This FORTRAN-backed webservice provides an interface to the 'gufm1' model as published by Andrew Jackson, Art R. T. Jonkers and Matthew R. Walker. From the abstract of their paper:
We present a new model of the magnetic field at the core-mantle boundary for the interval 1590-1990. The model, called gufm1, is based on a massive new compilation of historical observations of the magnetic field. The greater part of the new dataset originates from unpublished observations taken by mariners engaged in merchant and naval shipping.
For the period before 1800, more than 83000 individual observations of magnetic declination were recorded at more than 64000 locations; more than 8000 new observations are for the 17th century alone. The time-dependent field model that we construct from the dataset is parametrized spatially in terms of spherical harmonics and temporally in B-splines, using a total of 36512 parameters.
This webservice can be deployed to Heroku using the Fortran buildpack.
If you would like to play with this a bit, I have the service deployed at (gufm.herokuapp.com)gufm.herokuapp.com/?lat=52.452701&lng=13.232689&year=1900. In case of any problems check the service health at (Pingdom)[http://stats.pingdom.com/8xys4bppzbav/537892].