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electronic compass and altimeter question
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| I am buying a GPS for my 24 year old son. He does a lot of hiking,
some off roading, and maybe does one or two back packing trips per
year. I think I have narrowed my decision to either the 60C or 60CS,
or maybe the 76CS. I wanted some feedback on the importance of having
the electronic compass and altimeter. What bothers me about the CS is
not the minimal cost to have these features, but rather the large
decrease in battery life. The listed battery life is 30 hours for the
C and 20 hours for the CS. With back packing trips a long battery life
is important. If the electronic compass is turned off, is the battery
life the same for both units? My son does most of his hiking in the
mountains. Is the compass and altimeter good features to have, or are
they more of a fun toy?
Both the 60C and 60CS have been on the market for almost two years.
Does anyone know if there are going to be new units out soon
(especially units with more memory) that will make these obsolete?
Thanks
Bob?
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| Bob wrote:
> I am buying a GPS for my 24 year old son. He does a lot of hiking,
> some off roading, and maybe does one or two back packing trips per
> year. I think I have narrowed my decision to either the 60C or 60CS,
> or maybe the 76CS. I wanted some feedback on the importance of having
> the electronic compass and altimeter. What bothers me about the CS is
> not the minimal cost to have these features, but rather the large
> decrease in battery life. The listed battery life is 30 hours for the
> C and 20 hours for the CS. With back packing trips a long battery life
> is important. If the electronic compass is turned off, is the battery
> life the same for both units?
Yes, almost all of the extra power is for the compass and you can
choose to leave it turned off.
> My son does most of his hiking in the
> mountains. Is the compass and altimeter good features to have, or are
> they more of a fun toy?
I'd say they're definitely optional. All GPS units act as a good
compass while you're moving since they can measure your change in
position and derive the direction for that. But they can't tell
directions when standing still so the magnetic compass of the 's'
models is useful in that circumstance. But I'd expect a hiker to
always carry a regular compass anyway (electronics do fail
unexpectedly) so its mainly a slight extra convenience. Some (but not
all) geocachers who seek hidden caches based on coordinates like the
compass since you're moving too slow when getting very close to the
cache for the GPS directions to work well. But others prefer to use
the map display or use a separate regular compass.
The pressure altimeter is also an added feature that's rarely needed
but can be nice to have. Although all GPS receivers will give you
altitudes, sometimes you can only get 3 satellites due to mountains or
very thick tree cover and in that case the GPS stops giving altitude
readouts unless you have the pressure sensor. The other difference is
that only the models with this sensor let you display the elevation
profile of where you've hiked right on the screen. With other models
you can get this data later on your PC but not directly from the GPS.
I miss this mainly on bike rides when others on the ride keep asking
how many feet of total climb we had or which hill was the highest, etc.
When I'm using a model without the pressure sensor I can't check that
information until I get back to my PC. If your son likes to compare
elevation gains on different hikes then the 's' model might be worth it
just for this 'real-time' information advantage. And the pressure
sensor does make the altitude read-out more accurate as well. The
pressure readout can also be useful on extended backpacking trips as a
rough estimate of upcoming weather changes.
>
> Both the 60C and 60CS have been on the market for almost two years.
> Does anyone know if there are going to be new units out soon
> (especially units with more memory) that will make these obsolete?
Yes, there has already been a presumably premature announcement by a
few dealers (incl. Bass Pro) of new models, the 60C/CSx and 76C/CSx
with the x presumably standing for the TransFlash card that these
accept to eXpand the memory. The new models also use the reportedly
more sensitive SiRF III GPS receiver chip which should give somewhat
better reception in heavy woods. Apparently these models were supposed
to be announced by Garmin in about a month at the CES show in Las
Vegas. Just guesses at this time as to actual availability date - my
'wag' is around April.
| |
| Tom Hemp 2005-12-05, 2:48 am |
| I have the 60CS, the compass is useful and can be turned off(hold Page down)
to conserve batteries. The altimeter is useless, it requires to much
calibration to make it worth it and if the barometric pressure changes while
you are using it, out of calibration it goes. I now use the altimeter with
auto calibration off and force the unit to only use GPS altitude when
tracking. You must force the unit to use GPS altitude for every track you
start also, because it does not remember between tracks or unit power off, I
am not sure what resets it back to using the barometer again..
