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.