NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation plot
From: Lars Tornqvist
Date: 2002 Feb 6, 12:13 -0000
From: Lars Tornqvist
Date: 2002 Feb 6, 12:13 -0000
Hi If anyone is interested I've got an Excel spreadsheet which draw your Deviation courve and calculate the Coefficients. Let me know then I will download it on my home page. Lars ----- Original Message ----- From: Trevor KenchingtonTo: Sent: Tuesday, February 05, 2002 10:48 PM Subject: Re: Deviation plot > John Kabel wrote: > > > The best you can do is carefully navigate toward a known landmark a long > > distance off (to help average out swings of the bow) and take a number of > > readings of COMPASS heading (compass fixed on the boat) while at the > > same time recording TRUE heading (direction the GPS receiver is moving). > > Assuming you don't move about on the boat, and take a reasonable number > > of measurements to establish a meaningful average for the headings for each > > landmark, solving for deviation will be easy. What's reasonable? I would do > > eight or ten for each heading, and this would require assistance and some > > time. Just how bad do you think your compass deviation is? If bad, spend > > lots of time and do it right!! > > > > Doing this for six or eight landmarks spaced around the compass will get you > > a fairly accurate deviation chart. Graphing will give a smooth curve, better > > than a table. > > > > Do this on reasonably smooth water, so waves don't knock the boat heading > > about too much. It doesn't work on my Sea-Doo, big as it is. > > > > Note that this is possible only on powered craft. Anything with a sail relies on > > a keel and has leeway. I've tried, and I do not believe it to be possible to > > remove or solve for leeway. I would love to hear from someone who has been > > able to do that!! > > > I think the much simpler and more general solution was posted a few days > ago but I did not keep the message and so cannot say who provided it. To summarize: > > 1) Put your boat on any course you wish, with any amount of leeway and > any current running -- but with reasonably flat water so that you can > get accurate measurements. Hold its heading steady until the compass settles. > > 2) Select a visible, charted object at a moderate distance. Determine > its position using a WGS84 chart (or with appropriate corrections from > any other chart). > > 3) Enter that position into your GPS as a waypoint and have the GPS read > out the true bearing of the object/waypoint. > > 4) Record, simultaneously, the GPS bearing, the compass heading of your > boat (using the steering compass) and the compass bearing of the object > (also using the steering compass). The latter can be obtained directly > if the steering compass has sighting vanes. If not, use a pelorus or a > hand-bearing compass (which will need dual readings for heading and > bearing) to get the relative bearing and thence calculate the compass > bearing by the steering compass. > > 5) The difference between the GPS's true bearing and the compass bearing > is the compass error. Deducting the variation leaves the deviation of > your steering compass for the heading at the time the three angles were recorded. > > 6) Repeat for other headings. > > > In the absence of a GPS, the same thing can be done using two charted > objects to establish an LOP of known bearing and then recording heading > and the compass bearing to the two objects as you cross the LOP. > > > 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 >