My new GPS and elevation

Does that have the barometer? I can't imagine what sort of effects that would produce. A moving bare
eTrex reflects chiefly trees cutting out satellites, with the result that the day's refraction
correction assumption errors get modulated into the readout (if there were no errors, you'd get the
same height with any number of satellites; with errors, it averages and the average changes when a
satellite leaves). In the open, my bare eTrex does altitudes very nicely, going up when I go up and
down when I go down.

Every barometer I know of is suspectible to wind - not only wind hitting you but wind hitting the
rise ahead of you and therefore not reaching you. I have no idea how they try to filter that.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

GPS is inherently more accurate with lat/long than it is with elevations. This should be explained
in the documentation that came with your unit.

If you want good elevation info get a barometric instrument, like an Avocet Vertech watch.

Mike Yankee

(Address is munged to thwart spammers. To reply, delete everything after "com".)

Mine doesn't have the barometer. Does the position on the handlebars make a difference? Mine is
angled so I can read it easily. Does it need to be pointing straight up?

Ron Hardin said:

Does that have the barometer? I can't imagine what sort of effects that would produce. A moving
bare eTrex reflects chiefly trees cutting out satellites, with the result that the day's
refraction correction assumption errors get modulated into the readout (if there were no errors,
you'd get the same height with any number of satellites; with errors, it averages and the average
changes when a satellite leaves). In the open, my bare eTrex does altitudes very nicely, going up
when I go up and down when I go down.

Every barometer I know of is suspectible to wind - not only wind hitting you but wind hitting the
rise ahead of you and therefore not reaching you. I have no idea how they try to filter that.

Jim Sherman said:

Mine doesn't have the barometer. Does the position on the handlebars make a difference? Mine is
angled so I can read it easily. Does it need to be pointing straight up?

No, it's very indifferent to orientation.

Mine (the most primitive one, no compass no barometer) does great in the open, varying only as
satellites move, which is over several minutes, over about the range you see. But it tracks my ups
and downs concurrently, there not being much filtering I guess.

Blocked sky produces altitude swings, as I say.

Afternoons these days have a not-very-good cluster of satellites, which may be contributing. If the
front page shows which satellites you're tracking, and it's only 4 or 5 clumped together, you're on
the ragged edge. I've seen almost a dozen on good days, spread over the whole sky.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

Look in your setup menu or book for something like "Filters". There is a built in time constant
which means that it averages where you were with where you are over some period of time like 20
seconds or some minutes. You want the lowest time constant available so it does less averaging. Some
GPS units allow you to select this time constant.

Ed

"Jim Sherman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know
the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Any comments?

The only similar thing I can find is regarding the track log. I can adjust the interval of the bread
crumb by time or distance. It had been set at 0.02 miles. I'll change it to 0.01 to see if that
makes a difference but I expect you're referring to something else I can't find or don't even have.

I'll also check out the satellite page to see if this anomaly is only happening when I have only 2
or 3 satellites locked on.

Thanks.

Ed Kirstein said:

Look in your setup menu or book for something like "Filters". There is a built in time constant
which means that it averages where you were with where you are over some period of time like 20
seconds or some minutes. You want the lowest time constant available so it does less averaging.
Some GPS units allow you to select this time constant.

Ed

"Jim Sherman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know
the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Any comments?

Jim Sherman said:

I'll also check out the satellite page to see if this anomaly is only happening when I have only 2
or 3 satellites locked on.

One gives you nothing; two gives a surface in space; three gives a line in space (so a position, if
you assume an altitude); four gives a position in space (and so gives you altitude as well).

So four is the absolute minimum to even give an altitude measurement. If you have more, it can do
some averaging. Twelve is nice, but rare. Six or seven is typical.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

Jim Sherman <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know
the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Not surprising. The Legend has no altimeter. Elevation data is computed, but there are large
inaccuracies since the elevations are small relative to the distance from the satellites. Legend is
great for positional information, but do not rely on it for altitudinal info.

- rick

"Jim Sherman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know
the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Any comments?