Tom
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Tom Hemp wrote:
> I have the 60CS, the compass is useful and can be turned off(hold Page down)
> to conserve batteries. The altimeter is useless, it requires to much
> calibration to make it worth it and if the barometric pressure changes while
> you are using it, out of calibration it goes.
Actually the altimeter works very well in conjunction with the
auto-calibration. Sometimes it's off a little for the first 15 minutes
or so until it has enough time-averaged GPS data to calibrate properly.
After that it automatically compensates for weather changes
automatically by using the time-weighted average difference between the
GPS reading and the pressure sensor. But the actual recorded altitude
in the tracklog is always based on the pressure sensor - it just uses
the GPS for the automatic calibration if you have that option turned on
(which I recommend in almost all cases). This combination of GPS and
pressure sensor makes it work better than any pressure only altimeter
I've used in the past.
> I now use the altimeter with
> auto calibration off and force the unit to only use GPS altitude when
> tracking. You must force the unit to use GPS altitude for every track you
> start also, because it does not remember between tracks or unit power off, I
> am not sure what resets it back to using the barometer again..
Interesting since the 60cs firmware doesn't have any provision for
using only the GPS-based altitude. This does present problems when
flying in a pressurized aircraft since the pressure sensor is pretty
meaningless then (except for monitoring cabin pressure). You can get a
snapshot reading of the GPS altitude by going through the calibration
procedure but there's no option to have the unit continue to give you
the GPS altitude or to record it in the tracklog.
The ability to force the unit to just use GPS altitude has long been
requested of Garmin but they have never chosen to implement such a
feature.
| |
| Heinrich Pfeifer 2005-12-05, 5:48 am |
|
"Tom Hemp" <themp@nc.rr.com> wrote in
news:j6Pkf.1279$TU6.491738@twister.southeast.rr.com...
> I now use the altimeter with auto calibration off and force the unit to
> only use GPS altitude when tracking. You must force the unit to use GPS
> altitude for every track you start also, because it does not remember
> between tracks or unit power off, I am not sure what resets it back to
> using the barometer again..
please tell us *how* do you force the unit to use GPS altitude? I'm not
aware on such an option.
Anyway, for normal use (out of a pressurized aircraft cabin) I like the
barometric altimeter for its gain of precision.
--
Heinrich
http://www.gartrip.de
mail: new<at>gartrip.de
| |
| Tom Hemp 2005-12-05, 11:48 pm |
| The way I make it just use the GPS altitude is to:
1.) Select "Calibrate Altimeter" and press ENTER, this brings up the
Calibrate Altimeter Page
2.) It then asks "Do You Know The Correct Elevation?" Answer: NO
3.) It then asks "Do You Know The Current Pressure?" Answer: NO
4.) It then asks "Do You Want To Use The Current GPS Altitude?" Answer: YES
At this point it does uses the GPS Altitude, as I have tested this
during a track run as you can ask for the GPS Altitude from the satellite
page
and it matches what is on the Altimeter plot.
5.) You must do this prior to a track start or it will try and compensate
for barometric changes based on the weather. Not sure what resets this,
other
than the power off of the unit.
See page 66-67 of User's Manual for the GPS60CS
Back to the precision of the altimeter, for me it is basically useless as I
use it for kayaking along with hiking when I see a place to stop in the
kayak. Since I am out for 4-5 hours the barometric pressure changes base on
normal weather patterns affect my plot. In other words, with auto
calibration on and knowing the barometric or elevation at a start of a
paddle, I never end up at the end of a day at the same elevation that I
started at. With just using the GPS altitude it matches very close at the
end of the day since I am on a lake which usually has very good satellite
pickup. Thus, the plot in the track when using just GPS altitude may be
off, but in terms of how high I climbed it is very close to what I did. I
have checked this with a topo map when I get back home. I am interested in
how high I climbed based on the lake level.
For the precision to be accurate, the barometric pressure based on the
weather must be stable over the course of your track. For me this generally
does not happen. I have never tried auto calibration while the unit is off,
this may help, but the warning you get when you set this to make sure you
turn this off when you replace the batteries makes me leary of trying it.
Now to the original poster who ask about the altimeter, if I was to use this
only for hiking I would be hard pressed to know what the altitude is or what
the barometric pressure is at the start of a hike. Seems like a real pain,
when you can just use GPS altitude and be done with it.
