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Active 10 Altitude reading
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mikealder
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PostPosted: Tue Jan 08, 2008 8:36 pm    Post subject: Reply with quote

Most normal disposable batteries output 1,5V the NIMH output 1,2V the Lithium cells when new can be outputting around 1,8V each that is 5,4V applied to the device which is considerably higher than normal batteries, it is the over voltage that is causing the issue rather than the output being too low.
I still maintain the Lithium cells are by far the cheapest battery in terms of hours of use from a single set, I have experianced screen flicker but it soon goes away on my unit.
The LiPo cell looks interesting but with the cost of high power rechargable NIMH cells and a charger this might be the way to go, a couple of spare sets of cells in a box designed to carry them when setting out for a couple of days walking - Mike
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LostMike
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PostPosted: Fri Jan 18, 2008 5:44 pm    Post subject: Altitude Discrepancy Reply with quote

Though this thread seems to have moved on I don't think anyone has yet come up with why there should be a 40m height discrepancy.

In the hope that it helps I quote from the Ordnance Survey web site.

Quote:
Q27. Why does my handheld GPS display a height of 40m when I know I'm at sea level?

A. The model of the Earth's surface used by many handheld GPS receivers is often a mean fit for the entire world. Because of this the model may not fit Great Britain very well resulting in a height discrepancy. Height is always the least reliable element in a GPS position fix especially for a navigated position.
Unquote.
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robs1972
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PostPosted: Fri Jan 18, 2008 7:38 pm    Post subject: Reply with quote

I don't think that does help as your problem seems to be a difference in the OS elevation data rather than GPS derived height.

Does the very near area around this odd spot have any major difference too?
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LostMike
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PostPosted: Fri Jan 18, 2008 7:49 pm    Post subject: Altitude discrepancy Reply with quote

But the difference is between what the device says and what the OS map shows which is exactly what the OS quote refers to. Doesn't it?
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robs1972
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PostPosted: Fri Jan 18, 2008 7:59 pm    Post subject: Re: Altitude discrepancy Reply with quote

LostMike wrote:
But the difference is between what the device says and what the OS map shows which is exactly what the OS quote refers to. Doesn't it?


No...the Satmap's elevation figure comes from the OS elevation data in the map file. The OS quote is talking about GPS receivers that indicate where you are in 3 dimensions....which the Active 10 doesn't.
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LostMike
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PostPosted: Fri Jan 25, 2008 11:15 am    Post subject: Altitude Reply with quote

If it does take data from the OS mapping then its doing it in an odd was as I have just been looking at my map in "planning" mode (I had no Sat lock) and the height reported stayed constant over a substantial distance (maybe 1/2 mile) despite the cursor crossing several contour lines.

Odd me thinks!
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kidstypike
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PostPosted: Fri Jan 25, 2008 11:33 am    Post subject: Reply with quote

The height information that a GPS displays, comes from the satellites that you are locked onto, NOT from the map data. You need to be locked onto at least three satellites to receive height data. The height that you can see displayed when you are not locked onto ANY satellites, is probably the height retained from the last known location.

kidstypike
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robs1972
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PostPosted: Fri Jan 25, 2008 6:58 pm    Post subject: Reply with quote

kidstypike wrote:
The height information that a GPS displays, comes from the satellites that you are locked onto, NOT from the map data. You need to be locked onto at least three satellites to receive height data. The height that you can see displayed when you are not locked onto ANY satellites, is probably the height retained from the last known location.

kidstypike


The height that you can see displayed on this device when you are not locked onto ANY satellites, is derived from the map data at that point on the map....and that also applies when you ARE locked on to satelites in either GPS or Planning mode.

Mike...I've just tried mine in a random location and within 1km grid find various heights from 114m to 133m and many in between.
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kidstypike
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PostPosted: Fri Jan 25, 2008 7:14 pm    Post subject: Reply with quote

robs1972

Do you still get a height reading if you remove the map card?

kidstypike[/quote]
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robs1972
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PostPosted: Fri Jan 25, 2008 7:23 pm    Post subject: Reply with quote

Yes....it has elevation data in the base map and the height still changes as you move around in GPS or Planning mode on the base map.....scrolling is a hell of a lot quicker too!!
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kidstypike
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PostPosted: Fri Jan 25, 2008 7:46 pm    Post subject: Reply with quote

Quote:
Yes....it has elevation data in the base map and the height still changes as you move around in GPS or Planning mode on the base map.....scrolling is a hell of a lot quicker too!!


