Garmin Built in Chart Landmarks

On Mar 12, 8:46 pm, DaveC <david.cow....DeleteThis@comcast.net> 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/StaticFiles/Files/NautChrts_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.
 >> Stay informed about: Garmin Built in Chart Landmarks 

In article
<20abb461-c0a5-4c66-a884-edae8d469484.DeleteThis@s19g2000prg.googlegroups.com>,
DaveC <david.cowell.DeleteThis@comcast.net> 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 are:

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>
 >> Stay informed about: Garmin Built in Chart Landmarks 

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?

What model is the chart plotter and what version are the charts?

I use BlueChart V9.5 on a 76Cx handheld and have never found the
shorelines to be crude approximations as far as their shapes and
placement. And it has never placed me on a land mass when I was in
navigable (two feet or more) waters. I consider the BlueChart charting
to be very accurate and reliable.

In areas where the shorelines or channels shift a lot due to currents
and tides you will always see more variation but I don't expect any
chart or GPS to be right on the money in those conditions.

I do see crude depictions of shore lines and see myself placed on land
masses when I'm using the built in base map or a street/highway mapping
product like City Navigator North America.

If I put my cursor on a feature in BlueChart it will "highlight" the
feature. I can then query it for details but, as you say, that does not
include the lat/long position.

But the on and off action of the highlighting will never vary by more
than a one or two thousandths of a minute or a one tenth of a second,
depending on the display mode you are using. So the lat/long displayed
with the feature highlighted would be the location, to within a few
feet, of the feature's placement on the map.

Another consideration is the accuracy of the radar involved here. Even
if your friend was looking at a return from the mass of actual light
house structure itself or a reflector mounted on it, the range reported
by the radar would be much less accurate and can also include some
rounding errors from the displayed distance.

Sounds to me like you were being "nit picked". If he wants "play the
captain" and supervise you and enforce rules to that degree, he needs to
tell you what system will be used for the measurement, the radar or the
GPS, and give you a tolerance for variation. And for that guy, when you
get 5.5 miles out, he might turn out to be the guy that will then
criticize you for being too far off shore.

Jack
 >> Stay informed about: Garmin Built in Chart Landmarks 

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 >> Stay informed about: Garmin Built in Chart Landmarks 

Marc Heusser wrote:
> In article
> <20abb461-c0a5-4c66-a884-edae8d469484.TakeThisOut@s19g2000prg.googlegroups.com>,
> DaveC <david.cowell.TakeThisOut@comcast.net> 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.
 >> Stay informed about: Garmin Built in Chart Landmarks 

This message is not archived
 >> Stay informed about: Garmin Built in Chart Landmarks 

In article <marc.heusser-A0FD02.08481613032008.DeleteThis@news.uzh.ch>,
Marc Heusser <marc.heusser.DeleteThis@byeheusser.commercialspammers.invalid>
wrote:

> In article
> <20abb461-c0a5-4c66-a884-edae8d469484.DeleteThis@s19g2000prg.googlegroups.com>,
> DaveC <david.cowell.DeleteThis@comcast.net> 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 are:
>
> 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
 >> Stay informed about: Garmin Built in Chart Landmarks 

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.
 >> Stay informed about: Garmin Built in Chart Landmarks 

Jack Erbes wrote:
> 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?
>
> What model is the chart plotter and what version are the charts?
>
> I use BlueChart V9.5 on a 76Cx handheld and have never found the
> shorelines to be crude approximations as far as their shapes and
> placement. And it has never placed me on a land mass when I was in
> navigable (two feet or more) waters. I consider the BlueChart
> charting to be very accurate and reliable.
>
> In areas where the shorelines or channels shift a lot due to currents
> and tides you will always see more variation but I don't expect any
> chart or GPS to be right on the money in those conditions.
>
> I do see crude depictions of shore lines and see myself placed on land
> masses when I'm using the built in base map or a street/highway
> mapping product like City Navigator North America.
>
> If I put my cursor on a feature in BlueChart it will "highlight" the
> feature. I can then query it for details but, as you say, that does
> not include the lat/long position.
>
> But the on and off action of the highlighting will never vary by more
> than a one or two thousandths of a minute or a one tenth of a second,
> depending on the display mode you are using. So the lat/long
> displayed with the feature highlighted would be the location, to
> within a few feet, of the feature's placement on the map.
>
> Another consideration is the accuracy of the radar involved here.
> Even if your friend was looking at a return from the mass of actual
> light house structure itself or a reflector mounted on it, the range
> reported by the radar would be much less accurate and can also
> include some rounding errors from the displayed distance.
>
> Sounds to me like you were being "nit picked". If he wants "play the
> captain" and supervise you and enforce rules to that degree, he needs
> to tell you what system will be used for the measurement, the radar
> or the GPS, and give you a tolerance for variation. And for that
> guy, when you get 5.5 miles out, he might turn out to be the guy that
> will then criticize you for being too far off shore.
>
> Jack

I heartilly endorse what you say about Garmin's Bluecharts. They are precise
and accurate, although better at some zoom levels than others, as you would
expect. It sounds as though the OP is using a basemap, which is awful!


Dennis.
 >> Stay informed about: Garmin Built in Chart Landmarks 

Bruce in alaska wrote:
<snip>
>
> 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.
>

In radar, isn't some imprecision or lack accuracy also induced by things
such as the display of soft returns (gradually rising shorelines, marsh
lands, etc.) and where the logic in the radar actually has to decide the
placement the land mass trace on the display?

And it also has to make some sort of judgment call as far as the
"called" range does it not?

When I said that the radar was less precise, those were the kinds of
factors I was thinking of.

I know that in the real world someone would have to decide what
constitutes land mass and what is not and I think the placement of a
shoreline would by a radar be the same, a best guess estimation.

If I was seeing a bright spot from a reflector or sharply rising object
(cliff, building, etc.,) in amongst the traces of the returns from land
mass, in that spot I'd feel like I was determining a well defined point.

On NAVAID objects on the chart, lighthouses and the like have locations
that are very precisely known. I guess the only question is whether or
not the chartmakers have them placed accurately relative to the
coordinate system.

But I know that if I can look at a reference, like querying a data base
on the S-57 charts or looking at the Light List, and compare the cursor
highlighting action and coordinate display on the Garmin charts with the
locations in the reference, the objects will normally report very
similar locations. To within a matter of feet or yards in most cases.

I would expect to see pretty close agreement between a GPS and a radar
as far as estimating the range to a sharply defined object at a well
defined and known location.

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)
 >> Stay informed about: Garmin Built in Chart Landmarks 

DaveC wrote:
<snip>
> 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 <snip>

Thinking like a machinist, if they are defining the location to 1/10th
of a minute then they are defining the location in increments of 100 feet.

So it might be fair to say that the location is known to an accuracy of
plus or minus 50 feet.

I say that using a basis of 1,000 feet to a minute of angle. I know
that is not exact or true for both latitude and longitude but it is
convenient and not uncommon to think of it that way.

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)
 >> Stay informed about: Garmin Built in Chart Landmarks 

"DaveC" <david.cowell.TakeThisOut@comcast.net> wrote in message
news:20abb461-c0a5-4c66-a884-edae8d469484@s19g2000prg.googlegroups.com...
> 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.
 >> Stay informed about: Garmin Built in Chart Landmarks