Tom
"Heinrich Pfeifer" <nws@gartrip.de> wrote in message
news:dn11vp$adh$1@on
line.de...
>
> "Tom Hemp" <themp@nc.rr.com> wrote in
> news:j6Pkf.1279$TU6.491738@twister.southeast.rr.com...
>
>
> please tell us *how* do you force the unit to use GPS altitude? I'm not
> aware on such an option.
>
> Anyway, for normal use (out of a pressurized aircraft cabin) I like the
> barometric altimeter for its gain of precision.
>
> --
>
> Heinrich
> http://www.gartrip.de
> mail: new<at>gartrip.de
>
| |
| peter 2005-12-05, 11:48 pm |
| Tom Hemp wrote:
> The way I make it just use the GPS altitude is to:
> 1.) Select "Calibrate Altimeter" and press ENTER, this brings up the
> Calibrate Altimeter Page
> 2.) It then asks "Do You Know The Correct Elevation?" Answer: NO
> 3.) It then asks "Do You Know The Current Pressure?" Answer: NO
> 4.) It then asks "Do You Want To Use The Current GPS Altitude?" Answer: YES
> At this point it does uses the GPS Altitude, as I have tested this
> during a track run as you can ask for the GPS Altitude from the satellite
> page
> and it matches what is on the Altimeter plot.
As you note, this is the procedure for *calibrating* the pressure-based
altimeter. So it uses the current GPS altitude as the starting point.
But the actual recorded tracklog altitude data will still be coming
from the pressure sensor and it may or may not closely follow the GPS
altitude depending on how conditions change, the quality of GPS
reception, and on whether you have the auto-calibration feature turned
on.
| |
| Tom Hemp 2005-12-05, 11:48 pm |
| Ok, then this function really is only useful when no barometric weather
changes occur during a track. I must have been lucky the last few times I
went out. Based on this I would purchase the 60C and NOT get the sensors,
as I see from the 60C User's Manual you can get an elevation plot using the
GPS altitude when viewing the track. I wish I had know this prior to
purchasing.....
Tom
| |
| Phil Wheeler 2005-12-05, 11:48 pm |
| Tom Hemp wrote:
> Ok, then this function really is only useful when no barometric weather
> changes occur during a track. I must have been lucky the last few times I
> went out. Based on this I would purchase the 60C and NOT get the sensors,
I went with 60C, partly because the 60CS was not available at the time
... and partly because I have a wristwatch Altimeter/Compass (Casio)
which recharges itself from the Sun. So maximizing battery life in the
GPSR (I'm told the compass and altimeter suck down the battery) was an
important factor, too.
Phil
| |
| peter 2005-12-05, 11:48 pm |
| Tom Hemp wrote:
> Ok, then this function really is only useful when no barometric weather
> changes occur during a track. I must have been lucky the last few times I
> went out. Based on this I would purchase the 60C and NOT get the sensors,
> as I see from the 60C User's Manual you can get an elevation plot using the
> GPS altitude when viewing the track.
AFAIK the only elevation profile you can display on the 60c is one
that's created using the topographic data in Garmin's Topo24K maps (see
p.37 of the manual). So this only works for areas where you have these
particular maps loaded and these maps are only available for National
Parks and selected other recreational areas. That makes this feature
much more limited than the display of track elevation profiles on the
60cs and other Garmin models with a pressure sensor. And besides being
limited to only a few specific areas, it depends on the contour lines
of the map so it won't capture your actual movement. If you go up in a
plane/glider the elevation profile would show the changes in the
terrain below you (but only if the right maps were loaded), not your
actual vertical motions in the plane/glider.
> I wish I had know this prior to
> purchasing.....
If you're using the elevation profile plots available on your 60cs, and
it sounds like you are from your previous posts, then it's good you got
that model and not the 60c which wouldn't give you those plots.
