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In textbooks it is mentioned that

Magnetic anomaly = Observed Geomagnetic field - Theoretical Geomagnetic field

observed value is measured using instruments like proton precession magnetometer or flux gate magnetometer etc , whereas Theoretical values are computed by IGRF formula which takes coordinates , elevation of the point as input . (https://www.ngdc.noaa.gov/geomag-web/#igrfwmm )

My doubt is regarding error in anomaly calculation . In the observed part the error is in the range 0.1 - 1 nT depending on the instrument . IGRF error for total field is around 152 nT .

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(from : https://www.ngdc.noaa.gov/geomag/WMM/limit.shtml )

Therefore our total error in magnetic anomaly calculation is around 152-153 nT . In such case if our anomaly ( observed - theoretical ) is small (say around 30 nT ) , what would be the validity of such observation as the value we are trying to estimate is smaller than the margin of error .

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  • $\begingroup$ Are you using the IGRF or the WMM? From your question, it appears that you consider them two different names for the same thing, but they're actually two different models. Your first step should probably be to decide which model you want to use and make sure that you actually are using that model :). $\endgroup$
    – Pont
    Jul 24, 2017 at 19:10
  • $\begingroup$ I am using the following website : ngdc.noaa.gov/geomag-web/#igrfwmm . it says that "Magnetic field is calculated using the most recent World Magnetic Model (WMM) or the International Geomagnetic Reference Field (IGRF) model." Final error is mentioned in a column after giving lat,long and elevation as inputs . $\endgroup$
    – shrey
    Jul 25, 2017 at 3:26

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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)

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