NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextant accuracy
From: Peter Fogg
Date: 2003 Mar 18, 11:09 +1100
From: Peter Fogg
Date: 2003 Mar 18, 11:09 +1100
On a small boat out at sea I think of a fix within 10 nm of the actual position as being good, within 5 nm as excellent, and as the manual system of sight reduction I use only inputs and calculates values to the nearest nm or minute of arc then that is as good as can be got. In practice the various roundings can tend to cancel themselves out (most often) or accumulate so the final result without mistakes can indicate 2 or more rarely 3 nm of error. As others have pointed out; far from land in slow moving craft this is more than good enough. The big advantage of the system I use is its simplicity and compactness, everything needed being contained in the one slim book. I think the theoretical advantage of the systems that use and calculate values to the nearest tenth of a nm is misleading, all that extra bulk of books and work of calculation to achieve this result is worthless when the various practical difficulties of taking sights from small unstable platforms tend to lead to errors of many nms. From a known fixed position on land its a little different, and using my navigational calculator which does accept data to many decimal places and displays results to the nearest tenth of a minute of arc or nm the actual position can be located to that level of accuracy - within 185 metres. I can clearly remember (it wasn't all that long ago!) our class taking its first sights from a suburban headland overlooking the sea, the instructor checking the intercepts with his calculator and announcing 'You're in the carpark' where we were standing. It seemed like magic then and to some extent it still does. One practical problem I face taking sights from headlands is I may have to guess at the height of eye. Once I have established the position (sometimes the DR may be a guess too) then I believe it may be possible to calculate the dip by re-iteration. The key to this accuracy, the essential technique if accuracy is at all important, is to plot as many sights of the one body within 5 minutes of time as you can make and compare them with the slope of the body's apparent rise or fall, as detailed on this list previously. Otherwise you are simply taking a punt with any individual sight and its myriad of possible errors. As to whether CN has a place in coastal nav: I think it does. Along a coast with distinct features over a wide arc then it is quicker and simpler to plot 3 LOPs directly from them. But if you're out on the edge of the continental shelf, about 10 nm from land, as you may well choose to be to take best advantage of a current, the land could be just a smudge on the horizon and CN is the way to go. Even closer, you may have only one shore feature to use, so it can be very useful to observe the sun or moon to give you another LOP to play with. An LOP is an LOP, it doesn't matter where it comes from. You could just as well use a bearing to a radio source as a useful LOP to plot with the others. Lacking a Radio Direction Finder, let me say that the cheapest and nastiest of portable transistor radios are the best to swivel around to locate a bearing. The problem may be to find where the transmission tower is located - charts only tend to locate them if close to the coast, unfortunately.