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.