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Cellular forums Home > Archive > Garmin GPS > March 2006 > 2 Questions: Etrex Legend Altimeter Accuracy and which GPS to get: GPSMAP 60CSx or GPSMAP 60Cx
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2 Questions: Etrex Legend Altimeter Accuracy and which GPS to get: GPSMAP 60CSx or GPSMAP 60Cx
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| I have an Etrex Legend and I have always been wondering about it's
accuracy when it comes to altitude. Does anyone know how INaccurate it
is?
In addition I am thinking of getting another GPS with SiRF. I really
like the feature of the barometric altimeter and electronic compass,
but I have a manual very good compass and I don't know if the accuracy
of the altimeter makes that much of a difference since I am a not a
mountain climber. But, I really like the all in one, but there is a
price issue attached to the higher-end model.
Any suggestions?
Thanks
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| Jesse wrote:
> I have an Etrex Legend and I have always been wondering about it's
> accuracy when it comes to altitude. Does anyone know how INaccurate it
> is?
It can be way off if you have poor GPS reception (i.e. only 3
satellites or even more if the geometry is bad). So it's important to
check the satellite page if you're depending on the altitude to be
accurate. This is a good idea in any event since if the altitude is
off then the horizontal position will also be degraded.
But when I've been logging positions at sites of known elevation and
have at least 5 satellites received then 95% of my readings have been
within 35'.
This was with an eMap, but the receiver/antenna components are the same
according to Garmin's techsupport.
>
> In addition I am thinking of getting another GPS with SiRF. I really
> like the feature of the barometric altimeter and electronic compass,
> but I have a manual very good compass and I don't know if the accuracy
> of the altimeter makes that much of a difference since I am a not a
> mountain climber. But, I really like the all in one, but there is a
> price issue attached to the higher-end model.
Garmin has a good altimeter design combining the barometric sensor and
GPS for automatic calibration. But their magnetic compass sensor isn't
so good since it requires the unit to be held very level for reasonable
accuracy. To get SiRF you'd need to get the 60CSx or the 76 CSx. One
of the Lowrance models might be an option as well.
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| dold@XReXX2XQue.usenet.us.com 2006-03-17, 11:48 pm |
| In sci.geo.satellite-nav Arthur Hass <ed@ridersite.org> wrote:
> I have use a map76CS extensively making elevation profiles of running
> courses. The barometric altimeter is much more accurate than reported
> GPS altitude.
I would say more reliable than any one instantaneous GPS altitude.
It has less variation than GPS altitude, because it is not subject to
satellite vagaries.
It has more variation than a normalized series of GPS readings because it
is affected by barometric changes brought about by weather.
Over time, the auto calibration seeks to bring the indicated altitude in
accordance with the GPS altitude.
If the automatic calibration is turned off, the altitude can rise or drop
over a hundred feet with the passing of a rain storm while the unit is
sitting still.
> If the automatic calibration is turned off, these instances are almost
> non-existent. So, if I'm making a profile over a short period of time,
> I calibrate the altimeter at a known elevation and set automatic
> calibration to off.
If you are sitting in one spot with good GPS reception, might you just
accept the displayed altitude and turn calibration off?
Someone who is used to using any sort of barometric altimeter might be
impressed with the altimeter in a GPS, but I was dismayed to find that it
needs calibration far more often than I ever would have believed.
I have a GPSMap 60cs, and I recently made a track where an elevated
highway appears to be below sea level near the San Francisco bay.
I like the compass, although I usually have it turned off, to conserve
batteries, since it is useless if not held flat, or for automotive use.
Holding the page key turns it on, if I want to orient myself properly while
standing still. The Sight-and-Go feature is useful.
If I were buying today, I might forego the "s" model, and put the money
toward an "x" instead (which came out weeks after I got my cs).
http://www.tvnav.com/60cx.htm
--
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Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5
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| Thanks for everyone's feedback. BTW, how do you calibrate the
barometric altimeter?
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| Hmm. How often does one have to recalibrate?
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| dold@XReXX2XQue.usenet.us.com wrote:
:
: Over time, the auto calibration seeks to bring the indicated altitude
: in accordance with the GPS altitude.
Does this mean that it will be the same as the GPS [estimated] altitude?
:: If the automatic calibration is turned off, these instances are
:: almost non-existent. So, if I'm making a profile over a short
:: period of time, I calibrate the altimeter at a known elevation and
:: set automatic calibration to off.
If one has the GPS unit off and calibration off, does one have to
calibrate it again?
:
: If you are sitting in one spot with good GPS reception, might you just
: accept the displayed altitude and turn calibration off?
With the new SiRF units, you will tend to get good reception so the BA
is redundant then?
:
: Someone who is used to using any sort of barometric altimeter might be
: impressed with the altimeter in a GPS, but I was dismayed to find
: that it needs calibration far more often than I ever would have
: believed.
:
: I have a GPSMap 60cs, and I recently made a track where an elevated
: highway appears to be below sea level near the San Francisco bay.
It sounds like it's using the GPSs altitude since this is a similar
response I get with me Etrex Legend.
:
: I like the compass, although I usually have it turned off, to conserve
: batteries, since it is useless if not held flat, or for automotive
: use. Holding the page key turns it on, if I want to orient myself
: properly while standing still. The Sight-and-Go feature is useful.
I would tend to use the GPS in the car (most of the time).
:
: If I were buying today, I might forego the "s" model, and put the
: money toward an "x" instead (which came out weeks after I got my cs).
: http://www.tvnav.com/60cx.htm
I definitely want the x version, but I am still unclear if I should pay
the extra and go with the S(ensor) unit.
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| ***Correction: If one has the GPS unit off and calibration ***ON***,
does one have to
calibrate it again?
