this. Yesterday I noticed that prior to take off the alt showed -6. It had never done that before. Then I noticed that I was in an area that was lower then I usually take off from. Like a gully. Is that what caused it. I recalibrated the Accelerometer but saw no change.
What did the altimeter read once you armed the motors?

Arming the motors should reset the altimeter to zero. The barometer handles the rest as to providing data for the altitude readout.

Jeff
 

FlushVision

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Can someone tell me how the H determines it altitude. I used to know this. Yesterday I noticed that prior to take off the alt showed -6. It had never done that before. Then I noticed that I was in an area that was lower then I usually take off from. Like a gully. Is that what caused it. I recalibrated the Accelerometer but saw no change.
The altitude is determined (not very accurately I may add) by barometric data obtained by the TH's barometer. Before arming the motors it is not uncommon for the controller to display a height other than zero. Indeed, that height can easily be a negative value... -6 is not unusual. The height is set to zero once the motors are armed and the height AMSL (Above Mean Sea level) has no bearing on it: Taking off from a gully, say, will have no influence on that height: It is set to zero at whatever height AMSL you are once the motors have been started.
 

PatR

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FlushVision brought up a good point. Semantics, but they should be kept in mind. The H does not measure altitude, it estimates height above the take off elevation. Theoretically, take off ground level is always zero (in practice it can be off zero considerably) while height is estimated using a preconfigured pressure/temperature lapse rate.

So if you launched from a ground level elevation of 2000’ and climbed 400’ the H would be flying at a height of 400’ while being at 2400’ in altitude.
 
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Sometimes, for reasons that are not immediately clear, the reported altitude will change by around 50 ft in either direction when motors are first started but before you have left the ground. This may have consequences during descent later in the flight, so if it happens to you, best bet is to stop motors, wait a few seconds, then restart them, at which point altitude reading will usually reset and stay more or less constant, after which normal take-off can commence.
 
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Since the alt is sensed by barometric pressure, be aware that changes in temperature & humidity can affect it.
EG: flying from cold dense air to warm air (over a hot roof) or from warm air to humid air (like near a waterfall).
 

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