I use a gps in my car and I see the same thing. I think the fluxuations are due to the inherent
inaccuracy in the gps. They are very accurate but only within 10 feet or so in ideal conditions and
less if things are not perfect it is way better on lat/lon location than altitude. The accuracy can
and does change in very short intervals based on satellite signals, antenna position, etc. My car
unit's accuracy reading regularly swings between 7 foot and 20 foot accuracy.

I know how your feel. How quickly we get jaded with technology. I used to not know what city I was
in most of the time and now I sometimes find myself getting [censored] at a little box just because
it is not telling me EXACTLY where the dashboard of my car is.

STP

Rick Warner said:

Not surprising. The Legend has no altimeter. Elevation data is computed, but there are large
inaccuracies since the elevations are small relative to the distance from the satellites. Legend
is great for positional information, but do not rely on it for altitudinal info.

The distance of the satellites doesn't matter. It's unfavorable geometry - there are no satellites
below ground, so half the sky is unavailable for an altitude measurement. For position, you get
satellites on both sides, so to speak, spanning more than 90 degrees of angle.

And the best satellites from the point of view of that geometry, near the horizon, have the worst
refraction problems.

If there were a satellite overhead, and one straight below ground, and the time delays were equal,
you'd know that you're exactly halfway between them, the best possible measurement of altitude,
regardless of the distances involved. Your position on the other hand is completely undetermined
(except that altitude becomes position with a round earth and flat planes of position eventually).
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

Ron Hardin said:

there are no satellites below ground,

That's what they want you to think...

Ron Hardin <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

The distance of the satellites doesn't matter. It's unfavorable geometry - there are no satellites
below ground, so half the sky is unavailable for an altitude measurement. For position, you get
satellites on both sides, so to speak, spanning more than 90 degrees of angle.

Yes and no. Yes, the oppositional satellites would be best if time delay is used. But absent that,
when using trig calculations on small angles over long distances then the measurement error
becomes large.

- rick

Rick Warner said:

Quoted message said:

The distance of the satellites doesn't matter. It's unfavorable geometry - there are no
satellites below ground, so half the sky is unavailable for an altitude measurement. For
position, you get satellites on both sides, so to speak, spanning more than 90 degrees of angle.

Yes and no. Yes, the oppositional satellites would be best if time delay is used. But absent that,
when using trig calculations on small angles over long distances then the measurement error
becomes large.

It doesn't use angles at all. It doesn't sense angles. It uses only time differences.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

You may remember from analytic geometry that the locus of points with distance difference D from two
fixed points is a hyperbola.

The fixed points are two satellites. In 3-space, the surface traced by distance difference D is a
hyperboloid (D=0 gives a plane halfway between the satellites; for other D, it bends around one or
the other of the satellites. You're somewhere on this surface.

Mostly, the satellites being ``up there,'' the surfaces that you're found to be on run pretty
vertical through the ground. So intersecting them results in a lot of error in that vertical
direction (the in-plane direction, where the measurement is precisely indifferent - that's why your
on the surface).

It all comes from time difference D between various pairs though, nothing from angles.
--
Ron Hardin [email hidden]

On the internet, nobody knows you're a jerk.

I see same thing with my legend especially when I can't get a good fix on several satellites,
usually in forested areas.

I suggest you put your question on: sci.geo.satellite-nav The GPS techies are over there.

"Jim Sherman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know
the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Any comments?

Jim Sherman said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

Quoted message said:

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I get
to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the intermediate
steps. This leads to a peculiar effect on rolling hills. While descending, it will still be
displaying the previous rise and on the ensuing rise will be displaying the descent.

Quoted message said:

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While sitting
there for fifteen minutes, I watched the elevation range from +18 to -14. I don't know the
exact elevation but I sure know I was above sea level. No buildings or obstructions anywhere
near by. I tried this with WAAS enabled and disabled.

Quoted message said:

Any comments?

Probably not a lag, just the general dithering within the error bounds characteristic of GPS
altitude, especially if you're moving past a broken horizon which keeps cutting satellites in and
out of view.

