I would like to retrieve data from NOAA Magnetic Field Calculators (IGRF) for multiple locations by using longitude and latitude grid spacing; say every 5 degree latitude and every 10 degree longitude for the entire globe. Please can advise?
There are several web applications the can calculate IRGF from latitude / longitude coordinates
These sites are documented in the IGRF-12 online data products section of this paper:
Thébault, Erwan, Christopher C. Finlay, Ciarán D. Beggan, Patrick Alken, Julien Aubert, Olivier Barrois, Francois Bertrand et al. "International geomagnetic reference field: the 12th generation." Earth, Planets and Space 67, no. 1 (2015): 79.
Note that the same section also lists C and Fortran code that could be used in a programmatic solution.