I also use my GPS receivers both for kayaking and hiking. When
kayaking the GPS reception is almost always excellent and the altitude
profile that I get later on my PC from receivers without a pressure
sensor is quite good (i.e. essentially flat) - almost as good as that
from receivers with a pressure sensor. However, much of my hiking is
in terrain where most of the satellites in the sky are blocked and the
GPS receiver frequently reverts to 2D navigation or even loses lock
entirely. Under those circumstances the elevation profile based on GPS
measurement shows apparent cliffs that don't really exist but are a
result of not having valid altitude data for an extended period. Using
a receiver that also includes the pressure sensor (like the 60cs)
provides much more realistic and accurate elevation profiles in those
conditions.
| |
|
|
Tom Hemp wrote:
> Ok, then this function really is only useful when no barometric weather
> changes occur during a track. I must have been lucky the last few times I
> went out.
I have a weather station that allows me to record the barometer reading
over time on my computer. There are often long periods where there is
little change. Today, for example, the reading changed by only 0.03"
over 12 hours at my location. That equals about 24 feet on the
barometric altimeter. Hardly noticeable.
> Based on this I would purchase the 60C and NOT get the sensors
You seemed happy enough with the results you were getting when you
thought they were from GPS elevation. The fact that Peter told you
that you were actually getting data from the barometric sensor doesn't
change the fact that you were getting good results. Why not enjoy
that? If you are concerned that your readings will be off as the
pressure changes (and occasionally it *does* change enough in the
course of 4 to 5 hours to make a noticeable difference), you can repeat
the manual calibration to GPS elevation every couple of hours. That
will probably give you a blip in your profile at that point, but it
should keep you close to GPS elevation. The problem with doing this is
that the GPS elevation at any given moment may be off by quite a bit.
On the other hand, you really would be better off if you turned auto
calibration on, though you may not believe it. If you want to see why
I say you would be better off using auto calibration, check out this
excellent page on the subject by Chris Malcolm:
<http://www.dai.ed.ac.uk/homes/cam/g.../altgraphs.html> Maybe
it will convince you that you made the right purchase after all.
Even with auto calibration on, it is still a good idea to calibrate the
altimeter manually when you start out. Calibrating to a known
elevation is preferable, but if that isn't available, calibrating to
GPS elevation gets you in the ballpark.
| |
| Phil Wheeler 2005-12-06, 5:48 pm |
| peter wrote:
> Tom Hemp wrote:
>
>
>
> AFAIK the only elevation profile you can display on the 60c is one
> that's created using the topographic data in Garmin's Topo24K maps (see
> p.37 of the manual). So this only works for areas where you have these
> particular maps loaded and these maps are only available for National
> Parks and selected other recreational areas.
I'm very fortunate in that about 90% of my hiking is in So. Calif areas
covered by the 24K Topo West maps (San Bernardino NF .. which includes
the San Jacintos, Joshua Tree NP and Mojave National Preserve); Joshua
Tree is definitely my favorite area from Sept to May. Interesting thing
is that in Joshua Tree the trails shown are more accurate and extensive
than on the latest USGS topos.
When not within such coverage areas, I have found the US Topo series
(roughly equivalent to USGS 100,000:1 maps to be surprisingly useful.
Phil
| |
| David Lee 2005-12-06, 5:48 pm |
| Tom Hemp wrote...
> Ok, then this function really is only useful when no barometric weather
> changes occur during a track. I must have been lucky the last few times I
> went out. Based on this I would purchase the 60C and NOT get the sensors,
> as I see from the 60C User's Manual you can get an elevation plot using
> the GPS altitude when viewing the track. I wish I had know this prior to
> purchasing.....
Tom
I think that you misunderstand how the altimeter works - the whole point of
auto-calibration is to correct for changes in barometric pressure.
GPS altitude is inherently inaccurate - about 3 times worse than horizontal
accuracy due to considerations of satellite geometry and so (in the absence
of WAAS corrections) errors can be of the order of 150ft. Unfortunately you
generally want to know your altitude to a greater accuracy than your
horizontal position so this is not very satisfactory. A barometric
altimeter is much more accurate, provided it is calibrated for the local
barometric pressure (ie the ambient pressure corrected to sea level - as
displayed on a weather map). That is why you should calibrate using either
a known altitude or barometric pressure. If you don't know either of these
then you should set to the current GPS altitude, as you have been doing.
However if you have disabled auto calibration then you are starting with a
potentially inaccurate altitude and (because Garmin idiocy means that you
can't turn off the barometer) this will drift further with changes in the
barometric pressure.