Ok... I stand corrected, sorry to have doubted Sad

kidstypike
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djhpk
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PostPosted: Fri Jan 25, 2008 8:32 pm    Post subject: Active 10 Altitude Reading Reply with quote

As has been already noted, the basic road map which comes with all Active 10s is accompanied by "something" that enables it to give altitude readings even though the contour information is not present. Presumably, therefore, hidden in the system is some sort of altitude database. Most likely, this comes from a Digital Terrain Model (DTM).

If one looks at a trip profile as recorded in the trip log, one can see that it consists of a number of steps, rather than a continous variation of height. This suggests to me that the Active 10's DTM consists of "tiles" with the altitude constant in each tile.

Careful study of the altitude display and grid reference when scrolling around shows that the altitude indicated only changes when the last two digits of either Easting or Northing pass an even number of 10s (or 00). For example, only when either Easting or Northing goes from xx39 to xx41 (or any other reference where the 3rd digit is even) will the altitude reading change. Thus, the DTM tiles appear to be 200m square.

All the above is apparent in planning mode and was verified this afternoon while on a local walk in the area of SK 03 80.

Also, I checked that this seems to apply in planning mode, at least, in the vicinity of the summit of Snowdon where I was disappointed to see the altitude given as 1.02km!! Changing to mile/feet units this altitude was given to 4 figures in feet, pity the metric version did not keep to metres rather than km.

Obviously, the indicated altitudes could be made more precise by using a smaller tile size for the DTM and Ordnance Survey appear to have a DTM with 5m tiles but this would need vastly more storage space than the one with 200m tiles.
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kidstypike
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PostPosted: Fri Jan 25, 2008 8:48 pm    Post subject: Reply with quote

Quote:
Yes....it has elevation data in the base map and the height still changes as you move around in GPS or Planning mode on the base map.....scrolling is a hell of a lot quicker too!!


I would like to try to iron out any misunderstanding, regarding my comments on altitude readings:

On my Garmin Legend CX, the location & altitude at the CURSOR are shown in a dialogue box on the map screen. These values change as you move the cursor around on the map.

At the same time, you can have data fields on the map screen that show your ACTUAL location & altitude, these do NOT change as you move the cursor around.

If you do not have a satelite fix, then these data fields show blank.

It was these data fields I was refering to when I posted my original comments. So I would assume that the A10 works in a similar way, and indicates somewhere on the unit, the ACTUAL altitude, as computed by the satelites.

kistypike
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robs1972
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PostPosted: Fri Jan 25, 2008 9:36 pm    Post subject: Reply with quote

kidstypike wrote:
Quote:
Yes....it has elevation data in the base map and the height still changes as you move around in GPS or Planning mode on the base map.....scrolling is a hell of a lot quicker too!!


I would like to try to iron out any misunderstanding, regarding my comments on altitude readings:

On my Garmin Legend CX, the location & altitude at the CURSOR are shown in a dialogue box on the map screen. These values change as you move the cursor around on the map.

At the same time, you can have data fields on the map screen that show your ACTUAL location & altitude, these do NOT change as you move the cursor around.

If you do not have a satelite fix, then these data fields show blank.

It was these data fields I was refering to when I posted my original comments. So I would assume that the A10 works in a similar way, and indicates somewhere on the unit, the ACTUAL altitude, as computed by the satelites.

kistypike


Ok...I've just tried that and that does appear to be the case...the elevation is blank whilst I don't have a lock..then it comes up as 70m once a lock is obtained...I had set the other databox to grid reference and it still showed the location even before a lock was obtained. 70m is exactly the height Memory Map says I am too, as does Satmap in Planning mode.....although that doesn't conclusively prove that height is derived from the 3rd dimension of GPS...it could be a bug in the databox feature.

SO....tomorrow or Sunday I am going to experiment by cycling 10 miles down the road to my local suspension bridge (the Tamar) and see what height it says I'm riding at....Memory Map of course says that the middle of the span is 0m above sea level....then we may have an answer. Very Happy
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kidstypike
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PostPosted: Fri Jan 25, 2008 9:52 pm    Post subject: Reply with quote

Quote:
SO....tomorrow or Sunday I am going to experiment by cycling 10 miles down the road to my local suspension bridge (the Tamar) and see what height it says I'm riding at....Memory Map of course says that the middle of the span is 0m above sea level....then we may have an answer.


I like your way of thinking Laughing
Anquet mapping also says 0m.
Just watch out for that wind!

kidstypike
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