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 .
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 .