Altimeter

Submitted: Sunday, Oct 20, 2019 at 09:44
ThreadID: 139212 Views:1051 Replies:10 FollowUps:5
Hi everyone. I have a Galaxy Tab 8 where I have the Explore Oz app. I like it alot, but the altimeter put me below sea level when I get near the coast, and is about 15 meters out everywhere else. Has anyone found this on theirs and is it EO or the tab 8 fault? Is there a fix for it as I really dont like being 6' under, so to speak.
John Lowes

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Reply By: The Explorer - Sunday, Oct 20, 2019 at 10:10

Sunday, Oct 20, 2019 at 10:10
Hello, I think you will find its a figure provided by the GPS. These are notoriously bad at getting altitude correct based on my experience. You can only use the figure provided as a guide..i.e plus or minus x metres.

Cheers Greg
To penetrate this great unknown it would be necessary to first pass over the inhospitable regions described by Wells, Forrest & Giles - Carnegie 1896

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Reply By: Member - Jim S1 - Sunday, Oct 20, 2019 at 10:57

Sunday, Oct 20, 2019 at 10:57
Seems about right according to info available.

"The general rule of the thumb is that vertical error is three times the horizontal error. If a decent signal reception is available, a modern GPS receiver should be able to give elevation data accurate to a range of 10 to 20 meters (35 to 70 feet) post correction."

Cheers
Jim
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Reply By: TrevorDavid - Sunday, Oct 20, 2019 at 11:42

Sunday, Oct 20, 2019 at 11:42
John,

Just wondering if you are using your tab 8 with your mobile data turned off?

It is generally computer corrected very 20mins when in phone service.

If direct of the GPS you may well be getting as best your device can supply.




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TrevorDavid
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Follow Up By: Member - John L43 - Monday, Oct 21, 2019 at 22:12

Monday, Oct 21, 2019 at 22:12
Hi Trevor. It is just a wifi tab 8, so only using its own built in gps. I found on the weekend with 3 other cars that the gps was close at times and 20 meters out at other times. So, an error factor of 10-20 meters was consistent. Just to answer some of the other comments made by others.
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Follow Up By: Member - Andrew & Jen - Thursday, Oct 24, 2019 at 13:54

Thursday, Oct 24, 2019 at 13:54
John, 2 comments.
You say sometimes accurate but at other times 10 to 20m out. What's your problem? So long as you are on the right route, who cares about 10 to 20m! (If you were on a boat, I would have a different response as 20m in the wrong direction could have you on a reef)
Secondly and more importantly, location as indicated by ordinary GPSs wanders up to 20m radius all the time. If you have a GPS with a graphical tracking display and remain stationary for half and hour or so, you would observe the "reported location" tracking N, S, E and W seemingly randomly, sometimes out to 20m and at other times far less. Slowly a pattern would develop, with a distinct concentration near the centre of the screen. This would give you a more accurate idea of your "true" location. But would this be useful to help you find your way about in the bush? I doubt it. IMHO, a good pair of eyes is still the most useful accessory ?
Cheers Andrew
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Reply By: Member - Boobook - Sunday, Oct 20, 2019 at 12:25

Sunday, Oct 20, 2019 at 12:25
John, apart from being the least accurate of the measurements, height, or Z is the least understood parameter.

A raw GPS reading does not show height above sea level, it can't because the GPS system has no idea what sea level is. If you are using WGS 84, it will give you height above that model of the earth and sea level which on average across the whole world is the same as sea level. But in reality, can be a few meters or more different because the model earth shape is not the same as the real world's shape.

So, like your experience, the GPS can show that you are several m below WGS84 sea level, but still above the real sea level.

I am not sure how EO handles this, but in Memory Map, you can download the actual date for height above sea level across the world. You can either display that, or GPS height or both. They are rarely the same thing. The height above sea level is downloaded from satellite Dopler height data from NASA. I think Oziexplorer can do the same. It took me ages to figure out why Memory map showed both and they were different values.

