Magnetic Declination Info

This reference table is used to adjust the azimuth readings of downhole survey points that were recorded relative to magnetic north.

The table represents a number of rectangular coordinate windows (in lat/long), each with a magnetic declination value. To allow for temporal drift (due to the migration of the Earth's magnetic north pole), the windows are linked to a base year (epoch), along with the annual rate of change in the value of the magnetic declination. The windows can be of any size and may even overlap.

When doing azimuth adjustments for readings taken relative to magnetic north, the system will automatically locate the most appropriate coordinate window, retrieve the magnetic declination value for the base year and adjust this value based on the time difference between the base epoch year and the supplied survey date. Any coordinate conversions required to perform this task are done automatically.

These data are usually imported from text files; however, individual windows can also be added manually. The following parameters are entered in the editor:

Field	Description
Geodetic datum	Geodetic datum on which the lat/long windows are based
Base epoch	The year to which the reference data applies
Minimum latitude	Minimum latitude for the coordinate window (in decimal degrees)
Maximum latitude	Maximum latitude for the coordinate window
Minimum longitude	Minimum longitude for the coordinate window
Maximum longitude	Maximum longitude for the coordinate window
Magnetic declination	The magnetic declination value for the window (in decimal degrees)
Annual change	Rate of change (decimal degrees per year)
Data source	Source of the reference data

 

Magnetic declination data can be sourced from the internet by visiting sites such as; https://www.ngdc.noaa.gov/geomag-web/#igrfgrid