An attempt to obtain a detailed declination chart from the United States magnetic anomaly map

Abstract

Modern declination charts of the United States show almost no details. Greater detail may be of value to surveyors trying to follow old land deed descriptions, or to pilots of small planes or small pleasure boats operating in inland waterways. It would be extremely expensive to make adequate declination measurements needed for such a chart. It was hoped that declination details could be derived from the information contained in the existing magnetic anomaly map of the United States. This could be realized only if all of the survey data were corrected to a common epoch, at which time a main-field vector model was known, before the anomaly values were computed. Because this was not done, accurate declination values cannot be determined. In spite of this conclusion, declination values were computed using a common main-field model for the entire United States to see how well they compared with observed values. The provisional geomagnetic reference field for 1978.5 was used as the main-field model. The computed detailed declination values were found to compare less favorably with observed values of declination than declination values computed from the IGRF 1985 model itself. This result indicates that the computed anomaly elements or their combination with main-field values cannot be used as accurate anomaly values, but they may be used as an indication of where anomalies probably occur.