When auto calibration is enabled the receiver is continuously averaging the
barometric altitude with the current GPS value. The GPS value is given a
smaller weighting so effectively the barometric altitude is continuously
corrected to an averaged GPS value. This means that the altimeter can
accurately respond to rapid changes of altitude by measuring the change in
pressure but is corrected for longer term drifts due to weather related
changes in pressure giving you the best of both worlds.
If you travel some distance to your lake then it's important that you
calibrate the altimeter on arrival at the lake - even if you have to do this
using the GPS altitude. If you were to calibrate more accurately to a known
altitude before leaving home than any change in weather conditions with time
and location can lead to a large error in recorded altitude when arriving at
your start point. This error will be averaged out with time during your
trip, leading to a drift in apparent altitude during your paddling trip.
If you frequently return to the same place then your best bet would be to
record the altitude at a known point at the end of the day, when it should
be at it's most accurate (provided you haven't been subject to poor
satellite reception conditions of course) and use that value to calibrate
your GPS on your next visit. Even better take a measurement at the end of
each visit and average them all to give you an even more reliable altitude
reference point.
David
| |
| Phil Wheeler 2005-12-06, 5:48 pm |
| David Lee wrote:
> Tom Hemp wrote...
>
>
>
> Tom
>
> I think that you misunderstand how the altimeter works - the whole point of
> auto-calibration is to correct for changes in barometric pressure.
>
> GPS altitude is inherently inaccurate - about 3 times worse than horizontal
> accuracy due to considerations of satellite geometry and so (in the absence
> of WAAS corrections) errors can be of the order of 150ft.
Generally true. But if you have the 24K Topo maps loaded, you can read
the altitude based on the contours on the map (i.e., based on the
horizontal information). And that info is not subject to the vagaries
of a barometric altimeter. So I use the GPS 24K Topo alt data to reset
my wristwatch (or Thommen, if I'm using it) barometric altimeter.
But, as said in an earlier msg, most of my hiking these days is where
the 24K Topos have coverage .. a pretty unique situation due to the
relatively good coverage in So Calif.
In any case, I rely only on map and compass (with baro altimeter for
certain low-visibility situations) for land navigation. The 60C is more
for fun :)
Phil
| |
| Tom Hemp 2005-12-06, 11:49 pm |
| David, ok you have convinced me to try again. I will turn on auto
calibration and calibrate the altimeter when I start. Since I usually know
the lake level for the lake I am going to, I can use that at the start of a
paddle. I have just been getting such crazy elevation plots on the trips I
have been making, I was trying anything to fix it. And I was blaming this
on weather barometric changes, but who knows what really is happening.
Also, I am probably asking to much in terms of elevation accuracy based on
the last graph in the link you provided.
Thanks, Tom
| |
| Chull 2005-12-06, 11:49 pm |
| I was always of the opinion that 80 ft Straight Up is Much further than 80
ft on the level.
> Unfortunately you generally want to know your altitude to a greater
> accuracy than your horizontal position so this is not very satisfactory.
| |
|
|
David Lee wrote:
> If you travel some distance to your lake then it's important that you
> calibrate the altimeter on arrival at the lake - even if you have to do this
> using the GPS altitude. If you were to calibrate more accurately to a known
> altitude before leaving home than any change in weather conditions with time
> and location can lead to a large error in recorded altitude when arriving at
> your start point.
If auto calibration is turned on, change in the barometric pressure
would not be the cause of error at the end of a long drive. Auto
calibration works much faster than the fastest barometric pressure
change I've ever observed. However, a vehicle traveling at high speed
probably would be pressurized by air being forced into the interior
through the ventilation system. With auto calibration turned on, the
unit would try to adjust the altimeter to GPS altitude for the
pressurized condition inside the vehicle. Upon arriving at the
destination and getting out of the vehicle, the unit would experience
an immediate and sizeable pressure drop to the actual ambient pressure.
That *would* introduce an error that would take auto calibration a
while (probably 30 minutes to an hour) to readjust it to GPS altitude
(unless you recalibrate manually).
| |
|
| Roy wrote:
> However, a vehicle traveling at high speed
> probably would be pressurized by air being forced into the interior
> through the ventilation system. With auto calibration turned on, the
> unit would try to adjust the altimeter to GPS altitude for the
> pressurized condition inside the vehicle. Upon arriving at the
> destination and getting out of the vehicle, the unit would experience
> an immediate and sizeable pressure drop to the actual ambient pressure.
Your car must have a more impressive ventilation system than any I've
seen. :)
I've never observed any clear and reproducible change in barometric
altimeter reading upon opening a car door or changing speeds.
| |
| David Lee 2005-12-07, 5:53 am |
| Roy wrote...
> If auto calibration is turned on, change in the barometric pressure
> would not be the cause of error at the end of a long drive. Auto
> calibration works much faster than the fastest barometric pressure
> change I've ever observed. However, a vehicle traveling at high speed
> probably would be pressurized by air being forced into the interior
> through the ventilation system. With auto calibration turned on, the
> unit would try to adjust the altimeter to GPS altitude for the
> pressurized condition inside the vehicle. Upon arriving at the
> destination and getting out of the vehicle, the unit would experience
> an immediate and sizeable pressure drop to the actual ambient pressure.
> That *would* introduce an error that would take auto calibration a
> while (probably 30 minutes to an hour) to readjust it to GPS altitude
> (unless you recalibrate manually).
An error of 100ft corresponds to a pressure error of only about 4mB - that
sort of change in barometric pressure seems reasonable to me over the
distance of a car journey for a day out. Autocalibration is fine if you are
moving slowly (walking, cycling, canoeing etc) but no good at all for high
speed travel. I despair at the total idiocy of Garmin engineers who specify
soft/firmware. It's blindingly obvious to anyone with greater intelligence
than a termite that there are many cases when GPS altitude is most
appropriate - not to mention the obvious pressurised aircraft!
I expect some sycophantic Garmin apologist will accuse me of being a "Garmin
bashing troll" again for pointing out this obvious stupidity!
David
| |
|
|
peter wrote:
> Your car must have a more impressive ventilation system than any I've
> seen. :)
> I've never observed any clear and reproducible change in barometric
> altimeter reading upon opening a car door or changing speeds.
To be honest, I've never tried it without the ventilation fan being
turned on, so I would concede that the pressure increase resulting
solely from air being forced into the interior by driving at high speed
might not be enough to produce a large effect. When I've observed the
difference has always been with the ventilation fan on. Just a moment
ago, sitting in my parked car with the fan off and windows closed, the
barometric altimeter read 5300 ft (plus or minus a couple). I turned
the ventilation fan on to "High" and the reading dropped to 5280 in a
couple of seconds. Turned the fan back off and the reading rebounded
in a couple of seconds to 5300 again. Opening a window with the fan on
"High" also allowed the reading to rebound to 5300 in a couple of
seconds. I tried turning the fan on and off several times. Definitely
reproducible. The effect was less than I thought I remembered, but it
is observable.
I didn't have the engine on because I was in a closed garage. When the
engine is running and the alternator is generating electricity, the fan
runs noticeably faster, which might increase the effect a little. And
when driving at highway speed, the system would get a further boost
from the air being forced into the intake. I don't think it's too
far-fetched to assume that these additional effects could change the
reading by another 10 feet.
So I still contend that if a person is driving in a closed car at high
speed with the ventilation fan on high, it will affect the barometric
altimeter enough to notice, if perhaps not enough to matter. Switching
the controls from fresh air intake to recirculate (with the fan on) or
opening a window at highway speed also have definite effects. And I
don't mean a few feet; I'm talking tens of feet.
| |
|
|
David Lee wrote:
> An error of 100ft corresponds to a pressure error of only about 4mB - that
> sort of change in barometric pressure seems reasonable to me over the
> distance of a car journey for a day out.
I agree; but what makes you think auto calibration can't take care of
that? It can adjust a deliberate miscalibration of 100 feet in about
an hour. (I've tried it.) It could easily do it over the length of a
day.
> Autocalibration is fine if you are
> moving slowly (walking, cycling, canoeing etc) but no good at all for high
> speed travel.
I don't understand why you think that.
> I despair at the total idiocy of Garmin engineers who specify
> soft/firmware. It's blindingly obvious to anyone with greater intelligence
> than a termite that there are many cases when GPS altitude is most
> appropriate - not to mention the obvious pressurised aircraft!
>
> I expect some sycophantic Garmin apologist will accuse me of being a "Garmin
> bashing troll" again for pointing out this obvious stupidity!
