CHAPTER 2
GEODESY AND DATUMS IN NAVIGATION
GEODESY, THE BASIS OF CARTOGRAPHY
200. Definition equal and to which the direction of gravity is always perpen-
dicular. The latter is particularly significant because optical
Geodesy is the science concerned with the exact posi- instruments containing level devices are commonly used to
tioning of points on the surface of the earth. It also involves make geodetic measurements. When properly adjusted, the
the study of variations of the earth s gravity, the application vertical axis of the instrument coincides with the direction of
of these variations to exact measurements on the earth, and gravity and is, therefore, perpendicular to the geoid.
the study of the exact size and shape of the earth. These fac- The geoid is that surface to which the oceans would
tors were unimportant to early navigators because of the conform over the entire earth if free to adjust to the com-
relative inaccuracy of their methods. The precise accuracies bined effect of the earth s mass attraction and the
of today s navigation systems and the global nature of sat- centrifugal force of the earth s rotation. The ideal ocean
ellite and other long-range positioning methods demand a surface would be free of ocean currents and salinity chang-
more complete understanding of geodesy than has ever be- es. Uneven distribution of the earth s mass makes the
fore been required. geoidal surface irregular.
The geoid refers to the actual size and shape of the
201. The Shape Of The Earth earth, but such an irregular surface has serious limitations
as a mathematical earth model because:
The irregular topographic surface is that upon which
actual geodetic measurements are made. The measure- " It has no complete mathematical expression.
ments, however, are reduced to the geoid. Marine " Small variations in surface shape over time intro-
navigation measurements are made on the ocean surface duce small errors in measurement.
which approximates the geoid. " The irregularity of the surface would necessitate a
The geoid is a surface along which gravity is always prohibitive amount of computations.
Figure 201. Geiod, ellipsoid, and topographic surface of the earth, and deflection of the vertical due to differences in mass.
15
16 GEODESY AND DATUMS IN NAVIGATION
The surface of the geoid, with some exceptions, tends
a b
f = ----------- .
to rise under mountains and to dip above ocean basins.
a
For geodetic, mapping, and charting purposes, it is
This ratio is about 1/300 for the earth.
necessary to use a regular or geometric shape which closely
The ellipsoidal earth model has its minor axis parallel to the
approximates the shape of the geoid either on a local or glo-
earth s polar axis.
bal scale and which has a specific mathematical expression.
This shape is called the ellipsoid.
203. Ellipsoids And The Geoid As Reference Surfaces
The separations of the geoid and ellipsoid are called
geoidal heights, geoidal undulations, or geoidal
Since the surface of the geoid is irregular and the sur-
separations.
face of the ellipsoid is regular, no one ellipsoid can provide
The irregularities in density and depths of the material
other than an approximation of part of the geoidal surface.
making up the upper crust of the earth also result in slight
Figure 203 illustrates an example. The ellipsoid that fits
alterations of the direction of gravity. These alterations are
well in North America does not fit well in Europe; there-
reflected in the irregular shape of the geoid, the surface that
fore, it must be positioned differently.
is perpendicular to a plumb line.
Since the earth is in fact flattened slightly at the poles
and bulges somewhat at the equator, the geometric figure
used in geodesy to most nearly approximate the shape of the
earth is the oblate spheroid or ellipsoid of revolution.
This is the three dimensional shape obtained by rotating an
ellipse about its minor axis.
202. Defining The Ellipsoid
An ellipsoid of revolution is uniquely defined by spec-
ifying two parameters. Geodesists, by convention, use the
semimajor axis and flattening. The size is represented by
the radius at the equator, the semimajor axis. The shape of
the ellipsoid is given by the flattening, which indicates how
closely an ellipsoid approaches a spherical shape. The flat-
tening is the ratio of the difference between the semimajor
and semiminor axes of the ellipsoid and the semimajor axis.
See Figure 202. If a and b represent the semimajor and
semiminor axes, respectively, of the ellipsoid, and f is the
flattening,
Figure 203. The geoid and two ellipsoids, illustrating how
the ellipsoid which fits well in North America will not fit
well in Europe, and must have a different origin.
(exaggerated for clarity)
A number of reference ellipsoids are used in geodesy
and mapping because an ellipsoid is mathematically sim-
pler than the geoid.
204. Coordinates
The astronomic latitude is the angle between the
plumb line at a station and the plane of the celestial equator.
