NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Role of CN at sea, was Re: Averaging sights ... - Chart Errors
From: Trevor Kenchington
Date: 2004 Oct 17, 22:34 +0000
From: Trevor Kenchington
Date: 2004 Oct 17, 22:34 +0000
Lisa Fiene wrote: > In an article last year in Australia's 'Cruising Helmsman', the > following was contributed (on the writer's recent passage to the > Louisiades, which is on the south eastern tip of Papua New Guinea): > > "we sailed to a less frequently visited island (Rossel Island) at the > eastern end of the Louisiade archipelago, and had been concentrating on > visual navigation as we approaxhed the island. This was standard > practice because of hazardous bommies in the area, and we took only a > passing glance at the navigation system presentation. ...Two days > later, we moved to another bay nearby and with gentle sailing conditions > I noticed a major difference between the navigation system presentation > and what we were seeing outside 'the office', e.g. the relative location > of islands, headlands, bearings, etc. I initially suspected a GPS > error, and quickly started our backup GPS, but it confirmed our lat/long > position. ....By triangulation, I estimated the whole island, reefs > etc. were displaced by about .7nm to the east. [snip] It has often been pointed out that, in GPS navigation, you can know your own position with extreme precision but that the locations of the hazards around you are often much less well known. That is a reversal of the situation with non-electronic navigation, in which your estimates of your own position are generally much less precise than the chart. I don't see that as reducing the value of GPS relative to the non-electronic navigational tools but it does illustrate one reason why GPS navigation needs more skill than simply turning the system on and following the calculated bearing to the next waypoint. It is also a warning to navigators properly trained in non-electronic position-fixing that GPS is not simply an alternative way of finding your position. Some old certainties change in unexpected ways as technology advances. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus