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
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