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 The greater part of the new dataset originates from unpublished observations taken by mariners engaged in merchant and naval shipping.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 (the given link has arguments of an example query for Berlin, Germany in the year 1900).