XIII. GRAVITY MEASUREMENTS 



Geophysics Section 

 Survey Branch 



A. INTRODUCTION 



The gravity program of the U. S. Navy Hydrographic Office includes 

 measurements of the vertical component of the earth's gravitational 

 field on land and sea and probably will include measurements in the 

 air within the near future. These data are required for correcting 

 astronomically determined positions on the earth for observational 

 error due to deflection of the vertical, an effect created by the unequal 

 distribution of mass in the earth's crust. These data also are required 

 for use as an aid to navigation by submerged submarines and for calcu- 

 lating area deflections of the vertical for missile guidance systems. 



Measurements of gravity are expressed in gals (for Galileo) and 

 milligals, one gal being equal to an acceleration of one centimeter per 

 second per second. Accuracies in the order of one part per million are 

 readily obtainable. Variations in gravity are caused primarily by 

 topography, since variation is a function of the distance from the center 

 of the earth. Other factors contributing to variations in gravity are 

 the shape of the earth and the geologic structure of the earth's crust. 

 Values of gravity on the earth's surface range approximately between 

 978.0490 gals at the equator to 983.2213 gals at the poles (± 5,200 

 milligals). A one-foot change in elevation is equivalent to a 0.094- 

 milligal change in gravity on land or a 0.068-milligal change under 

 water. For geodetic and navigation applications, measurements in 

 the order of 0.1 milligal are considered to be sufficiently accurate. 



Measurements of gravity are accomplished generally by one of three 

 methods: dropped ball, pendulum, or spring gravimeter. Only the last 

 is used by the Hydrographic Office. Absolute measurements of gravity 

 are complicated and required at only a few places in the world to estab- 

 lish a primary reference for all other measurements. The absolute 

 measurement is accomplished by accurately timing the acceleration of 

 a mass (ball, cylinder, etc.) falling over a known distance in an evac- 

 uated chamber. The National Bureau of Standards currently is engaged 

 in constructing the necessary equipment for a new and more precise 

 determination of absolute gravity by this method. 



Fortunately geodesy generally is concerned only with measuring 

 the difference in gravity from one place to another, and such measure- 

 ments are fairly simple to make. Relative gravity measurements are 



XIII- 1 



