referred to the points located on the International Ellipsoid directly 
under the observation points. These deviations will be due to the alti- 
tude of an observation point in respect to the altitude of its projection 
point located on the Ellipsoid, the topography and the type, that is, the 
density of rocks present near the point of observation, and on the mag- 
nitude of the isostatic compensation. It is assumed in the commercial 
and semi-rigorous scientific surveys, that in correction for altitude 
one meter in height corresponds to the change in acceleration of grav- 
ity equal to +0.3086 milligals, where one milligal is equal to 1/1000 of 
a gal. The observed gravity corrected on altitude is called Free-Air 
Gravity, and the difference between the Free -Air Gravity value at the 
point of observation and the value of gravity at its projection point 
located on the Ellipsoid is referred to as Free-Air correction. Simi- 
larly Free -Air Gravity corrected on the average density and topography 
of rocks located between the altitudes passing through the point of ob- 
servation and its projection on the Ellipsoid, or sea level, is referred 
to as Bouguer gravity. The difference between the Bouguer gravity of 
an observation point and the gravity of its projection point located on 
the Ellipsoid is referred to as Bouguer anomaly. The Bouguer gravity 
corrected for certain assumptions based on the Theory of Isostasy, 
minus the value of gravity at the projection point located on the EUip^- 
soid, is referred to as isostatic anomaly. In general, it is assumed 
that isostatic anomaly, neglecting the errors in computation and as- 
sumptions, is due to the incomplete compensation of the blocks of the 
earth's crust floating in a heavier, semi-plastic substratum. According 
to Airy, an English scientist of the middle of the 19th century, the blocks 
of the earth are floating as icebergs in the ocean: the higher the ave- 
rage altitude, the greater the submergence (Mountain Roots' Theory). 
In this article, for the convenience of plotting and under- 
standing, Free -Air Gravity and Bouguer Gravity are assumed to be t 
values of gravity before the gravity corrections are applied for altitt 
and density layers respectively in variance with the common usage. 
Gravitational methods are used for both scientific and 
utilitarian purposes. 
understanding 
shape 
the 
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