Calculating error in measuring geomagnetic Anomalies using IGRF
The key point here is that both theoretical models quoted - IGRF or WMM - are of the field from the Earth's core only, and only the largest scale parts of it at that. Only the WMM provides an error estimate with these values (as described above), and that error is a combination of "commission errors" from the approximation of the model to reality, and "omission errors" from various magnetic field sources which are not included in the model.

To calculate a magnetic anomaly (usually considered the field from the Earth's crust) you would remove all the other known field sources from the observed field:


  anomaly field = observed field - (core field + external fields)