NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Manual calculation of compass variation
From: Bill B
Date: 2004 Oct 17, 17:10 -0500
From: Bill B
Date: 2004 Oct 17, 17:10 -0500
> Richard, > I was surprised that a vessel needs to be going 5-10 km/hr to get a GPS > track accurate to better than one degree, and am wondering what the > accuracy of the track is for low speeds, which might easily be > encountered in a sailing vessel; I suppose this information is included > in GPS manuals, but I don't have one. A couple of points: When steering by a compass and fixes, the navigator can calculate set, drift, leeway and wind-induced surface current. The GPS shows COG and SOG, therefore the navigator has to work backwards to determine what direction the lubber line was headed. If in doubt, point your nose north and walk laterally to your right. The GPS will tell you you are going east. In fact you are, but your nose is pointing north. It is not a compass. As to speed vs., accuracy, my Garmin 76 gives no such warnings. In walking and sailing tests I have not found it to be off due to low speed. I do find it to be off in other areas: 1. When measuring course and distance between points, my manual tells me it is giving me the distance on the great circle route as opposed to along the rhomb line. 2. I have calculated short courses (30-60 nm) using a polyconic-projection chart (Great Lakes), various sailings, and trig. Also with my GPS. I found that usually all the traditional calculations agreed to the tenth of a degree. My unit reads out in whole degrees. What surprised me was that the GPS unit uniformly rounded DOWN to the nearest whole degree, even with calculations of ddd.6 to.9. Even on a 30-60 nm run, that is pretty far of course. Practically speaking, who is going to steer perfect ddd.d in a sailboat with chop? Still, if you took the GPS information without verification, hardly optimal. My other problem is that friends with boats new to them who have made GPS point-and-shoot their standard, and now leave their paper charts stowed. They have no idea their new wing keel has different leeway charactersitics than their old fin keel boat. On the Great lakes set and drift are generally not a consideration. Because of short fetch and ever-changing wind patterns, wind-induced surface currents are neglibable unless (Lake Michigan) it has been blowing out of the north for a day or more. So a perfect testing ground for determining leeway under various conditions and points of sail. But they either no longer care or don't know how to work the it out "backwards." Sad. A student learning to fly does not start out under the hood, but must become proficient in VFR first. E-nav, IMHO, starts too many boaters out under the hood, or seduces skippers who should know better. Bill