15
CHAPTER 2
GEODESY AND DATUMS IN NAVIGATION
GEODESY, THE BASIS OF CARTOGRAPHY
200. Definition
Geodesy
is the science concerned with the exact posi
tioning of points on the surface of the earth. It also involves
the study of variations of the earth’s gravity, the application
of these variations to exact measurements on the earth, and
the study of the exact size and shape of the earth. These fac
tors were unimportant to early navigators because of the
relative inaccuracy of their methods. The precise accuracies
of today’s navigation systems and the global nature of sat
ellite and other longrange positioning methods demand a
more complete understanding of geodesy than has ever be
fore been required.
201. The Shape Of The Earth
The irregular
topographic surface
is that upon which
actual geodetic measurements are made. The measure
ments, however, are reduced to the
geoid
. Marine
navigation measurements are made on the ocean surface
which approximates the geoid.
The
geoid
is a surface along which gravity is always
equal and to which the direction of gravity is always perpen
dicular. The latter is particularly significant because optical
instruments containing level devices are commonly used to
make geodetic measurements. When properly adjusted, the
vertical axis of the instrument coincides with the direction of
gravity and is, therefore, perpendicular to the geoid.
The geoid is that surface to which the oceans would con
form over the entire earth if free to adjust to the combined
effect of the earth’s mass attraction and the centrifugal force
of the earth’s rotation. The ideal ocean surface would be free
of ocean currents and salinity changes. Uneven distribution
of the earth’s mass makes the geoidal surface irregular.
The geoid refers to the actual size and shape of the
earth, but such an irregular surface has serious limitations
as a mathematical earth model because:
• It has no complete mathematical expression.
• Small variations in surface shape over time intro
duce small errors in measurement.
• The irregularity of the surface would necessitate a
prohibitive amount of computations.
Figure 201. Geiod, ellipsoid, and topographic surface of the earth, and deflection of the vertical due to differences in mass.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document16
GEODESY AND DATUMS IN NAVIGATION
The surface of the geoid, with some exceptions, tends
to rise under mountains and to dip above ocean basins.
For geodetic, mapping, and charting purposes, it is nec
essary to use a regular or geometric shape which closely
approximates the shape of the geoid either on a local or glo
bal scale and which has a specific mathematical expression.
This shape is called the
ellipsoid
.
The separations of the geoid and ellipsoid are called
geoidal heights
,
geoidal undulations
, or
geoidal
separations
.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '10
 kolmogrov
 Geodesy, datum, World Geodetic System, geodetic survey, MODERN GEODETIC SYSTEMS

Click to edit the document details