It is the latitude which results directly from observations of
celestial bodies, uncorrected for deflection of the vertical
Figure 202. An ellipsoid of revolution, with semimajor
component in the meridian (north-south) direction. Astro-
axis (a), and semiminor axis (b).
nomic latitude applies only to positions on the earth. It is
GEODESY AND DATUMS IN NAVIGATION 17
reckoned from the astronomic equator (0°), north and south fers from the corresponding astronomic longitude by the
through 90°. prime vertical component of the local deflection of the ver-
The astronomic longitude is the angle between the tical divided by the cosine of the latitude. The geodetic
plane of the celestial meridian at a station and the plane of coordinates are used for mapping.
the celestial meridian at Greenwich. It is the longitude The geocentric latitude is the angle at the center of the
which results directly from observations of celestial bodies, ellipsoid (used to represent the earth) between the plane of
uncorrected for deflection of the vertical component in the the equator, and a straight line (or radius vector) to a point
prime vertical (east-west) direction. These are the coordi- on the surface of the ellipsoid. This differs from geodetic
nates observed by the celestial navigator using a sextant and latitude because the earth is approximated more closely by
a very accurate clock based on the earth s rotation. a spheroid than a sphere and the meridians are ellipses, not
Astronomic observations by geodesists are made with perfect circles.
optical instruments (theodolite, zenith camera, prismatic Both geocentric and geodetic latitudes refer to the ref-
astrolabe) which all contain leveling devices. When proper- erence ellipsoid and not the earth. Since the parallels of
ly adjusted, the vertical axis of the instrument coincides latitude are considered to be circles, geodetic longitude is
with the direction of gravity, and is, therefore, perpendicu- geocentric, and a separate expression is not used.
lar to the geoid. Thus, astronomic positions are referenced Because of the oblate shape of the ellipsoid, the length
to the geoid. Since the geoid is an irregular, non-mathemat- of a degree of geodetic latitude is not everywhere the same,
ical surface, astronomic positions are wholly independent increasing from about 59.7 nautical miles at the equator to
of each other. about 60.3 nautical miles at the poles.
The geodetic latitude is the angle which the normal to A horizontal geodetic datum usually consists of the
the ellipsoid at a station makes with the plane of the geodet- astronomic and geodetic latitude, and astronomic and geo-
ic equator. In recording a geodetic position, it is essential detic longitude of an initial point (origin); an azimuth of a
that the geodetic datum on which it is based be also stated. line (direction); the parameters (radius and flattening) of
A geodetic latitude differs from the corresponding astro- the ellipsoid selected for the computations; and the geoidal
nomic latitude by the amount of the meridian component of separation at the origin. A change in any of these quantities
the local deflection of the vertical. affects every point on the datum.
The geodetic longitude is the angle between the plane For this reason, while positions within a given datum are
of the geodetic meridian at a station and the plane of the directly and accurately relateable, those from different datums
geodetic meridian at Greenwich. A geodetic longitude dif- must be transformed to a common datum for consistency.
TYPES OF GEODETIC SURVEY
205. Triangulation each location. The lines are then connected by a series of
adjoining triangles forming quadrilaterals extending from
The most common type of geodetic survey is known as each end. All angles of the triangles are measured repeated-
triangulation. Triangulation consists of the measurement ly to reduce errors. With the longitude, latitude, and
of the angles of a series of triangles. The principle of trian- azimuth of the initial points, similar data is computed for
gulation is based on plane trigonometry. If the distance each vertex of the triangles, thereby establishing triangula-
along one side of the triangle and the angles at each end are tion stations, or geodetic control stations. The coordinates
accurately measured, the other two sides and the remaining of each of the stations are defined as geodetic coordinates.
angle can be computed. In practice, all of the angles of ev- Triangulation is extended over large areas by connect-
ery triangle are measured to provide precise measurements. ing and extending series of arcs to form a network or
Also, the latitude and longitude of one end of the measured triangulation system. The network is adjusted in a manner
side along with the length and direction (azimuth) of the which reduces the effect of observational errors to a mini-
side provide sufficient data to compute the latitude and lon- mum. A denser distribution of geodetic control is achieved
gitude of the other end of the side. in a system by subdividing or filling in with other surveys.
