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Author Topic:   GPS Error
Jersey Jim posted 04-05-2001 11:36 AM ET (US)   Profile for Jersey Jim   Send Email to Jersey Jim  
Installed the new Lowrance X15mt with gps package. Estimated position error is at best 40 feet, sometimes as high as 70 to 80 feet.When I compare my hand-held Garmin unit it tracks the same satellites but with estimated position error as low as 12 feet at the same time the Lowrance shows epe of 45ft. Lowrance tells me the unit is within
acceptable range so there is nothing I can do.
I don't know anything about this but I know the members of this board will tell me if I should be concerned. I can't believe a $100 hand-held is more accurate.
Thanks for your suggestions
Peter posted 04-05-2001 02:11 PM ET (US)     Profile for Peter  Send Email to Peter     
One question to ask is does Lowrance and Garmin calculate EPE in the same way? Is the determination of EPE done through with a standard algorithm used by all manufacturers? Is it safe to assume that the Lowrance has a 12 channel receiver? FYI, my 8 channel Garmin handheld now that SA is turned off will obtain about 40 EPE at best. My 12 channel Garmin handheld will usually achieve 11 EPE at best under the same conditions.
triblet posted 04-05-2001 05:17 PM ET (US)     Profile for triblet  Send Email to triblet     
The different manufacturers use different
algorithm's to come up with their accuracy
number, which is generally wildly
pessimistic. And they absolutely positively
won't discuss how they calculate it.

DOP (dilution of precision) is, I belive a
standard algorithm based on satellite
geometry.

Consumer GPS units of the same type all have
about the same accuracy on the open ocean.
There are three
basic types on the market now: 12-channel
w/o any differential, 12 ch + DGPS, 12 ch +
WAAS. The first is the least accurate (the
spec is I think 15 m 95% of the time, and
practice more like 10 m 95%). DGPS is
1-5 m 95%, WAAS 3m 95%. The jury is still
out on whether DGPS or WAAS is more accurate,
but either one lets me nail the top of Eric's
Pinnacle (about 10' across).

Nonconsumer units (survey grade stuff) can
be a good deal more accurate, but would be
ver expensive (10K) overkill on a boat.

The units do vary in their abilities under
tree cover, but I don't remember the details

Chuck

lhg posted 04-05-2001 07:20 PM ET (US)     Profile for lhg    
Chuck - My older (1995) Lowrance GPS is a five channel. Would this be a lot less accurate than a 12 channel? I do have a Differential receiver added to it.

Incidentally, I have recently entered waypoint Lat/Lons, saved ON LOCATION from a previous Lowrance LORAN, into my 12 channel Lowrance DGPS, and found the old Loran waypoints to be off by as much as 1/2 mile! I am amazed at the difference. I had thought Loran was pretty accurate on an absolute Lat/Lon basis.

triblet posted 04-06-2001 09:06 AM ET (US)     Profile for triblet  Send Email to triblet     
Your old unit should be close to as accurate
as the current 12-channel units, but not
quite. 5 birds is sufficient for a fix.
If there are more, as 12-channel receiver
will have some redundancy. With DGPS,
your 5-channel unit should be fine.

LORAN isn't terribly accurate as far as the
lat/long, but is quite repeatable. In other
words, it's consistently inaccurate. The
repeatability made it popular with fishermen
because they could go back to the same site
over and over. In the days when GPS SA was
on, LORAN was far more repeatable than GPS.
I don't hear anyone saying that anymore, and
I keep expecting the Coasties to shut down
LORAN-C. Plan of Record was 12/31/00, but
I don't think it happened.

Chuck

bigz posted 04-06-2001 09:56 AM ET (US)     Profile for bigz    
As Chuck pointed the only thing 4 or more channels gives you is redundancy they do not make the GPS more accurate, they do provide back up and error correction if it looses contact with any of initial satellite fixes --- adding DGPS to any GPS and I'm sure the new "fuzzy WAASIE" will provide excellent positioning.


Larry in a simple statement no to the 5 vs. 8 or 12 --- 5 coupled with your DGPS set up, like ours, is highly accurate ------ the LGC-1 GPS receiver can actually pick up more than 5 satellites (tracks all that are in view) but it only uses up to 4 to determine your position.

