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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Auroras/compass readings
From: Trevor Kenchington
Date: 2004 Oct 17, 23:18 +0000
From: Trevor Kenchington
Date: 2004 Oct 17, 23:18 +0000
Charles Seitz posted: > More than 90% of the geomagnetic field is generated by the > Earth's outer core. It is this portion of the geomagnetic field that is > represented by the Magnetic Field Models. So, when a chart (Canadian Hydrographic Service chart in my case) shows a compass rose with 20 degree West variation, is that drawn from a model which ignores 10% of the geomagnetic field, or is it based on empirical measurements in the vicinity of the point where the compass rose is printed? If the former, some of the missing 10% is due to transient currents in ionosphere and magnetosphere, which will be too short-lived for their effects to be reflected on a chart. Some will be due to magnetized bedrock beneath the water, which will usually be too localized to be reflected on anything but very large-scale charts. But at least some of the 10% must apply over large enough areas and long enough periods to be included in charted values of variation. Is it there? Put another way: How large an error can there be in the variation shown on charts, aside from the short-term effects of solar flares, the local effects of iron ore, and the like? 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