The measured side of the base triangle is called a base- There are four general classes or orders of triangula-
line. Measurements are made as carefully and accurately as tion. First-order (primary) triangulation is the most precise
possible with specially calibrated tapes or wires of Invar, an and exact type. The most accurate instruments and rigorous
alloy highly resistant to changes in length resulting from computation methods are used. It is costly and time-con-
changes in temperature. The tape or wires are checked pe- suming, and is usually used to provide the basic framework
riodically against standard measures of length. of control data for an area, and the determination of the fig-
To establish an arc of triangulation between two wide- ure of the earth. The most accurate first-order surveys
ly separated locations, the baseline may be measured and furnish control points which can be interrelated with an ac-
longitude and latitude determined for the initial points at curacy ranging from 1 part in 25,000 over short distances to
18 GEODESY AND DATUMS IN NAVIGATION
approximately 1 part in 100,000 for long distances. network of vertical control points. From these, the height of
Second-order triangulation furnishes points closer to- other positions in the survey can be determined by supple-
gether than in the primary network. While second-order mentary methods. The mean sea-level surface used as a
surveys may cover quite extensive areas, they are usually reference (vertical datum) is determined by averaging the
tied to a primary system where possible. The procedures are hourly water heights for a specified period of time at spec-
less exacting and the proportional error is 1 part in 10,000. ified tide gauges.
Third-order triangulation is run between points in a There are three leveling techniques: differential, trig-
secondary survey. It is used to densify local control nets and onometric, and barometric. Differential leveling is the
position the topographic and hydrographic detail of the ar- most accurate of the three methods. With the instrument
ea. Triangle error can amount to 1 part in 5,000. locked in position, readings are made on two calibrated
The sole accuracy requirement for fourth-order triangu- staffs held in an upright position ahead of and behind the in-
lation is that the positions be located without any appreciable strument. The difference between readings is the difference
error on maps compiled on the basis of the control. Fourth-or- in elevation between the points.
der control is done primarily as mapping control. Trigonometric leveling involves measuring a vertical
angle from a known distance with a theodolite and comput-
206. Trilateration, Traverse, And Vertical Surveying ing the elevation of the point. With this method, vertical
measurement can be made at the same time horizontal angles
Trilateration involves measuring the sides of a chain of are measured for triangulation. It is, therefore, a somewhat
triangles or other polygons. From them, the distance and direc- more economical method but less accurate than differential
tion from A to B can be computed. Figure 206 shows this leveling. It is often the only practical method of establishing
process. accurate elevation control in mountainous areas.
Traverse involves measuring distances and the angles In barometric leveling, differences in height are deter-
between them without triangles for the purpose of comput- mined by measuring the differences in atmospheric
ing the distance and direction from A to B. See Figure 206. pressure at various elevations. Air pressure is measured by
Vertical surveying is the process of determining eleva- mercurial or aneroid barometer, or a boiling point ther-
tions above mean sea-level. In geodetic surveys executed mometer. Although the accuracy of this method is not as
primarily for mapping, geodetic positions are referred to an el- great as either of the other two, it obtains relative heights
lipsoid, and the elevations of the positions are referred to the very rapidly at points which are fairly far apart. It is used in
geoid. However, for satellite geodesy the geoidal heights must reconnaissance and exploratory surveys where more accu-
be considered to establish the correct height above the geoid. rate measurements will be made later or where a high
Precise geodetic leveling is used to establish a basic degree of accuracy is not required.
Figure 206. Triangulation, trilateration, and traverse.
GEODESY AND DATUMS IN NAVIGATION 19
DATUM CONNECTIONS
207. Definitions of the points given with respect to one datum will differ
from those given with respect to the other. The differences
A datum is defined as any numerical or geometrical can be used to derive transformation formulas. Datums are
quantity or set of such quantities which serves as a refer- connected by developing transformation formulas at com-
ence point to measure other quantities. mon points, either between overlapping control networks or
In geodesy, as well as in cartography and navigation, two by satellite connections.
types of datums must be considered: a horizontal datum and Many countries have developed national datums which
a vertical datum. The horizontal datum forms the basis for differ from those of their neighbors. Accordingly, national
computations of horizontal position. The vertical datum pro- maps and charts often do not agree along national borders.
vides the reference to measure heights. A horizontal datum The North American Datum, 1927 (NAD 27) has
may be defined at an origin point on the ellipsoid (local datum) been used in the United States for about 50 years, but it is
such that the center of the ellipsoid coincides with the Earth s being replaced by datums based on the World Geodetic
center of mass (geocentric datum). The coordinates for points System. NAD 27 coordinates are based on the latitude and
in specific geodetic surveys and triangulation networks are longitude of a triangulation station (the reference point) at
computed from certain initial quantities, or datums. Mead s Ranch in Kansas, the azimuth to a nearby triangu-
lation station called Waldo, and the mathematical
208. Preferred Datums parameters of the Clarke Ellipsoid of 1866. Other datums
throughout the world use different assumptions as to origin
In areas of overlapping geodetic triangulation net- points and ellipsoids.
works, each computed on a different datum, the coordinates The origin of the European Datum is at Potsdam,
Figure 208. Major geodetic datum blocks.