If EO can display height above sea level you should use that, otherwise, accept that your displayed GPS height can be different to your height above sea level depending on where you are.

When you are away from the sea, you rarely notice this, unless you are a sign showing altitude above sea level, but if it says 1355m and your GPS shows 1345m you tend to dismiss the difference.

Ausgoid 09 goes some way to address this if it is supported.

Then, of course, there is the actual tide which could be higher or lower than the average.


Tony
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Reply By: RMD - Sunday, Oct 20, 2019 at 19:15

Sunday, Oct 20, 2019 at 19:15
John.
While the signals from satellites are read by your gps until they vary in distance from each satellite across the face of the earth, the gps algorithm determines your position, sort of, BUT it is far less accurate to determine the difference of your feet in relation to your head. Presumably it is above the feet, BUT the time shift distance between your feet and head is so small it is almost negligible and the signal is the same time length. Until there is a reasonable distance change in Z, there is no real signal time difference to be detected unlike a few steps forward or back.
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Reply By: Member - Andrew & Jen - Monday, Oct 21, 2019 at 19:48

Monday, Oct 21, 2019 at 19:48
John, to get an accurate altitude measurement you need at least 2 satellites high in the sky and idealy separated by at least 45 degrees. Satellites nearer the horizon give more accurate positions but more inaccurate altitude.
Cheers Andrew

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Reply By: Member - Warren H - Monday, Oct 21, 2019 at 20:01

Monday, Oct 21, 2019 at 20:01
Don't some apps estimate altitude using a combination of the barometer, gps (x,y)/ map position and gps (z)? Incidently, won't the next generation of gps used in autonomous vehicles be using differential gps so accuracy will be cm?
NT Pajero
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Follow Up By: The Explorer - Monday, Oct 21, 2019 at 23:43

Monday, Oct 21, 2019 at 23:43
Some gps units have barometers – need regular calibration (you need to enter known altitude when calibrating) and it is independent of other info (gps) - possibly not super acurate if weather changes. “Next generation” gps already exists but a bit expensive for recreational use (so no need to wait for an autonomous vehicle).

Cheers Greg
To penetrate this great unknown it would be necessary to first pass over the inhospitable regions described by Wells, Forrest & Giles - Carnegie 1896

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Reply By: Member - John L43 - Monday, Oct 21, 2019 at 22:17

Monday, Oct 21, 2019 at 22:17
Thanks to everyone who made comments. Seems I cant do much about the altitude in this unit, and it really doesnt matter that much anyway. Just as well I can breathe under the sand at the beach!!!!!.
I also use my iphone8 with motion X GPS, when I am out and about, which has other benefits over ExploreOz.
Anyway, all is well and I am grounded.
John Lowes

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Reply By: Dave B18 - Tuesday, Oct 22, 2019 at 05:52

Tuesday, Oct 22, 2019 at 05:52
In Oziexplorer you need to select the option:
'Adjust the altitude for Geoid separation'
The Samsung GPS chips are known to be poor and slow.
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Follow Up By: Member - John L43 - Tuesday, Oct 22, 2019 at 09:40

Tuesday, Oct 22, 2019 at 09:40
I dont have Oziexplorer, only ExplorOz traveller. However, I did look for any setting like you say, but couldnt find any. In the samsung setting, I have it set to "high accuracy" in the gps settings.
John Lowes

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Reply By: Maia L - Thursday, Oct 24, 2019 at 04:52

Thursday, Oct 24, 2019 at 04:52
Hi...I m going thru the manual and its says it needs a control signal or some sort of switch connected to an ignition source. What are you guys using for a ignition source? Do you think an Add-A-Fuse to a reliable source would be sufficient enough?
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Follow Up By: Frank P (NSW) - Thursday, Oct 24, 2019 at 07:32

Thursday, Oct 24, 2019 at 07:32
This has nothing to do with this topic. Maybe you inadvertently posted in the wrong thread?
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