Be cautious when using any geomagnetic declination information. There are both spatial and short-term temporal variations not accounted for by model magnetic fields. The r.m.s. value for a predicted declination is typically better than 0.5 degrees but the actual error at a given location and time could be larger and is influenced by local geology, cultural features and activity, and ionospheric and magnetospheric processes. While a compass might be useful for initial orientation of a sundial, satellite dish, etc., final alignment is best done using the "signal" itself whether it be a TV picture or the sun's shadow. -- Richard Langley Professor of Geodesy and Precision Navigation
On Thu, 12 Nov 1998, Fernando Cabral wrote: > > >Peter Mayer wrote: > >> On 11/11/98 Greg Milsom wrote, >> >Today I learned that one of the reasons my new dial was not as exact as >> >I had hoped was because the magnetic variation I was using to align the >> >dial was several years old and that magnetic variation does in fact >> >change over time. > >You can get precise magnetic declination from several sites in the Internet.In >the end of this message I put a message from Tom McHugh that >points toward geomag, a DOS-based program that calculates the >magnetic declination for several years, any coordenates. > >I think among the the URLs I used are: http://geomag.usgs.gov/ > >On the other hand, I don't think I would use a magnetic compass >to orient a sundial. I think it is too error prone. I would use the >sun itself instead. > >I don't know your coordenates and I can not get any hints from your >e-mail address (since it is not based on the ISO (country) schema). >If you are in the Northen Hemisphere, this is a good season to >find the North/South line using the sun since it is now approaching the >tropic in the Southern Hemisphere (Capricorn) and it moon shadow >in the Northen Hemisphere is quite long. > >Here is what I would do: use Luke's sun calculator to find >the transit time at your longitude.http://www.gcstudio.com/suncalc.html > >Put a short, vertical gnomon. Make sure it is really vertical. >With a precise watch mark the shadow at transit time. The >sun will be due South, the shadow points towards the North. > >> Dear Greg, >> It does change, alas. A while ago I created an Excel spreadsheet, >> based on the equations in Meus' book which seemed to get correct answers. >> I only set it up to calculate single days, but that could be modified. >> Since it was created solely for my own amusement, it comes with little or >> no documentation, but if you want to play with it, let me know (and what >> encoding standard your e-mail browser uses) and I can try to send it to >> you. >> > >Peter > >I am interested in this Excel Spreadsheet. I am right now implementing >fer j. de vries' algorithms (http://iaehv.nl/users/ferdv/definit.htm). One of >the >problems I have is checking every step. >So, a spreadsheet like your may be useful for me to check the results of >my program. > > >> Peter Mayer >> >> ---------------------------------------------------------------------- >> Peter Mayer | [EMAIL PROTECTED] >> Politics Department | 'phone:+61.8 8303 5606/5610 >> University of Adelaide | FAX: (+61.8) 8303 3446 >> Adelaide, SA 5005 | Time Zone: GMT +9.5 hrs (April-October) >> AUSTRALIA | GMT +10.5 hrs(November-March) >> ---------------------------------------------------------------------- > > >Subject: > Fw: Fw: Magnetic North Pole > Date: > Mon, 28 Sep 98 21:11:26 -0400 (EST) > From: > "Tom Mchugh" <[EMAIL PROTECTED]> > To: > [EMAIL PROTECTED] > References: > 1 , 2 > > > > >Fernando, > >I found these e-mails on the subject of magnetic north, dating >from November last year (see quoted material below). I hope >this helps you > >Tom McHugh > >R. R. 1, Box 896 >Fort Fairfield, ME 04742 >USA > >46N45' 13" >67W48' 42" > > > >**************************************************************************** >****************** >---------- >> Date: Saturday, November 08, 1997 15:25:45 >> From: Richard Langley >> To: k.schwarzinger >> Cc: Sundial List >> Subject: Re: Fw: Magnetic North Pole >> >> On Sat, 8 Nov 1997, k.schwarzinger wrote: >> >> > Richard, >> > >> > thank you very much for your answer about the magnetic North >> > Pole. >> > >> > It is very practically to found the magnetic declination for each >> > point of the earth and every year. >> > >> > I have entered the geographical coordinates of my home : >> > Latitude: 47 Degees 14 Min (North) and Longitude: -11 Degrees >> > 27 Min (East) and the year 1997. >> > I got the message : >> > >> > "The magnetic declination in 1997 at Latitude 47 Degees 14 Min >> > N and Longitude -11 Degrees 27 Min W (!) = 0 Degrees >> > 40 Min E. >> > >> > Why -11 W (West?) ! Why is 'East' not indicated ? >> > >> > Karl. >> > [EMAIL PROTECTED] >> >> Karl: >> >> It is very common in North America (except in geodetic circles!) to treat >west >> longitudes as positive. The software was written primarily for the use of >> Canadians hence the sign bias. The geodetic convention is to treat east >> longitudes as positive. Of course, then North Americans would have -66 >E! :-) >> >> >> By the way, there is another geomagnetic field program that can be >accessed on >> the Internet. It's hosted by the U.S. Geological Survey and is available >via >> telnet: neis.cr.usgs.gov. Log in ID is QED. This software is not >> "North-American-centric." >> >> Here's the information on how to run the on-line program: >> >> GEOMAG provides values of the elements and parameters of the Earth's >magnetic >> field. The values are estimates based on mathematical models. Values of >the >> following elements and their rates of change are available: >> >> D - Declination (also called compass variation; east declination is >> considered positive, west declination negative) >> I - Inclination (also called dip; downward inclination is considered >> positive, upward inclination negative) >> H - Horizontal intensity >> X - North component >> Y - East component >> Z - Vertical intensity (considered positive downward, negative upward) >> F - Total intensity >> >> Positions of the Magnetic Poles and parameters of the centered and >eccentric >> geomagnetic dipoles are also available. >> >> You will be prompted to select a field model and to enter the date and >> geographic coordinates (latitude, longitude, and elevation) of locations >of >> interest. If you are unsure of how to respond to any prompt, enter a >question >> mark (?) to get help. If you would like the program to back up to the >> previous prompt, enter a caret (^) (that is, the SHIFTed-6 key, not the >> UP-ARROW key). If a default response (shown in brackets) is available >and is >> suitable, you may select it simply by pressing RETURN. To quit the >program, >> press CTRL-Z (hold down the Control key and press Z). >> >> (Reminder: Enter <?> for help, <^> to back up, <CTRL-Z> to quit.) >> >> Options: >> 1 = Field Values (D, I, H, X, Y, Z, F) >> 2 = Magnetic Pole Positions >> 3 = Dipole Axis and Magnitude >> 4 = Magnetic Center >> >> -- Richard Langley >> Professor of Geodesy and Precision Navigation >> > >> >> >============================================================================ >== >> = >> Richard B. Langley Internet: [EMAIL PROTECTED] or >[EMAIL PROTECTED] >> >> Geodetic Research Laboratory BITnet: [EMAIL PROTECTED] or >> [EMAIL PROTECTED] > >> >> Dept. of Geodesy and Geomatics Engineering Phone: (506) 453-5142 > >> >> University of New Brunswick FAX: (506) 453-4943 > >> >> Fredericton, N.B., Canada E3B 5A3 Telex: 014-46202 > >> >> Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ >> >============================================================================ >== >> = >> >> >**************************************************************************** >******************************************************** > >**************************************************************************** >******************************************************** >>On Fri, 7 Nov 1997, k.schwarzinger wrote: > >>> Dear all, >>> >>> is there in the INTERNET a website, which gives informations >>> about the curent value from the declination of the magentic >>> North Pole ? >>> >>> Are there still informations about the situation of the magnetic >>> North Pole ( INTERNET or literature ) ? >>> >>> Thanks for any help. >>> >>> Regards >>> Karl. > >>http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html > > >>=========================================================================== >==== >> Richard B. Langley Internet: [EMAIL PROTECTED] or >[EMAIL PROTECTED] >> Geodetic Research Laboratory BITnet: [EMAIL PROTECTED] or >> [EMAIL PROTECTED] > >> Dept. of Geodesy and Geomatics Engineering Phone: (506) 453-5142 > >> University of New Brunswick FAX: (506) 453-4943 > >-- >Fernando Cabral Padrao iX Sistemas Abertos >mailto:[EMAIL PROTECTED] http://www.pix.com.br > mailto:[EMAIL PROTECTED] >Fone: +55 61 321-2433 Fax: +55 61 225-3082 >15? 45' 04.9" S 47? 49' 58.6" W >19? 37' 57.0" S 45? 17' 13.6" W > > > =============================================================================== Richard B. Langley E-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ ===============================================================================