Well, I'm not one of them. I am among the many who have complained to
Garmin about it.
| |
| David Lee 2005-12-08, 5:49 pm |
| Roy wrote...
>
> I agree; but what makes you think auto calibration can't take care of
> that? It can adjust a deliberate miscalibration of 100 feet in about
> an hour. (I've tried it.) It could easily do it over the length of a
> day.
You are totally missing the point. There is no doubt that the receiver will
have calibrated itself by the end of the day (assuming decent satellite
reception). Tom was complaining about the difference in reported altitude
between starting his kayaking/hiking trips and returning to the same point
at the end of the day. I was assuming that his problem was that his
receiver had not had sufficent time to adjust its altimeter calibration for
differences in barometric pressure encountered during the high speed car
journey from his home to his starting point so that the altitude reported at
the start of the trip would be in error - then becoming increasingly
accurate throughout the day. However on re-reading his original posting I
note that he didn't say whether the receiver had been switched on at all
during the car trip. If not then there could be an even larger error when
he turned it on at the start of his trip, since the latest calibration could
be out of date by days or weeks as well as referring to a different
location.
[color=darkred]
> I don't understand why you think that.
Well think about it a bit harder! The barometric altimeter measures ambient
pressure and so can give you a very accurate indication of altitude provided
that you know the local barometric pressure so that it requires constant
recalibration to a known pressure or altitude to correct for weather related
changes in pressure. The GPS altitude on the other hand measures the
altitude directly and so will always be measuring the correct altitude.
However GPS altitude is very inaccurate and the errors can vary rapidly with
satellite reception conditions and so can only give you a high degree of
accuracy if you average the measurements over a sufficiently long period of
time. Clearly autocalibration of the altimeter can only succeed provided
that the barometric pressure does not change significantly over the
averaging period. Since pressure changes at a given location generally
occur fairly slowly, this is not normally a problem provided you do not move
too far during the averaging period. However when travelling at high speed
you may be moving sufficiently rapidly between areas of differing barometric
pressure such that the calibration can never come to equilibrium and all
altitude measurements during the journey will be inaccurate to an unknown
and varying degree.
David
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David Lee wrote:
> Clearly autocalibration of the altimeter can only succeed provided
> that the barometric pressure does not change significantly over the
> averaging period. Since pressure changes at a given location generally
> occur fairly slowly, this is not normally a problem provided you do not move
> too far during the averaging period. However when travelling at high speed
> you may be moving sufficiently rapidly between areas of differing barometric
> pressure such that the calibration can never come to equilibrium and all
> altitude measurements during the journey will be inaccurate to an unknown
> and varying degree.
I think you have overestimated the variation of barometric pressure
with distance. Over a distance of 25 miles, the pressure varies hardly
at all. Even over a distance of 75 miles, which is about the maximum
distance most people could realistically drive in an hour, the
variation would be small enough that any change due to distance
travelled would be sufficiently slow for the auto calibration to adjust
within its normal limitations. Even if distance is combined with a
changing barometer, the rate of change would still be gradual enough
for auto calibration to adjust for.
I agree that it is likely that the altimeter is likely to be out of
calibration at the end of a long, high-speed drive if the unit is on
with auto calibration; but not because of change in barometric pressure
with distance. I contend that the miscalibration would be due to false
ambient pressure experienced by the unit due to pressurization of the
interior by the ventilating system or the effects of an open window.
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| Tom Hemp 2005-12-09, 5:49 pm |
| I went through three stages with altimeter calibration:
1.) I had been setting the altitude from my home which I know is accurate to
also setting the barometric pressure from my home barometer that is very
accurate prior to going kayaking. I tried both ways on different trips. I
would then turn on the unit and drive to the lakes(10-40 miles) depending on
which lake. You all in this forum informed me this was not the best way to
get accurate readings based on various things so I move to step 2.
Auto calibration was on.
2.) I left the unit off till I got to the lake and then entered the lake
level which I looked up prior to going paddling to calibrate the altimeter.
Auto calibration was on.
In doing the above two ways I was getting very strange track profiles,
meaning my starting altitude usually never matched when I returned to the
take out on the lake. I would take hikes at various points on the lake and
usually when I got back to the boat, again the elevation was off. Sometimes
by hundreds of feet. Here is an example of a track with calibration done at
the put in based on the lake elevation.
http://www.thehemps.com/ JordanLake...
