Navigation circle

I want to expand on something I hinted at in a reply to Skip's preventative measures post.

You know how the position circle on a GPS gets larger and smaller depending on the sattelite coverage? You keep the circle off the bad spots, not the center of the circle because you could be anywhere within it.

There should be another, mental, circle around the GPS location or the pencil fix on your chart and it should be projected forwards along the track. This circle gets larger and smaller depending upon many factors. This circle should be kept clear of anything hard or areas of breaking water.

Some factors that make the circle bigger (No priority implied):

Falling tide

Swell

Hard bottom

Fatigue

Imminent bad weather

Poor visual back up for GPS

Poor quality chart coverage

Possible equipment problems

Sail settings that may hinder maneuverability

Poor visibility

Etc., etc.

Factors that make the circle smaller:

Local knowledge

Good visibility and weather conditions

Rising tide

Soft bottom

Etc, etc.

Running these factors through your mind, at least in the background, is valuable not only for establishing the size of your buffer but for increasing overall situation awareness.

From Skip's account, I see their real error as treating their navigation
position as a point and letting the circle get too small. Based on assurances from others that the entrance was doable, they pressed on. It was doable, with good weather and either local knowledge or fresh and alert when the navigation circle might be 100 yards. Their navigation circle at the time should have been about ten miles in diameter.

The instructions to the watch stander in a situation like that shouldn't be, "Wake me if we get close to something or you think you need me." It should be, "Wake be when we are within ten miles of ____"

In conditions like that I wouldn't count on being woken up. The watch stander could fall asleep or zone out. Figure out the shortest time that the edge of your navigation circle could touch anything bad with a worst case calculation of speed and course. Set a loud alarm for that time. You might also be able to do it with various alarm settings. Best to do all three in challenging conditions. You'll sleep better.

--
Roger Long

.

Relevant Pages

  • Re: Navigation circle
    ... You know how the position circle on a GPS gets larger and smaller depending ... the navigation circle might be 100 yards. ... The instructions to the watch stander in a situation like that shouldn't be, ...
    (rec.boats.cruising)
  • Re: iPhone location
    ... It was showing a largeish circle centred on the house. ... presumably it indicates WiFi rather than anything else, ... On the iPhone the blue target circle appears to mean "phone signal ... It means that you have a GPS fix. ...
    (uk.comp.sys.mac)
  • Re: OT - Wasnt a church or school available?
    ... Imagine a circle where the circle passes through the NAVSTAR GPS ... not triangles or triangulation. ... You have skills. ...
    (misc.survivalism)
  • Re: OT - Wasnt a church or school available?
    ... Imagine a circle where the circle passes through the NAVSTAR GPS ... not triangles or triangulation. ... You have skills. ...
    (misc.survivalism)
  • Re: Firmware bugs on 76CSx (and probably on all C*x models): No Great Circle routes!
    ... I don't think great circle or "direct" has anything to do with it. ... The estimated landing time is likely based on some average speed and bearing. ... new 76CSx gps. ... even without using an external antenna under the window shade. ...
    (sci.geo.satellite-nav)