Altitude/height

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Altitude/height

Author	Message
Mikernav	Mikernav posted 12/3/2013 12:45:54 AM Topic Details Share Topic
Group: Forum Members Posts: 34, Visits: 129	What is the calculation basis for altitude and height in SD? Eg, is altitude based on 1013 QNH and height based on altitude less terrain height? Trying to understand discrepancy between altimeter set to RPS and SD's altitude. Tags Altitude calculation basis height
	Reply Like 88
micromike	micromike posted 12/3/2013 9:01:21 AM Post Details Share Post
Group: Forum Members Posts: 19, Visits: 1.4K	my SD shows exactly the same as my Garmin set to above sea level Mike Cheers Mike Anything i say is my opinion you can take it or leave it
	Reply Like 105
Tim Dawson	Tim Dawson posted 12/3/2013 12:54:57 PM Post Details Share Post
Group: Forum Members Posts: 8K, Visits: 9K	SkyDemon uses the GPS so altitude is derived absolutely rather than with reference to any pressure setting. The altitude given is above local mean sea level. The height reading gives your height above the highest nearby terrain point elevation. The resolution of terrain points in SkyDemon is currently 30 arc seconds.
	Reply Like 98
Mikernav	Mikernav posted 12/3/2013 5:51:15 PM Post Details Share Post
Group: Forum Members Posts: 34, Visits: 129	Thanks Tim.
	Reply Like 69
TimT	TimT posted 12/16/2019 6:11:22 PM Post Details Share Post
Group: Forum Members Posts: 88, Visits: 92	+x Tim Dawson - 12/3/2013 12:54:57 PM SkyDemon uses the GPS so altitude is derived absolutely rather than with reference to any pressure setting. The altitude given is above local mean sea level. The height reading gives your height above the highest nearby terrain point elevation. The resolution of terrain points in SkyDemon is currently 30 arc seconds. At what frequency do you calculate and update Height AGL? Looking at the Height read-out during a rapid descent (or rapidly rising terrain), it appears to get updated every 2 seconds or so. Is the resolution and algorithm still the same as in 2013? When you say "30 arc seconds", do you mean 30 arc seconds with regards to 1 degree longitude? That would be approx 1km then. The observed behaviour - as at today - doesn't seem to match. To give an example, when I - flying straight & level - approach a ridge line it shows the constant height above the bottom of that valley, all while the terrain is closing in below me up to including the moment when I am above the ridge, and only then changes to a new value for height above the ridge. We are talking here maybe 4 seconds from overhead the foot of the ridge to being overhead the top of the ridge. Much less than 1 km (I'd say 1 to 2 arc seconds max).
	Reply Like 2
grahamb	grahamb posted 12/16/2019 7:08:55 PM Post Details Share Post
Group: Forum Members Posts: 565, Visits: 27K	+x TimT - 12/16/2019 6:11:22 PM +x Tim Dawson - 12/3/2013 12:54:57 PM SkyDemon uses the GPS so altitude is derived absolutely rather than with reference to any pressure setting. The altitude given is above local mean sea level. The height reading gives your height above the highest nearby terrain point elevation. The resolution of terrain points in SkyDemon is currently 30 arc seconds. At what frequency do you calculate and update Height AGL? Looking at the Height read-out during a rapid descent (or rapidly rising terrain), it appears to get updated every 2 seconds or so. Is the resolution and algorithm still the same as in 2013? When you say "30 arc seconds", do you mean 30 arc seconds with regards to 1 degree longitude? That would be approx 1km then. The observed behaviour - as at today - doesn't seem to match. To give an example, when I - flying straight & level - approach a ridge line it shows the constant height above the bottom of that valley, all while the terrain is closing in below me up to including the moment when I am above the ridge, and only then changes to a new value for height above the ridge. We are talking here maybe 4 seconds from overhead the foot of the ridge to being overhead the top of the ridge. Much less than 1 km (I'd say 1 to 2 arc seconds max). Surely the position refresh rate depends on the GPS source. A tablet’s inbuilt GPS chip may refresh at a different rate to an external device providing position data to a tablet. When you compound that with the size of the terrain tiles it’s not surprising that there may be jerky behaviour
	Reply Like 2
Tim Dawson	Tim Dawson posted 12/17/2019 11:41:31 AM Post Details Share Post
Group: Forum Members Posts: 8K, Visits: 9K	The only significant change from when this thread was started is that we now have a high-resolution terrain feature. If this is enabled, and high-resolution tiles have been downloaded for the area, the reading will be more accurate than the low-resolution tiles. I still wouldn't expect it to necessarily be able to show a different value every second, in level flight over terrain of rapidly-changing elevation. It's not designed to replace a radio altimeter.
	Reply Like 3