20 GEODESY AND DATUMS IN NAVIGATION
Germany. Numerous national systems have been joined earlier Principal Triangulation of Great Britain (1783-1853).
into a large datum based upon the International Ellipsoid of Tokyo Datum has its origin in Tokyo. It is defined in
1924 which was oriented by a modified astrogeodetic meth- terms of the Bessel Ellipsoid and oriented by a single astro-
od. European, African, and Asian triangulation chains were nomic station. Triangulation ties through Korea connect the
connected, and African measurements from Cairo to Cape Japanese datum with the Manchurian datum. Unfortunately,
Town were completed. Thus, all of Europe, Africa, and Tokyo is situated on a steep slope on the geoid, and the single-
Asia are molded into one great system. Through common station orientation has resulted in large systematic geoidal sep-
survey stations, it was also possible to convert data from the arations as the system is extended from its initial point.
Russian Pulkova, 1932 system to the European Datum, and The Indian Datum is the preferred datum for India and
as a result, the European Datum includes triangulation as several adjacent countries in Southeast Asia. It is computed
far east as the 84th meridian. Additional ties across the on the Everest Ellipsoid with its origin at Kalianpur, in cen-
Middle East have permitted connection of the Indian and tral India. It is largely the result of the untiring work of Sir
European Datums. George Everest (1790-1866), Surveyor General in India
The Ordnance Survey of Great Britain 1936 Datum from 1830 to 1843. He is best known by the mountain
has no point of origin. The data was derived as a best fit be- named after him, but by far his most important legacy was
tween retriangulation and original values of 11 points of the the survey of the Indian subcontinent.
MODERN GEODETIC SYSTEMS
209. Development Of The World Geodetic System of Defense World Geodetic System 1972 (WGS 72).
Further refinement of WGS 72 resulted in the new World
By the late 1950 s the increasing range and sophistica- Geodetic System of 1984 (WGS 84). As of 1990, WGS 84 is
tion of weapons systems had rendered local or national being used for chart making by DMA. For surface navigation,
datums inadequate for military purposes; these new weap- WGS 60, 66, 72 and the new WGS 84 are essentially the same,
ons required datums at least continental in scope. In so that positions computed on any WGS coordinates can be
response to these requirements, the U.S. Department of De- plotted directly on the others without correction.
fense generated a geocentric reference system to which The WGS system is not based on a single point, but
different geodetic networks could be referred and estab- many points, fixed with extreme precision by satellite fixes
lished compatibility between the coordinates of sites of and statistical methods. The result is an ellipsoid which fits
interest. Efforts of the Army, Navy, and Air Force were the real surface of the earth, or geoid, far more accurately
combined leading to the development of the DoD World than any other. The WGS system is applicable worldwide.
Geodetic System of 1960 (WGS 60). All regional datums can be referenced to WGS once a sur-
In January 1966, a World Geodetic System Committee vey tie has been made.
was charged with the responsibility for developing an im-
proved WGS needed to satisfy mapping, charting, and 210. The New North American Datum Of 1983
geodetic requirements. Additional surface gravity observa-
tions, results from the extension of triangulation and The Coast And Geodetic Survey of the National Ocean
trilateration networks, and large amounts of Doppler and op- Service (NOS), NOAA, is responsible for charting United
tical satellite data had become available since the States waters. From 1927 to 1987, U.S. charts were based
development of WGS 60. Using the additional data and im- on NAD 27, using the Clarke 1866 ellipsoid. In 1989, the
proved techniques, the Committee produced WGS 66 which U.S. officially switched to NAD 83 (navigationally equiva-
served DoD needs following its implementation in 1967. lent to WGS 84 and other WGS systems) for all mapping
The same World Geodetic System Committee began and charting purposes, and all new NOS chart production is
work in 1970 to develop a replacement for WGS 66. Since the based on this new standard.
development of WGS 66, large quantities of additional data The grid of interconnected surveys which criss-crosses
had become available from both Doppler and optical satellites, the United States consists of some 250,000 control points,
surface gravity surveys, triangulation and trilateration surveys, each consisting of the latitude and longitude of the point,
high precision traverses, and astronomic surveys. plus additional data such as elevation. Converting the NAD
In addition, improved capabilities had been developed 27 coordinates to NAD 83 involved recomputing the posi-
in both computers and computer software. Continued re- tion of each point based on the new NAD 83 datum. In
search in computational procedures and error analyses had addition to the 250,000 U.S. control points, several thou-
produced better methods and an improved facility for han- sand more were added to tie in surveys from Canada,
dling and combining data. After an extensive effort Mexico, and Central America.
extending over a period of approximately three years, the Conversion of new edition charts to the new datums,
Committee completed the development of the Department either WGS 84 or NAD 83, involves converting reference
GEODESY AND DATUMS IN NAVIGATION 21
points on each chart from the old datum to the new, and ad- adjustment of the graticule is the only difference between
justing the latitude and longitude grid (known as the charts which differ only in datum. All charted features re-
graticule) so that it reflects the newly plotted positions. This main in exactly the same relative positions.
