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I’m a new prospector who was given the task of learning a GEM GSM-19T Proton Magnetometer. I’ve learned how to set it up, take base readings, and take walking surveys. However when setting up a base reading, I’ve been given the prompt to change the Datum number. The last user had it set to 53113.00. According to the manual, the Datum represents the offset or shift imposed to the Diurnal Correction. I want to upload the survey data to a computer but I want to make sure that this number is correct before doing so. I don’t know how to find this Datum, and am wondering if there’s a way I can figure it out.

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I have used the GEM magnetometers for 25 years as a professional geophysicist.
You can use the total field intensity of an IGRF model, or you can use the average value of all of the readings of the base station mag. For example you can use the NOAA online service. Just enter the date and latitude and longitude, and you will get the magnetic field parameters.

Either way, don't change the value and don't move the base station during the survey.
That being said, the "datum" entered into the Gem magnetometer is not used in the diurnal corrections. During post processing, the user overrides that value when diurnally correcting using GemlinkW software.
So no need to worry about the datum you enter into the mag. Still, it is still useful know the magnetic field, as this is used for tuning the magnetometer. But, that is another story.

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