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Old 21 May 2009, 16:21   #11
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are you sure? most gps will give altitude readings - and I think they all need to take it into account to accurately calculate long/lat.
But that segment doesnt seem to be accurate,. my last 2 boats' systems showed an unlikely figure when I was hauled up on the hard well above sea level (200ft or so) , could ofcourse just have been my gear but I suppose a hill climber could advise us on that one (If any exist on a RIB site )
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Old 21 May 2009, 16:28   #12
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But that segment doesnt seem to be accurate,. my last 2 boats' systems showed an unlikely figure when I was hauled up on the hard well above sea level (200ft or so) , could ofcourse just have been my gear but I suppose a hill climber could advise us on that one
I would say its reasonably accurate in the "vertical" direction. All GPS signals are of course subject to error - I think the "vertical" error is supposed to be a bit bigger than the "horizontal" error - but still fairly accurate. Of course it does depend on visibility of the sky - and its generally more likely to be obscured on land than at sea.

{Edit see here for some techy info: http://gpsinformation.net/main/altitude.htm }
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Old 21 May 2009, 16:37   #13
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The problem with altitude is that it works to an average around the world. The GPS model makes assumptions about the shape of the earth and WGS84 has defined that shape to be an ellipsoid, with a major and minor axis Those dimensions are in reality only an approximation to the real shape. Ideally, the GPS world-shape would correspond precisely to "sealevel" everywhere in the world but there are very few places where the WGS84 ellipsoid definition coincides with sealevel. The discrepancy is averaged out to be zero, but can be as large as 300ft in some isolated locations.
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Old 21 May 2009, 16:45   #14
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The problem with altitude is that it works to an average around the world. The GPS model makes assumptions about the shape of the earth and WGS84 has defined that shape to be an ellipsoid, with a major and minor axis Those dimensions are in reality only an approximation to the real shape. Ideally, the GPS world-shape would correspond precisely to "sealevel" everywhere in the world but there are very few places where the WGS84 ellipsoid definition coincides with sealevel. The discrepancy is averaged out to be zero, but can be as large as 300ft in some isolated locations.
That makes sense Ian .. I was further north when I observed this .. so begs the question,.. given a spherical planet shape, how do pilots flying over the poles cope with this .. or is it the old story that the military system is more accurate than the stuff we get access to ?
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Old 22 May 2009, 05:36   #15
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To clarify a little on aircraft - above a certain altitude ( which I forget right now as I dont often get that high on one engine !) you fly at a given ' flight level' based on a pressure setting of 1013, there is also a prescribed rule dictating heading & level to ensure some degree of automatic seperation if all the transponders packup ! Beneath this & assuming you are talking to someone ( ie an airfiled) on the radio or even slightly sensible you take the local pressure setting ( before you take off , or as you move around the world ) - either QNH or QFE - one being height above a a certain place ( usually an airfield) or sea level, pressure is updated regularly to account for the weather ) . This will hopefully allow all aircraft to use the same pressure setting - so will all see the same altitude shown on the altimeter , and this also ensure they dont fly into 'tall things' -mountains,spinaker tower, buildings etc by accident as they will know the minimun safe altitude to fly at - this is shown on thier chart anyway - abit like water depth you see on your boating charts.

GPS as has been said is very good & is used , but backed up by trusted methods of navigation - in my case a line on chart & good bit of compass dead reckoning ............OH and you can always just follow the motorway network in the UK.
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Old 22 May 2009, 05:45   #16
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I am a hill walker!

Well a climber anyway

I don't know why but the error on the vertical seem to be really bad. I use a garmin hand-held and I often get accuracy down to 5 or 10 meters. I have watched the altitude reading change by over 100 meters while I was walking along on the flat and level!

Mike
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Old 22 May 2009, 05:51   #17
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The big problem is that everything you base altitude on changes - the earth , sea level , pressure - its always a comparison to a variable - thats why they were still arguing over the height of Everest till very recently - where do you measure from ?
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Old 22 May 2009, 06:58   #18
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how do pilots flying over the poles cope with this .. ?
I don't know, so I spoke to a pal who's a pilot, and his answer was that they don't rely too much on altitude readings unless it becomes a sensitive issue - ie they're getting near the ground! On airfield approach, they are guided by ground radar, and a sonar gadget which seems to do pretty much the same as a depth sounder on a boat. Over mountains they combine eyeball, altimeter, and GPS, and if vis is bad they increase their safety margin.

I have to say that he's wound me up about stuff before, but this sounds as though he may be telling the truth for once
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Old 22 May 2009, 08:07   #19
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The military wouldn't let the system go down, simply because most of its weapons can't use a compass and chart!! Most long-range air to ground weapons use Inertial Navigation (IN) updated by GPS to increase accuracy - but IN on it's own isn't good enough to get them to where they need to be.

The accuracy (in 2 dimensions) on my GPS is often within 3 metres, nearly always within 6 metres - and the military signal is more accurate still.

The altitude accuracy on a GPS depends on how many satellites you have locked in - 12 will give quite good results, 3 or 4 will only give you a 2D fix - some units will tell you that on a status bar.

I've also been told (but don't know if it's true) that WAAS only corrects/adjusts the 2D position, and not altitude.

D...
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Old 22 May 2009, 11:31   #20
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or is it the old story that the military system is more accurate than the stuff we get access to ?
Used to be that consumer GPS was downgraded in performance; that, I believe, went away several years ago.

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