The U.S. Geological Survey has published three new magnetic data maps and a map of the Mount St. Helens area that was devastated by the May 18 eruption.

One of the magnetic data maps shows lines of equal magnetic declination for the 50 states and the locations of the more than 100 stations and observatories at which measurements are made. Magnetic declination, also called compass variation, is the angle between true north and magnetic north. The earth's magnetic field constantly changes, and the USGS replots the U.S. data every 5 years. The latest map is based on measurements through 1979 and is projected by model estimates through January 1980. Designated Map I-1283, it measures 32 x 40 inches, and displays 48 states on one side (scale of 1:5,000,000) and Alaska and Hawaii on the other (scale of 1:7,500,000).