Magnetism is an ever present phenomenon in our daily lives. More so in some geo/cartographic contexts. Providing a way to add magnetic information to our GRASS projects might be helpful.
Here is a gathering of ideas, in order to produce geomagnetic modules or scripts to GRASS
The California Soil Resource Lab has some quite interesting information for production of a Detailed Magnetic Declination map.
As Dylan points out in his post, Stefan Maus has put together an excellent website on the Historical main field change and declination. (This sentence shamelessly pasted from Dylan's blog)
The USGS has a collection of software, however it dates of 1997. This Open-File Report (97-725) is named Potential-Field Geophysical Software for the PC, version 2.2 and supersedes Open-File Report 95-77, from 1995, A bibliography of some geophysical computer programs, databases, and maps from the U.S. Geological Survey, 1971-1994.
The Brazilian National Observatory (ON) also had its own geomagnetic model, called ELEMAG, which agrees with the IGRF, but is calibrated to work only in Brazil. I (Pmarc) however, have not been able to find it on their website, on the other hand, Google was able to find some versions online.
- Rotating a north arrow symbol by a given amount can be accomplished using the d.graph module or in a ps.map plot using a specified rotation angle.
- The d.rast.arrow module may be of use for visualizing gravity fields.
- Module capable of computing the magnetic field (all 7 components) over a location, for a given date or range of dates. It seems to me (Pmarc) that the most appropriate output would be a vector map. The starting point could be the IGRF model provided by NOAA (link above)