Setting the RTH altitude when flying uphill

[rfc]

If your home point is at a location several hundred feet below your destination to shoot (i.e. up a hill), is it true that you need to establish the elevation of the target, then add the height to ascend to in the event of an RTH? In other words, the RTH altitude is based on MSL, not AGL.

 

[GrammatonxXXxCleric]

RTH alt. is based on take off alt. If the drone was flown to a higher alt. than set RTH alt. it will just RTH at the last alt. it was at. If it is lower than the set RTH alt. it will climb to it.

 

[msinger]

In other words, the RTH altitude is based on MSL, not AGL.

The RTH altitude is always the altitude above the takeoff location. The Phantom does not know where the ground is located, so AGL is not an option.

 

[solentlife]

OK ... to be exact .... Altitude reading of AC is based on Barometric pressure.

So if the pressure is 1000mb at take off point ... it will read 1000 as zero altitude.

The rate of change of pressure with height is approx -1.8mb per 50ft / 15m.

It is generally accepted that the contour of pressure will be level and therefore you set RTH altitude to allow for rise of ground from your take off position. For example if you set RTH alt at 30m and take off ... there is a hill of 35m between you and the AC ... RTH will have it hit the hill on auto RTH. So you set your RTH alt to allow for that ... ie set RTH alt at over 35m ... plus any trees etc. that may be on the hill.

BUT in reality actually pressure contours often follow ground level for a reasonable height ... so your zero reading of 1000mb could in fact rise up / down according to ground level. BUT DO NOT RELY OR COUNT ON THIS !!

Second - the Adv and Pro P3's have height sensors pointing down .... to assist with hovering close to ground ... and don't forget GPS also has an altitude function when more than 4 sats locked on. But these two items will not stop you hitting that hill !!

So regardless of physics and environment contours - set your RTH altitude to take account of Hills / Trees / objects in flight path.

Nigel