[PUP] "UNCHARTED" ROCKS [Was: Lady Anne (W40) Hits Rock In Glacier Bay]

[Ron Rogers]

Just to be clear; the rock under discussion was on the chart. Arild and I
were discussing how to deal with charts based upon old surveys and before
the advent of LORAN and GPS. The datum used then was also different. A good
chart in the Caribbean based upon a British Navy survey in the 1800's was
only 100 yards off. That's darned good so long as you don't set your
autopilot based upon that chart's position of an opening in a reef! Although
there are now better privately created charts, all you can expect from an
electronic chart is the correct datum for GPS use; unless of course you
learn through experience otherwise. You can also and should mark up your
paper charts with both public and your own corrections.

 

The further you are from commercial shipping lanes, the less likely a recent
survey has been performed. Local sources suggest the last Glacier Bay
surveys are 50 to 70 years old. It is unlikely that the rock has moved,
although you are betting that: they sounded the full dimension of the
obstruction, their survey was accurate, and that the datum has since been
converted to a GPS compatible datum recognized by your chartplotter or
software.

 

Not too long ago on www.panbo.com he recounted how Garmin was out surveying
a local lake for a series of fishing charts. They were in whalers loaded
down with gear and going back and forth across the lake.

 

There is one thing that I do that you may wish to do from the comfort of
your homes. I do it in Coastal Explorer and also use Google Earth. You can
compare airborne and satellite imagery to your paper and electronic charts.
In almost every case, the often undated images will be newer than the chart.


 

 

I use the imagery for shoreline erosion, shifting sands (much less reliable)
and the location of rocks. You are only forming impressions and not absolute
facts. But if you want to know what points of land are suspect and where a
body of water is shoaling, these methods work. Unless removed by man or a
dramatic natural event, rocks are rocks.

 

Ron Rogers