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| Jesse 2006-03-18, 11:48 pm |
| Thanks
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| Jesse wrote:
> dold@XReXX2XQue.usenet.us.com wrote:
> :
> : Over time, the auto calibration seeks to bring the indicated altitude
> : in accordance with the GPS altitude.
>
> Does this mean that it will be the same as the GPS [estimated] altitude?
Not exactly. The strictly GPS-determined altitude will jump around
more from one moment to the next. Auto-calibration works over about a
30 minute time constant so after some period with auto-calibration on
it'll indicate the altitude that the GPS alone *would show if allowed
to average for about 30 minutes*.
>
> :: If the automatic calibration is turned off, these instances are
> :: almost non-existent. So, if I'm making a profile over a short
> :: period of time, I calibrate the altimeter at a known elevation and
> :: set automatic calibration to off.
>
> If one has the GPS unit off and calibration off, does one have to
> calibrate it again?
With the auto-calibration off you'd want to calibrate any time you
think the barometric pressure may have changed
> :
> : If you are sitting in one spot with good GPS reception, might you just
> : accept the displayed altitude and turn calibration off?
>
> With the new SiRF units, you will tend to get good reception so the BA
> is redundant then?
SiRF doesn't do anything that magical to reception. If most satellites
are hidden behind a mountain then the accuracy of the GPS solution will
still be degraded.
> :
> : Someone who is used to using any sort of barometric altimeter might be
> : impressed with the altimeter in a GPS, but I was dismayed to find
> : that it needs calibration far more often than I ever would have
> : believed.
> :
> : I have a GPSMap 60cs, and I recently made a track where an elevated
> : highway appears to be below sea level near the San Francisco bay.
>
> It sounds like it's using the GPSs altitude since this is a similar
> response I get with me Etrex Legend.
I've noticed that the geoid-ellipsoid table in my eMap is off by about
16' relative to the USGS value for the SF Bay area. Tends to make the
unit say that most of my kayaking is done underwater. You can check
the NMEA GGA sentence to see what value the unit is using to convert
between the ellipsoid and geoid heights.
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| dold@XReXX2XQue.usenet.us.com 2006-03-19, 5:48 pm |
| In sci.geo.satellite-nav peter <prathman@comcast.net> wrote:
> SiRF doesn't do anything that magical to reception. If most satellites
> are hidden behind a mountain then the accuracy of the GPS solution will
> still be degraded.
My 60cs is much better than my eTrex, but the 60cx is supposed to be so
much better than the 60cs that, by inference, my eTrex should be virtually
unable to get a lock on the flat plains of Mongolia.
I do recall that the eTrex wasn't as good as a GPS12, but they all seem to
work in normal usage for me.
> I've noticed that the geoid-ellipsoid table in my eMap is off by about
> 16' relative to the USGS value for the SF Bay area. Tends to make the
> unit say that most of my kayaking is done underwater. You can check
> the NMEA GGA sentence to see what value the unit is using to convert
> between the ellipsoid and geoid heights.
That would explain I-80 in Richmond being 7 feet underwater. I hadn't
considered that the internal references would be incorrect that close to a
major body of water. I just thought it was an anomaly of the GPS altitude,
not improved by the barometer.
--
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Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5
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| GSV Three Minds in a Can 2006-03-19, 5:48 pm |
| Bitstring <dvk91p$136$1@blue.rahul.net>, from the wonderful person
dold@XReXX2XQue.usenet.us.com said
>In sci.geo.satellite-nav peter <prathman@comcast.net> wrote:
>
>
>My 60cs is much better than my eTrex, but the 60cx is supposed to be so
>much better than the 60cs that, by inference, my eTrex should be virtually
>unable to get a lock on the flat plains of Mongolia.
Only if there is a leaf on the horizon did my eTrex have trouble. Flat
plains, no leaves, no problem.
My 60C was 10x better than my original eTrex. The 60CSx is only 5x
better than the 60C. There is not much scope for more improvement - when
you can lock everything above the horizon, you're about done (yes, at
the bottom of a deep canyon you are stuffed - welcome to geometry).
<snip>
>
>That would explain I-80 in Richmond being 7 feet underwater. I hadn't
>considered that the internal references would be incorrect that close to a
>major body of water. I just thought it was an anomaly of the GPS altitude,
>not improved by the barometer.
Sadly 'sea level' is a rather lumpy moveable feast. Even in a tiddly
little island(s) like the UK there are multiple datums required in order
to get 'mean sea level' somewhere close to where the water actually
lives.
--
GSV Three Minds in a Can
Contact recommends the use of Firefox; SC recommends it at gunpoint.
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| Chris Malcolm 2006-03-22, 11:48 pm |
| In sci.geo.satellite-nav Arthur Hass <ed@ridersite.org> wrote:
> Jesse wrote:
[color=darkred]
> No, it uses the GPS alt. to recalibrate - not sure how. But, it seldom
> re-calibrates in my experience. However, when it does, there is usually
> a spike (10-20 feet) in the elevation profile.
When good enough GPS altitude is available, it calibrates continuously
at every reading, i.e. every sec or whatever the reading rate is. It
temporarily stops calibrating if it loses 3D lock and goes to 2D, or
if the EPE is too large. When good 3D lock becomes available again it
compensates for the temporary loss by a temporary period of very rapid
recalibration. This might be what caused the spikes you observed.
> It takes an hour or more to hone in on
> the correct altitude when set to auto-calibrate (if off significantly
> when turned on and not calibrated).
It's unlikely to take more than half an hour. For an explanation of
how it works and graphs of the performance of autocalibration see
http://www.dai.ed.ac.uk/homes/cam/g.../altgraphs.html
--
Chris Malcolm cam@infirmatics.ed.ac.uk +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]
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