In planning for my trip up the Inside Passage, I'm noting several charted
Magnetic Disturbance Areas. Some are as large as 10 miles across with a
possible disturbance of 7 deg. The largest I note is near/ S.E. of Cape
Northumberland, Duke Is.. This might be encountered while transiting to and
from Ketchikan.

To my knowledge, I have never operated a boat with magnetic compass in such
a disturbance area. I assume that this wouldn't be a problem in clear
weather where visual course keeping might be used. I don't think I would
want to rely on AP set to follow a magnetic heading..

However, would it be appropriate to set a waypoint and allow the GPS steer
through these areas and would this be a safe method at nite or in reduced
visibility..

Opinions welcome.


--
Steve
s/v Good Intentions

On Tue, 12 Apr 2005 10:56:58 -0700, "Steve" wrote:

However, would it be appropriate to set a waypoint and allow the GPS steer
through these areas and would this be a safe method at nite or in reduced
visibility..

===================================

It's certainly appropriate to set a "from" and "to" waypoint and steer
to minimize XTE (Cross Track Error). That's pretty common even in
good visibility and with no compass anomalies. At night you also need
a good radar, spotlight and decent 7x50 binoculars to be reasonably
safe. Add in a pair of night vision binoculars for even more peace of
mind.

It all depends on the actual opriginal 'survey' (probably lead line
from a acual sailing ship in the mid 1800s) .... hence Im usualy
'suspicious' when using a GPS in 'tight' courses. I guess what Im
tring to say is that gps can give very precise positions but when
related to ancient charts .... you can get into a hell of a problem in
a hell of a hurry because the precision of gps can mathematicallly
overwhelm the data that was orginally taken (optically and
mechanically). Plus there is/can be an extreme 'resolution' anomaly
when using gps and electronic charts (and based using charts that were
originally done with lead-lines, etc. ..... that original data that
appears on your chart may NOT be so accurate). .....
This is probably the cause of the US submarine that recently ran
aground in the middle of the pacific ocean ..... using gps and
electronic charts .... based on original data that had a LOT of
(tolerance) error.. GPS & electronic charts are probably OK when used
at LOW 'magnification'' but, if you use them at very high magnification
(resolution) expect to hit a few 'bottoms', etc.

SO .... in areas of magnetic disturbances its really best to verify
with VISUAL sites to make your fix .... as thats the method that was
used in making the original charts. Sextant held parallel to the
horizon, pelorous, site bearing compasses, etc. ..... will match the
charting and gps / electronic charts MAY NOT match the original (and
still valid) map survey. In otherwords dont TRUST electronic data
when applied to charts that were made using 'mechanical' means.

In article , Steve
wrote:

In planning for my trip up the Inside Passage, I'm noting several charted
Magnetic Disturbance Areas. Some are as large as 10 miles across with a
possible disturbance of 7 deg. The largest I note is near/ S.E. of Cape
Northumberland, Duke Is.. This might be encountered while transiting to and
from Ketchikan.

To my knowledge, I have never operated a boat with magnetic compass in such
a disturbance area. I assume that this wouldn't be a problem in clear
weather where visual course keeping might be used. I don't think I would
want to rely on AP set to follow a magnetic heading..

However, would it be appropriate to set a waypoint and allow the GPS steer
through these areas and would this be a safe method at nite or in reduced
visibility..

Opinions welcome.