Re: T&T: GPS elevation
Some GPS machines may give elevation but I'm not sure I would trust the
results. This afternoon I placed my venerable Garmin 48 on my front
porch to see if I would have waterfront property when Global warming
melts the icecaps. The GPS 48, in most respects an admirable handheld,
located the lat, long coordinates of the house with great precision but
told me that it was at an elevation of 26 feet. Since we live on the
crest of a small hill, a fact that I am reminded of everytime I slip
slide my way down the driveway after every minor snowfall or ice storm,
I am sure that the indication was wrong. I measured the elevation a
couple of hours later and this time I got a reading in excess of 600
feet, surely another error.
A topo map of the ares shows the actual elevation as about 486 ft. Even
if the icecaps all melt, the sea level would only rise about 75 ft. I
might as well scrap the plans for building a dock in front of my house.
Larry Z
-----Original Message-----
From: Lawrence Zeitlin
Some GPS machines may give elevation but I'm not sure I would trust the
results. >snip< The GPS 48, in most respects an admirable handheld,
located the lat, long coordinates of the house with great precision but
told me that it was at an elevation of 26 feet.
A topo map of the ares shows the actual elevation as about 486 ft.
REPLY
A long time ago I was talking to Garmin on the subject of altitude
readouts.
I was informed that the altitude calculation is a very low priority on
marine application products.
If you compare an eTrek used for hiking etc. you will find it gives
somewhat better results because the altitude calculations has a higher
priority in that model.
Bottom line, unless you use a survey grade GPS with the relevant
differential correction algorithms you are not going to get decent
accuracy to the nearest meter.
In view of the fact a meter plus or minus is a significant variable when
talking about water front property the whole exercise is a moot point.
Sorry Roger; yet another OT thread derailed with logic. <VBG>
Now to get this thread back on track for marine related use, The
University of New Brunswick did some field tests using GPS to measure
wave heights in Bay of Fundy. Quite a feat given the extremely high tide
range. But the tests were successful. This is the kind of
instrumentation used for ODAS buoys that report wave heights offshore.
You didn't think they used a depth sounder did you? <VBG>
BTW don't use precise data unless the HDOP value is less than 1.5 on
your GPS.
And if you can't access the HDOP (Horizontal Dilution Of Precision)
value on your display all bets are off regarding accuracy closer than 10
meters.
Cheers
Arild
(SNIP) A long time ago I was talking to Garmin on the subject of altitude
readouts. I was informed that the altitude calculation is a very low
priority on marine application products. Arild
I'm surprised they seem to differentiate chips for hiking, marine, and
flying. Seems to me I recall Garmin makes a lot of aeronautical GPS
instrumentation where elevation would be a understandable priority and I'm
amazed they wouldn't simply use the same chip-set?
Regards....
Phil Rosch
Old Harbor Consulting
M/V "Curmudgeon" MT44 TC
Currently lying Bond Creek, NC
Trust me Phil, their aviation products are no better at calculating altitude
than their marine products! If you think about the geometry involved, to
calculate lat-long, they need a good number of satellites positioned around
the clock nearly overhead. But to calculate altitude, they need a good
number of satellites positioned down low nearly at the horizons. The
cheaper handheld designs are not optimized to pick up the weaker signals of
satellites far away almost at the horizon. So most designs attempt to
calculate altitude with stronger satellite signals emanating from nearly
overhead, and that produces wide variability and many errors.
Bob Peterson
-----Original Message-----
From: trawlers-and-trawlering-bounces@lists.samurai.com
[mailto:trawlers-and-trawlering-bounces@lists.samurai.com] On Behalf Of
trawlerphil
Sent: Monday, March 13, 2006 8:07 AM
To: trawlers-and-trawlering@lists.samurai.com
Subject: Re: T&T: GPS elevation
(SNIP) A long time ago I was talking to Garmin on the subject of altitude
readouts. I was informed that the altitude calculation is a very low
priority on marine application products. Arild
I'm surprised they seem to differentiate chips for hiking, marine, and
flying. Seems to me I recall Garmin makes a lot of aeronautical GPS
instrumentation where elevation would be a understandable priority and I'm
amazed they wouldn't simply use the same chip-set?
Regards....
