If the variances are minimal then I don't really need them. All I am doing is providing an automatically calculated magnetic declination for any location in NZ. I know that the total variance across the country is only about 2-3 degrees (although this can be quite significant over long distances), but wasn't sure of the scale of local anomalies.
Is the IGRF model very different from the WMM model? I had a quick look at the IGRF coefficients and they look fairly similar. Cheers, Phil. ----- Original Message ----- From: "Phil Scadden" <[EMAIL PROTECTED]> To: "NZ Borland Developers Group - Delphi List" <[EMAIL PROTECTED]> Sent: Tuesday, April 20, 2004 11:37 AM Subject: Re: [DUG] Converting some code from C > > When you say that you can calculate for any year, surely those calculations > > are dependent on coefficients that are only valid for the current epoch (or > > for past epochs). The coefficients are recalculated every 5 years because > > simple extrapolation is too innacurate. Haven't looked at the IGRF files, > > but assumed that they also work of coefficients. Is that the case? > > The IGRF files contain coefficients for each 5 year epoch. "any" year sorry > means any year within that range. > > > The WMM-2000 model is a Low-Resolution model - you don't happen to have a > > high resolution model for NZ do you? > > Now this is an issue as to what you are trying to achieve. The global > magnetic model is supposed to represent the mantle-modified main field, > free of crustal effects. You map crustal effects by comparing what you > actually measure at an xy point and subtract the global model. This leaves > you with magnetic anomaly map which is what geophysicists are extremely > interested in because it highlights crustal differences. I coded the IGRF > model precisely for this purpose - so I can produce anomaly maps from > all the total field measurements I have to hand. > > Now higher-resolution total-field maps (the actual magnetic field at xy) do > exist for NZ but I would be very curious as to why you would want one. The > local anomalies differ from IGRF by amounts so small that a good > magnetometer is needed to detect them. > > > Cheers, > > Phil. > > > > ----- Original Message ----- > > From: "Phil Scadden" <[EMAIL PROTECTED]> > > To: "NZ Borland Developers Group - Delphi List" <[EMAIL PROTECTED]> > > Sent: Tuesday, April 20, 2004 9:10 AM > > Subject: Re: [DUG] Converting some code from C > > > > > > > > where the m is not used in the loop and should have just been > > initialised > > > > outside it! There's more, but you get the idea. </rant> It also doesn't > > help > > > > that I'm translating code that carries out calculations that are well > > above > > > > my comprehension (calculates the 2000 Epoch World Magnetic Model for a > > given > > > > Lat, Long, Altitude, and Year using spherical-harmonic Gauss > > coefficients - > > > > whatever they are). Cheers, Phil. > > > > > > Well those calculations are also available in Fortran if you prefer. I > > > translated those earlier in the year into VBA for ArcGIS, but was using > > the > > > IGRF files so can calculate for any year, any lat/long. > > > > > > ---------------------------------------------------------- > > > Phil Scadden, Institute of Geological and Nuclear Sciences > > > 764 Cumberland St, Private Bag 1930, Dunedin, New Zealand > > > Ph +64 3 4799663, fax +64 3 477 5232 > > > > > > _______________________________________________ > > > Delphi mailing list > > > [EMAIL PROTECTED] > > > http://ns3.123.co.nz/mailman/listinfo/delphi > > > > > > > _______________________________________________ > > Delphi mailing list > > [EMAIL PROTECTED] > > http://ns3.123.co.nz/mailman/listinfo/delphi > > > ---------------------------------------------------------- > Phil Scadden, Institute of Geological and Nuclear Sciences > 764 Cumberland St, Private Bag 1930, Dunedin, New Zealand > Ph +64 3 4799663, fax +64 3 477 5232 > > _______________________________________________ > Delphi mailing list > [EMAIL PROTECTED] > http://ns3.123.co.nz/mailman/listinfo/delphi > _______________________________________________ Delphi mailing list [EMAIL PROTECTED] http://ns3.123.co.nz/mailman/listinfo/delphi
