"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.