IMPACTS ON NAVIGATION
211. Datum Shifts of recent surveys. Currently issued charts of some areas are
based on surveys or use data obtained in the age of sailing
One impact of different datums on navigation appears ships. The lack of surveyed control points means that they
when a navigation system provides a fix based on a datum cannot be properly referenced to modern geodetic systems.
different from that used for the nautical chart. The resulting In this case there may be a note that says: Adjustments to
plotted position may be different from the actual location WGS cannot be determined for this chart.
on that chart. This difference is known as a datum shift. A few charts may have no datum note at all, but may
Another effect on navigation occurs when shifting be- carry a note which says: From various sources to (year).
tween charts that have been made using different datums. If In these cases there is no way for the navigator to determine
any position is replotted on a chart of another datum using the mathematical difference between the local datum and
only latitude and longitude for locating that position, the WGS positions. However, if a radar or visual fix can be very
newly plotted position will not match with respect to other accurately determined, the difference between this fix and a
charted features. This datum shift may be avoided by re- satellite fix can determine an approximate correction factor
plotting using bearings and ranges to common points. If which will be reasonably consistent for that local area.
datum shift conversion notes for the applicable datums are
given on the charts, positions defined by latitude and longi- 212. Minimizing Errors Caused By Differing Datums
tude may be replotted after applying the noted correction.
The positions given for chart corrections in the Notice to To minimize problems caused by differing datums:
Mariners reflect the proper datum for each specific chart and
edition number. Due to conversion of charts based on old da- " Plot chart corrections only on the specific charts and edi-
tums to more modern ones, and the use of many different tions for which they are intended. Each chart correction
datums throughout the world, chart corrections intended for is specific to only one edition of a chart. When the same
one edition of a chart may not be safely plotted on any other. correction is made on two charts based on different da-
These datum shifts are not constant throughout a given tums, the positions for the same feature may differ
area, but vary according to how the differing datums fit to- slightly. This difference is equal to the datum shift be-
gether. For example, the NAD 27 to NAD 83 conversion tween the two datums for that area.
results in changes in latitude of 40 meters in Miami, 11 " Try to determine the source and datum of positions of
meters in New York, and 20 meters in Seattle. Longitude temporary features, such as drill rigs. In general they are
changes for this conversion are about 22 meters in Miami, given in the datum used in the area in question. Since
35 meters in New York, and 93 meters in Seattle. these are usually positioned using satellites, WGS is the
Most charts produced by DMA and NOS show a da- normal datum. A datum correction, if needed, might be
tum note. This note is usually found in the title block or in found on a chart of the area.
the upper left margin of the chart. According to the year of " Remember that if the datum of a plotted feature is not
the chart edition, the scale, and policy at the time of produc- known, position inaccuracies may result. It is wise to al-
tion, the note may say World Geodetic System 1972 low a margin of error if there is any doubt about the
(WGS-72) , World Geodetic System 1984 (WGS-84) , or datum.
World Geodetic System (WGS). A datum note for a chart " Know how the datum of the positioning system you
for which satellite positions can be plotted without correc- are using (Loran, GPS, etc.) relates to your chart.
tion will read: Positions obtained from satellite navigation GPS and other modern positioning systems use the
systems referred to (REFERENCE DATUM) can be plot- WGS datum. If your chart is on any other datum, you
ted directly on this chart. must apply a datum correction when plotting the
DMA reproductions of foreign chart s will usually be GPS position of the chart.
in the datum or reference system of the producing country.
In these cases a conversion factor is given in the following Modern geodesy can support the goal of producing all
format: Positions obtained from satellite navigation sys- the world s charts on the same datum. Coupling an elec-
tems referred to the (Reference Datum) must be moved tronic chart with satellite positioning will eliminate the
X.XX minutes (Northward/Southward) and X.XX minutes problem of differing datums because electronically derived
(Eastward/ Westward) to agree with this chart. positions and the video charts on which they are displayed
Some charts cannot be tied in to WGS because of lack are derived from one of the new worldwide datums.
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