We experienced great accuracy on the trip up the ICW last Spring with no DGPS (had 1200 plus miles to play with it)! When we installed DGPS (which by the way can only use one one station at a time even if there is an overlap of coverage) later on it was amazing the pin point accuracy --did some testing last summer. If I can locate my notes will post them.

We have a shorter trip in store this Spring only about 600 miles and we'll have Loran, GPS with DGPS --- should be interesting --- (chuckle --- if the Atlantic cooperates we will have a shorter and faster trip -- outside than inside after Hatteras)

Maerd posted 04-06-2001 11:13 AM ET (US)     Profile for Maerd    
I had saved a site from a earlier posting that has GPS data www.joe.mehaffey.com and has a forum. I would be interested in knowing what handheld / fixed GPS everyone would choose if money is not a factor. New Leica any good? I currently do not own one but will probably buy a handheld this season. Thanks
triblet posted 04-06-2001 04:56 PM ET (US)     Profile for triblet  Send Email to triblet     
Handheld: Garmin eTrex Vista. I have one on
backorder. WAAS. 24M flash builtin for
charts (that's a MESS of charts -- My 162
is more than accurate with 3M). Barometric
altimeter (yawn, I know how high the surface
of the ocean is), fluxgate compass.

The eTrex Legend is a good choice too (8M,
no altimeter, no fluxgate, UGLY iMac blue).

There's also a new GPSMAP 76 worth looking
at (not available yet, but RSN).

Chuck

Chuck Tribolet

caddis posted 04-07-2001 05:37 PM ET (US)     Profile for caddis  Send Email to caddis     
Jersey Jim,

Other than the gps accuracy, how do you like the new Lowrance? I am about to buy one. Here is what I am thinking, though. Like you, I have a handheld Garmin (a III+). The gps interface on the X-15 is NMEA. The gps kit for the X-15 is basically just an antenna. There should be a way to attach the Garmin to the X-15, using a self made cable, and have it furnish the signal to the X-15. It looks like I would have to buy an NMEA cable for each piece, cut the ends off, and attach them together to get them to fit the respective units. Does this make sense to anyone? I don't want to have to buy the Lowrance gps attachment until it is WAAS capable.

Just a thought.

Caddis

triblet posted 04-07-2001 06:56 PM ET (US)     Profile for triblet  Send Email to triblet     
That ought to work. Maybe. The GPS upgrade
for the Lowrances includes the GPS,
cartography on CD, an 8MB memory card, and
the PC interface cable. I'm not 100% certain
you'll actually be able to use it as a GPS.

I'm not a big fan of combo GPS/fishfinders.
When I'm looking for the dive site, I'm using
both the GPS and the depth finder
intensively. The GPS puts me on the
pinnacle, the fishfinder tells me where to
drop the hook. I have like having two
screens.

For the Garmin, get the power/data cable.
Then you don't have to cut the ends off.

Chuck

Maerd posted 04-07-2001 07:40 PM ET (US)     Profile for Maerd    
Just went to www.joe.mehaffey.com and they review the Garmin Vista / Legend and there is a link to worlds largest GPS store. The review is way over my head but I think I will buy the Vista. Thanks
Jersey Jim posted 04-08-2001 06:56 AM ET (US)     Profile for Jersey Jim  Send Email to Jersey Jim     
Caddis,
After fooling with the X-15 for a week, I don't know how to answer your question. After I paid the extra $ for the dual (50/200) 'ducer, a friend on the Lowrance Pro Staff called and said DO NOT use the dual
but switch to the straight 200 'ducer which I did. The dual 'ducer does not do well in shallow water applications.
I am not very skilled in this and I have
had a heck of a time getting the gps to send data to my Icom (dsc) radio. Lowrance was of no help on this, in fact, customer service gave me incorrect info. Today, the gps "hunts" to acquire position- 1 second locked on- the next second lost.
I replaced a Furuno dgps and Zercom fish-finder with the X15 and I already miss the simplicity and accuracy of the Furuno. I
think after I survive the learning curve I will be pleased but I'd download the manual
and go for a ride on a boat that is using the unit befor you decide. There is a lot to be said for separate units. I'd rather be fishing.
triblet posted 04-08-2001 10:49 AM ET (US)     Profile for triblet  Send Email to triblet     
Well, I have neither of these units, but
I did get my Garmin 162 to talk to my
Standard Horizon Spectrum. Lowrance's
manual is online, but Icom's isn't, so I
can only conjecture about that side.