--
Chris Malcolm [email hidden] +44 (0)131 651 3445 DoD #205 IPAB, Informatics, JCMB, King's
Buildings, Edinburgh, EH9 3JZ, UK [dai.ed.ac.ukcamOpen ↗]

For whatever reason, I feel like contributing my $0.02 worth. Ignore as you desire! :o)

Funny how few people (granted, there have been some good answers here) seem to understand how a GPS
works! I've been reading this thread and that is one of two things that becomes apparent. The second
is that many people become dependent on GPS without necessarily having backup. ("Danger... Danger,
Will Robinson... Danger"😉 If you are going to rely on a GPS, at least learn how they work. And,
please don't go wandering about the wilderness without a map and a compass and the knowhow to use
them! Here's a reasonably good GPS primer:

colorado.edugps f.htmlOpen ↗

Generally, the discussion here has been good (i.e., fairly accurate). Just keep in mind that your
GPS has to get time-dependent equational satelite postion information from each satelite (contrary
to popular belief, the space vehicles are not in stationary geosynchronous orbit but rather trace
precisely prescribed orbits daily) and then by reading the time stamp differential between multiple
(read "4 or more"😉 satelites, can determine position, and to a lesser degree, altitude. Once in a
while, annomolies and blunders occur -- often because of local interference (noise); the signals the
GPS is receiving are rather weak. On rare occassions, you might encounter a satelite blunder that
sends receivers into chaos due to a satelite sending eroneous information!

You should, in general, not use a GPS for better than gross determination of altitude. (Note, many
of the new GPS units have both an electronic compass and altimeter built in.) You should not expect
better than about 20 meters accuracy (often can be around 100m) without DGPS. If using the GPS in
combination with maps, know the geodetic datum of the map for proper display or you'll be perplexed
beyond belief. Know also that many areas can have dead spots at certain times and some locations;
deep canyons for example can almost never be navigated by GPS. Additionally, signals can be locally
or broadly disrupted at any time, either with intent (military or homeland security reasons) or due
to unintentional interference.

end of $0.02 worth... for more, send contribution *hehe*!

Best wishes, Doug

David Storm said:

I see same thing with my legend especially when I can't get a good fix on several satellites,
usually in forested areas.

I suggest you put your question on: sci.geo.satellite-nav The GPS techies are over there.

"Jim Sherman" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I bought the Garmin Etrex Legend but the elevation acts very strangely.

1. When moving, the elevation display can't keep up with me. I understand that sine I AM moving
there could be a lag but it seems to want to display every altitude I have been at. When I
get to the top of a hill and stop it will cycle through the elevations (209, 210, 211, 212,
213,...) instead of just calculating the present elevation and bypassing all the
intermediate steps. This leads to a peculiar effect on rolling hills. While descending, it
will still be displaying the previous rise and on the ensuing rise will be displaying the
descent.

2. Yesterday, I took a break at a bench by the Potomac river across from Washington. While
sitting there for fifteen minutes, I watched the elevation range from +18 to -14. I don't
know the exact elevation but I sure know I was above sea level. No buildings or obstructions
anywhere near by. I tried this with WAAS enabled and disabled.

Any comments?

--
[email hidden] Spammers be DAMNED! Friendlies: change notmail to hotmail to reply. Just
keeping the web bots and crawlers from filling our inboxes with junk! May they eternally spam
addresses that swamp their servers with bounced mail!

Doug said:

For whatever reason, I feel like contributing my $0.02 worth. Ignore as you desire! :o)

I believe it's a "glutton for punishment" issue. <G>

Quoted message said:

many people become dependent on GPS without necessarily having backup.

Yikes...venturing into unknown territory far from home, depending on ONE of anything to get you
to safety?

Quoted message said:

("Danger... Danger, Will Robinson... Danger"😉 If you are going to rely on a GPS, at least learn
how they work. And, please don't go wandering about the wilderness without a map and a compass and
the knowhow to use them! Here's a reasonably good GPS primer:

And if you're not going to make the effort to learn how to use a map and compass, at least make sure
to have a them, and in an emergency, you can use the "go only one direction" strategy. This is not
nearly as good as learning how to properly use the map and compass.

Quoted message said:

end of $0.02 worth... for more, send contribution *hehe*!

Hmm...another $0.02 a good contribution?

Quoted message said:

Best wishes, Doug

--
Rick Onanian