XIV. GEOMAGNETIC MEASUREMENTS 



Floyd Woodcock and A. Joseph Heckelman 

 Geomagnetic s Branch 



A. INTRODUCTION 



World charts of all elements of the earth 1 s magnetic field are 

 published by the Hydrographic Office. These charts are brought to 

 epoch by the application to old data of known or estimated secular 

 change rates and by the inclusion of new data. Magnetic data compiled 

 from foreign agencies and various U. S. agencies, including the 

 Hydrographic Office, are maintained in a central U. S. depository. At 

 present, airborne geomagnetic surveys by the Hydrographic Office 

 are virtually the only available source of new data for the ocean 

 areas of the world. 



Magnetic Variation (Variation of the Compass) Charts are published 

 each five years, for Epochs 1955, I960, 1965, etc. Horizontal Intensity, 

 Vertical Intensity, Magnetic Inclination (Dip), and Total Intensity Charts 

 are published each decade, for Epochs 1955, 1965, 1975, etc. Charts 

 published subsequent to Epoch 1955 reflect the new data obtained by 

 Hydrographic Office surveys over the ocean areas. 



Magnetic information is important to a variety of military applica- 

 tions, some of which are: (1) USW, ASW, and mine warfare, (2) weapons 

 systems, (3) guidance, (4) navigation, direction and position fixing, 

 (5) degaussing, and (6) compass swinging. All nautical and aeronautical 

 charts include magnetic declination information needed by navigators. 



The Hydrographic Office receives frequent requests for magnetic 

 data from persons and agencies engaged in geophysical research. 

 Studies of interactions between the geomagnetic field and charged 

 particle motions in the upper atmosphere and space are advanced by 

 a thorough knowledge of terrestrial magnetism. 



In many cases, limitations on the applications of geomagnetism are 

 imposed by nature itself. Irregular temporal changes and anomalies 

 in some cases are limitations. In addition, inadequate knowledge of 

 geomagnetism often imposes still stricter limitations. The latter, 

 however, can be looked upon as a challenge. 



B. DEFINITIONS OF TERMS 



Some of the terms used in geomagnetic measurements are defined 



XIV- 1 