IGPS.gdb
3.) At the lake put in, I would calibrate the altimeter by answering no to
elevation and pressure and I turned auto calibration OFF.
I then went to step 3 based on a miss-read of how this all works. I thought
when you answered no to elevation and pressure on calibration then it would
use GPS elevation. I was wrong on that. But when I did this the last two
trips I got very good results as can be seen by this.
http://www.thehemps.com/ FallsLakeH...50II
.gdb
Tom
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| Good idea making the tracks available. Using them I was able to figure
out what was going on, at least in this case.
Tom Hemp wrote:
> Here is an example of a track with calibration done at
> the put in based on the lake elevation.
> http://www.thehemps.com/ JordanLake...
IGPS.gdb
I looked up the elevation for B. Everett Jordan Lake in Natl Geographic
Backwoods Explorer and on several web sites. (Here's one:
<http://en.wikipedia.org/wiki/Jordan_Lake> All sources gave the
surface elevation as around 216 or 217 ft. Yet the elevation at the
beginning of your track is about 150. So it seems that the starting
elevation you entered at put in was about 67 feet lower than the
correct one.
Looking at your track, it appears that auto calibration gradually
brought the elevation up over the course of the day until at the end,
the altimeter was indicating around 250 ft, which is about 32 ft above
the correct elevation of 217. That difference could be due to error in
the GPS elevation being on the high side at that time. As you may
recall from Chris Malcolm's web page, if given enough time, auto
calibration eventually causes the barometric altimeter to follow the
6-hour cycle of variation in GPS elevation, but at reduced amplitude.
Correct me if you think I'm wrong, but I believe that solves the
mystery of why your ending elevation was 100 feet above your starting
elevation. In this case, the source of the error was your initial
input of a seriously incorrect elevation, and the reason your track
looks strange is because auto calibration corrected the error.
There are a couple of things about this that surprise me. One is the
length of time it took for auto calibration to correct the initial
miscalibration. I once did an experiment with my 76CS in which I
deliberately miscalibrated the altimeter by 100 feet. 70 minutes later
it had adjusted the barometric altimeter to match the average GPS
elevation (which I plotted from readings taken every 2 minutes). I
don't know why it seems to have taken yours so much longer.
The other thing that is strange is at 5.25 miles on your track, the
elevation jumps from 205 ft at point 335 to 251 ft at point 336, just
14 seconds later. It looks like you were on shore at the time,
returning from waypoint "POND". Is it possible that something at that
location could have interfered with the satellite reception? Can you
think of anything else that might have happened at that location?
> 3.) At the lake put in, I would calibrate the altimeter by answering no to
> elevation and pressure and I turned auto calibration OFF.
>
> I then went to step 3 based on a miss-read of how this all works. I thought
> when you answered no to elevation and pressure on calibration then it would
> use GPS elevation. I was wrong on that. But when I did this the last two
> trips I got very good results as can be seen by this.
> http://www.thehemps.com/ FallsLakeH...50II
.gdb
In this case the starting elevation, set equal to GPS elevation by your
calibration, was pretty close to the lake elevation I get from
Backwoods Explorer. But since you didn't have auto calibration on, you
could have entered any value you wanted to, and the profile would still
*look* good *relative to itself*, because with auto calibration off,
the main source of error is change in barometric pressure. (It must
have been pretty stable that day.) Of course I'm not suggesting that
people calibrate to an assumed elevation; just trying to illustrate a
point. In this case, I think you would have gotten as good or better
results if you had auto calibration on, because your initial
calibration was to GPS elevation, so it was in the ballpark. Without a
large calibration error to begin with, your tracks should look much
better.
To avoid problems and get better results in the future, here is what I
suggest:
If you don't know the starting elevation, just calibrate to GPS
elevation, which is what you were doing in this case (step 3), but turn
autocalibration on. In any one reading of GPS elevation, the error may
be larger than you would like, but with auto calibration on, it will
home in on a more average GPS elevation in probably 30 minutes.
If you believe that you know your starting elevation, compare it to the
GPS elevation reported by your receiver before calibrating the
altimeter. That should help you avoid large errors such as the one in
the case at B. Everett Jordan Lake.
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