# Calculating error in measuring geomagnetic Anomalies using IGRF

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 .

• 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 :). – Pont Jul 24 '17 at 19:10
• 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 . – shrey Jul 25 '17 at 3:26