Surveying question: How do you adjust for true magnetic north over a period of years?

I have heard that true magnetic north fluctuates a matter of degrees over time. When attempting to apply a surveyors' map to the actual terrain using a magnetic compass, I see that this could present a problem in accuracy, if true, depending on what year the map was made and what year the compass is being used. In this specific case, the map was made June 12 1978. The location is SW Virginia, I think roughly latitude 37 degrees by -80 longitude.Surveying question: How do you adjust for true magnetic north over a period of years?
YOU WILL NEED TO GET THE EPHEMERIS TABLES OF MAGNETIC DECLINATION FOR THE TIME PERIODS YOU WISH TO ADJUST............THE DECLINATION CHANGES AS YOU RISE IN LONGITUDE....HERE IN FLORIDA THERE IS NOT MUCH CHANGE DUE TO OUR LONGITUDE BEING CLOSE TO THE EQUATORSurveying question: How do you adjust for true magnetic north over a period of years?
Hi, I talked to my Dad he is a civil engineer, he said yes that's true.that you have to check the ';Isogonic'; chart for the year that the map was made, it will show the variable seperation between true North and magnetic north for different parts of the country, you will have to make adjustments from there.