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..
On Tue, 12 Apr 2005 10:56:58 -0700, "Steve" <estevew@hctc.com> 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..
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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 <CLmdnV9Zl5hllsHfRVn-iQ@whidbeytel.com>, Steve <estevew@hctc.com> 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..>
