I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?

In article
,
DaveC wrote:

I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?

It certainly should be in the right location, but does not need to be.

Ways to check a

Radar, should be correct to 1% of distance.

Google Earth for photographs, often detailed enough down to 10 metres.
(Sometimes better than charts, as they lack some features like harbours
in some places of the earth.)

I'd hate to have such a lousy chart (if what you describe proves true)
but it never hurts to be wary.

HTH

Marc

--
remove bye and from mercial to get valid e-mail
http://www.heusser.com

Marc Heusser wrote:
In article
,
DaveC wrote:
I was recently off the coast of Mexico and using a Garmin
chartplotter for position.
[snipped]
Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?

It certainly should be in the right location, but does not need to be.
[snipped]
I'd hate to have such a lousy chart (if what you describe proves true)
but it never hurts to be wary.

I have a Garmin plotter and BlueChart for the Adriatic and
have consistent GPS positional errors on some charts - even
0.5nm difference switching between charts of the same area
but different scales.

I have tried differing datums but the errors remain -
presumably transcribed from the original charts. However, a
friend with C-Map does not have those errors and I would
have thought both would have used the same source data,
either British Admiralty charts or official Italian
hydrographic institute ones.

BrianH.

In article ,
Marc Heusser d
wrote:

In article
,
DaveC wrote:

I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?

It certainly should be in the right location, but does not need to be.

Ways to check a

Radar, should be correct to 1% of distance.

Google Earth for photographs, often detailed enough down to 10 metres.
(Sometimes better than charts, as they lack some features like harbours
in some places of the earth.)

I'd hate to have such a lousy chart (if what you describe proves true)
but it never hurts to be wary.

HTH

Marc

Even IF the Radar was off in it's calibration, some.... It wouldn't be
20% off. (5.5 miles to 4 miles) The roundtrip timing of a Radar Pulse
is very precise, and not subject to anything but the "Speed of Light".
Any error in distance display in the radar is due to the calibration of
the Range Rings, and depending on the type of display, (digital vs
Analog) the calibration should easily be within 1%, as Marc has stated.
I would suspect that Garmin doesn't have very good Waypoint Calibration
on their BaseMap that comes with most units. I know my GPS3+ BaseMap
is off on coastline parameters here in alaska, by more than.5 miles.

--
Bruce in alaska
add path after fast to reply

DaveC wrote:
I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?
I have found that GPS even DGPS is not totally reliable for navigation.
I would contact Garmin and see what they have to say.

--

Rick
Fargo, ND
N 46°53'251"
W 096°48'279"

Remember the USS Liberty
http://www.ussliberty.org/





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On Mar 12, 8:46 pm, DaveC wrote:
I was recently off the coast of Mexico and using a Garmin
chartplotter for position.
.....

I've been doing some research since my primary post. The NGA site has
some great information (pamphlet at
http://www.nga.mil/MSISiteContent/St..._GPS_index.htm
) and the NGA Light list for outside the US. To be fair the Garmin
chartplotter didn't have the dedicated Mexico Blue chart (a special
chip I believe) but only the built in chart/map. The reference Garmin
gave in the database was a Russian chart that was last updated in 2006
(this is out of memory) BUT my question still remains does Garmin use
the WGS coordinates of landmarks per the NGA light list for those
waypoints shown on those charts? The accuracy of our GPS position was
only as good as the DGPS would allow but I'd like to think the
landmark waypoint was correctly defined. Of note was the fact our
chartplotter happily rattled off positions to the 1000th of a minute
although everyone concedes the accuracy of GPS is only to the nearest
10th in those waters and the NGA only gives postitions to the 10th.
The cape we were looking at was Cabo Corrientes which is just north
of Cabo Roca Negra, the NGA has it defined at

15080 G 3482 Cabo Corrientes. 20° 24.0´ N 105° 42.8´ W Fl.W. period 6s
fl. 1s, ec. 5s 305 93
18 White truncated pyramidal octagonal tower, house with red cupola;
20.
RACON O(- - -)
The radar even had the RACON signal on screen so there was no
confusion about the landmark being the radar target.

DaveC wrote:
I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?


Do you really think Garmin goes out and create's charts? NOPE... and
they can't legally correct them. I went over this for 15 years in the
bahamas where I proved they were off by 1/2 mile in many cases... then I
found out the bahama govt charts were off by the same amount. They
finally fixed it in the last major release by dumping the govt charts
and adopting the Explorer charts (new electronic soundings by an
incredible non-govt company)

Many offshore charts are still based on leadline soundings from 50 to
100 years ago. I bet if you compare it to the Govt charts they will be
exact.

"DaveC" wrote in message
...
I was recently off the coast of Mexico and using a Garmin
chartplotter for position. My friend came up and chided me for being
inside the 5 mile buffer he prefered to be off the coast. I insisted
we were at five miles based on the GPS reported distance to the Punta
Negra lighthouse which is a built-in landmark/waypoint, He'd looked at
the radar and it said 4 miles. I suggested that although the GPS had a
lousy shoreline it would have to have accurate landmarks i.e
lighthouses and that maybe his radar needed calibration. Who is right?
We all know the built-in charts for the Garmins have generally
straight lines and don't closely follow the shores but are the
landmarks off too? We've often found ourselves anchored somewhere on
the chart's shore. Garmin reports all the specific data for a
lighthouse such as you'd find on a light list but don't actually give
the LAT/LONG for the site so ... the ASSUMPTION is that they're
correct on the chart. Is that too much to ask?

I cannot vouch for present Blue Charts, but I was in the identical area some
four years ago and found that the Garmin Blue Charts of that day were
reproductions of existing paper charts, and as such, were very inaccurate.
In particular, the areas around Puerto Vallarta were very poorly reproduced
and highly inaccurate in their agreement with actual GPS position data.