Wire X-15 COM2 transmit (blue) to the
receive (may be labeled +receive) connection
on the radio. Alternatively, you could
wire it to COM1 transmit (yellow). Wire the
shield around the
blue wire to the NMEA ground (may be labeled
-receive) on the radio. On the X-15
Configure NMEA menu, check all the GPS boxes
and none of the Sonar boxes. Interestingly,
I didn't see any place to select the bits
per second (often incorrectly called baud).
Maybe there's something on the radio side.
Mine runs at 4800 bps.

I just noticed that the X-15 generates NMEA
2.0 sentences. My Garmin generates NMEA 2.3.

Are you indoors when the GPS is having
trouble locking on? If you are outside with
a clear view of the sky, it should have no
problem once it locks on the first time.

I love the line in the X-15 manual: "Never
use this GPS reciever while operating a
vehicle." What do they think a boat is?

Chuck

Jersey Jim posted 04-08-2001 11:30 AM ET (US)     Profile for Jersey Jim  Send Email to Jersey Jim     
Thank you for the help. Chuck, the wiring was correct- all it took was to configure port 2 to send nema output and to set the baud at 4800. I also moved the boat and the unit locked on. Thank you for doing what customer service could not.
triblet posted 04-08-2001 12:30 PM ET (US)     Profile for triblet  Send Email to triblet     
BTW, this stuff is way harder than it should
be. I have a masters degree in electrical
engineering, and it took me about a day of
fooling with it to get to work. Every unit
seems to be just a little bit different, esp.
in terminology.

Please pass your results back to customer
service to help the next guy.

Chuck

Ed Stone posted 04-08-2001 11:32 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
I downloaded the most recent info into my
168 garmn. Today I was cleaning the boat
and marked the driveway as home on the gps.
About an hour later I turned the garmin back
on and searched for the nearest waypoints.
The distance constantly changed from 1.5ft
to 14ft.
What is the proper way to check for
accuracy?
Ed Stone
triblet posted 04-09-2001 12:03 AM ET (US)     Profile for triblet  Send Email to triblet     
First: have you loaded the latest firmware
into your 168? That will add WAAS and
improve the accuracy. Or is that what you
meant by "latest info"? And after you load
the new firmware, you have to enable WAAS
(hit menu on the panel with the signal
strength bars), then let it take about an
hour to figure out where the new bird is.
Once it locks in WAAS, you will see the
letter D on the signal strength bars.

What you did is one way of knowing what the
accuracy is. Note that when the new position
was 14' from the waypoint, you only know
an upper bound on the error. It might be
as little as 7' (if the actual postion is
between the position captured by the waypt
and the position the GPS now has.

And if you have a way to hook
the 168 up to a computer AND it's antenna
at the same time, there's a program called
SAwatch that will draw pretty pictures.
http://www.huntting.com/sawatch/index.html
of the position wandering around.

Earlier in this thread, I said I thought DOP
was a standard alogrithm. While looking for
the SAwatch URL, I found an explanation of
DOP:
http://www.ualberta.ca/~norris/gps/DOPnontech.html

Chuck

Ed Stone posted 04-09-2001 10:47 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
Hey Chuck,
I'm now chasing fish around the gulf with the
latest technology.
Thanks,Ed Stone
triblet posted 04-10-2001 09:17 PM ET (US)     Profile for triblet  Send Email to triblet     
And the fish are probably still laughing
just as hard.

One of the funniest things I've ever seen
was a couple of fellows fishing North
Monastery beach from a small boat. We were
diving, and I'd noticed the boat when we
walked in (this was pre-Montauk, I don't do
beach diving much any more) and we happened
to swim by where their bait was. The fish
were swimming up, "sniffing" the bait, and
swimming away without taking it. It sure
seemed like the fish had their noses in the
air as they swam away.

Chuck

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