In
https://docs.generic-mapping-tools.org/latest/supplements/mgd77/mgd77magref.html
references:
http://denali.gsfc.nasa.gov/cm/
http://www.iugg.org/IAGA/iaga_pages/pubs_prods/igrf.htm
It seems to be broken.
I think they should be:
https://web.archive.org/web/20111021003705fw_/http://core2.gsfc.nasa.gov/cm/CM4_A.html
or https://ftp.space.dtu.dk/pub/nio/CM4/
and https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html
There are two new set of coefficients:
http://www.spacecenter.dk/files/magnetic-models/CM5/
http://www.spacecenter.dk/files/magnetic-models/CM6/
I want to see if I can adapt those coefficients to work with mgd77magref. But before I try, I just want to check if someone has already adapted it to work with GMT or has found another open source program that does the equivalent job.