Phil Rosch
Old Harbor Consulting
M/V "Curmudgeon" MT44 TC
Currently lying Bond Creek, NC
bob@peterson.org
At 08:29 AM 3/13/2006 -0800, you wrote:
Trust me Phil, their aviation products are no better at calculating altitude
The last time I tried a Garmin Map 76S on a SW Airlines flight, I got
about 31,787 feet at cruise. I would estimate that our actual
altitude was 32,000. I figured that was pretty good accuracy. Speed
was about 400 knots and the speed update was a bit slow, after
throttle changes which you can hear.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
-----Original Message-----
................. If you think about the geometry involved, to
calculate lat-long, they need a good number of satellites positioned
around the clock nearly overhead. But to calculate altitude, they need a
good number of satellites positioned down low nearly at the horizons
Bob,
You really don't what the satellites overhead if you are looking for an
accurate lat/lon position, while you do want them overhead if you are trying
to determine altitude!
GPS lat/lon positions are calculated by first calculating a circle of
position for the receiver relative to each satellite, and then looking for
where the circles intersect.
To calculate a more accurate lat/lon, you want the satellites to be as close
to the horizon (but widely spaced around the compass) as you can, so the
circles of position intersect as close to 90 degrees as possible, not with
the birds overhead. This is no different from doing celestial navigation,
where the ideal celestial bodies to shoot are not overhead, but are widely
spaced. While GPS and celestial computations are radically different in how
they calculate the circles of position, getting a lat/lon position from the
circles is done in a similar fashion.
Kevin Redden
-----Original Message-----
From: trawlerphil
(SNIP) A long time ago I was talking to Garmin on the subject of
altitude readouts. I was informed that the altitude calculation is a
very low priority on marine application products. Arild
I'm surprised they seem to differentiate chips for hiking, marine, and
flying. Seems to me I recall Garmin makes a lot of aeronautical GPS
instrumentation where elevation would be a understandable priority and
I'm amazed they wouldn't simply use the same chip-set?
REPLY
Aeronautical equipment has to undergo rigorous testing and certification
before it is type approved. By comparison consumer goods do not require
such effort. There are two distinct function blocks of software inside
any GPS receiver. The reception of signal and calculation of essential
data. Then there is the display software or GUI that drives the
particular display on each piece of equipment. For instance the software
that drives the pixels showing a little mannikin planting a flag every
time you mark a waypoint or save a MARK in the eTREK model does not
exist in a chart plotter.
And for marine equipment there is a data stream formatted for NMEA
output. Not needed in Aeronautical applications.
When I said priority, what I was referring to was which piece of
information was calculated first by the numeric processor. Lat Long
comes first, then speed, direction, bearing to waypoint, display of
time, altitude etc.
Marine applications is essentially 2 dimensional so altitude is lowest
priority, whereas in aeronautical applications altitude is a much higher
priority.
The physical chips may be identical but not necessarily the software.
Regards
Arild
I have never had any confidence in the altitude readings on our little
Garmin 128 because it never says zero. Even with a tide change I would
expect it to say zero once in awhile if we are on salt water <g>. Anyway,
last summer we were in the Gulf of Alaska in 8 to 10 foot swells on the
beam. The swells were far enough apart that the motion was mostly "elevator
up - elevator down" However, we took an occasional deep roll and all eyes
would go to the Inclinometer (Divorce-o-meter, as it is affectionately
called in our family.) We have certainly been in worse so we just looked
for small distractions to pass the time. Watching the elevation change on
the GPS as we climbed and slid down each hill was entertaining. It seemed
fairly accurate by our eyeball estimation and the motion was slow enough
that it kept up pretty well.
Cheap Thrills.
Eric Thoman
Abyssinia
www.kayaktransport.com
Monday, March 13, 2006, 8:29:47 AM, Bob wrote:
BP> Trust me Phil, their aviation products are no better at calculating altitude
BP> than their marine products! If you think about the geometry involved, to
BP> calculate lat-long, they need a good number of satellites positioned around
BP> the clock nearly overhead. But to calculate altitude, they need a good
BP> number of satellites positioned down low nearly at the horizons. The
BP> cheaper handheld designs are not optimized to pick up the weaker signals of
BP> satellites far away almost at the horizon. So most designs attempt to
BP> calculate altitude with stronger satellite signals emanating from nearly
BP> overhead, and that produces wide variability and many errors.
BP> Bob Peterson
It is true that the altitude error is greater than the horizontal error, but you have the directions mixed up.
GPS works by calculating the distance from each of several satellites. To get best horizontal accuracy, you want the satellites distributed around the horizon, and for best vertical accuracy, you want the satellites overhead (or underneath - but the earth gets in the way).
Because we don't have access to satellite signals from all directions (including down), vertical errors are typically about three times the horizontal error, if I recall correctly.
The elevation is calculated as part of the general position calculation, not as a separate item, so I don't believe the claim that "Garmin puts a lower priority on altitude calculations of marine units."
--
Peter Bennett, VE7CEI Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca