With the increasing accuracy and
reliability of navigational receivers
this part of the cbt module will try to
establish a better understanding of the
mathematical definition of the earth's
surface and mapping of this curve shape
onto a plane surface chart projections
constitute the way of representing the
Earth's surface in a plane distortions
are impossible to avoid when
representing a surface with double
curvature in a plane surface different
methods for representing this are
designed to keep certain properties of
the elements soil surface undistorted
the different possibilities are conform
the charts are angular correct IE any
angle on the chart is exactly the same
on the earth for example makita or
transverse Mercator projection
equivalent the areas on the chart are
correct
ie the ratio between any area on the
chart and the corresponding area of the
earth is correct charts used at sea for
navigation are normally made indicator
or transverse Mercator projection on a
Mercator chart a straight line between
two points represents the course between
the points chart projections are
classified according to the type of
projection surface used the three
commonly used surfaces are the plane
polar projection the code Lambert
projection the cylinder mikaido
projection most navigation charts are
constructed using a cylinder projection
the two most commonly used to these are
the makita and the transverse Mercator
Giada see is the science concerned
the exact positioning of points on the
surface of the earth
it also involves the study of the exact
size and shape of the earth the study of
variations of the Earth's gravity and
the application of these variations to
exact measurements on the earth this
figure represents a vertical slice
through the earth
it shows the uneven surface of the earth
but for survey purposes and they are
reduced to the geoid the geoid is a
surface along which gravity is always
equal and to which the direction of
gravity is always perpendicular since
the geoid refers to the actual size and
shape of the earth the uneven
distribution of the Earth's mass makes
the g idol surface irregular 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 introduced
small errors in measurement the
irregularity of the surface would
necessitate a prohibitive amount of
computations for geodetic mapping and
charting purposes it is necessary to use
a regular or geometric shape that
closely approximates the shape of the
geoid which has a specific mathematical
expression this shape is called an
ellipsoid which is referred to as a
spheroid rotating an ellipse about its
minor axis forms an ellipsoid g-o-d
cysts use the term flattening to
indicate how much an ellipsoid departs
from a spherical shape for the earth
this flattening has a ratio of about 1
300 since the surface of the geoid is
irregular and the surface of an
ellipsoid is regular many different
ellipsoids have been developed to
provide a best-fit to the geoid for
different areas of the earth's surface
the figure shows 2 different ellipsoids
one fits well in Europe but not in
Africa and the other fits well in Africa
but not in Europe before a particular
lips I'd can be used for positional
calculations its relationship to the
geoid must be defined and the definition
of such a relationship is known as the
datum the ideal geodetic datum for a
worldwide reference system is one that
is truly geocentric and oriented through
the two poles and the Greenwich Meridian
derived from this other following two
groups of datums local datums that are
based on convenience and best fit a
certain area satellite datums that are
used for global references the first
satellite datum established in 1972 was
based on the Navy navigation satellite
system this system was also known under
the name transit in 1989 the NSS
satellite datum was replaced by the
global positioning system based world
geodetic survey 84 wgs84 datum wgs84 is
a global dating system based on many
points fixed with great accuracy
the result is an ellipsoid that is
centered on the Earth's center of mass
and that fits the geoid far more
accurately than any other dusk
wgs 84 is the reference datum for gps
and this relationship can be the source
of significant error for navigators
wishing to plot a GPS fix on a chart
that is not reference to wgs 84 in order
to convert positions on one datum two
positions on another it is necessary to
establish the relationship between a
number of known points that are common
to both datums
most maritime charts contain a source
data diagram or explanatory notes that
provide information on the origin scale
date and limits of the hydrodynamic
surveys used in compiling the chart
these notes also include information
about the origin of the topographical
data the projection used the datum to
which positions on the chart are
referred some charts will have a note
satellite-derived positions this
normally means that if a gps is used for
positioning positions obtained from the
gps must be adjusted before they are
used together with the map this is a
specific example taken from chart ba
2449 remember that no transformation is
perfect because both the original datum
and the datum to which positions are
adjusted have inherent weaknesses that
vary over their area of coverage the
parameters used within gps receivers for
the transformations from one datum to
another are likely to be meaning values
for the whole area and their application
particularly in the extremities of the
area of an accurately defined datum
could introduce errors of hundreds of
meters in conventional navigation IE
plotting ship's position manually onto
the chart it is better to keep the gps
receiver reference to wgs 84 datum and
to apply the datum shift values shown on
the chart to the wgs 84 position output
from the receiver scene from The
Navigators point of view all charts
should have been referenced to wgs 84
however this is not likely to happen for
many years
there are several reasons for this
wgs 84 is a relatively new datum even
with sufficient economic backing it
would take a considerable period of time
to replace existing charts with new
additions using the new datum lack of
control data in some parts of the world
various political reasons in several
countries delay the process some charge
carrier note indicating that datum shift
values for gps derived positions cannot
be determined
these are charts for which in sufficient
detail is known about the horizontal
datum it is important to note that in
the worst cases such as isolated oceanic
islands positions may be several miles
different from those derived from GPS in
such cases Mariners are advised to make
greater use of traditional methods of
observational position fixing when
closing sure or navigating in the
vicinity of dangers the relative
positions of features may be more
reliable for navigation than the use of
unadjusted satellite-derived positions
on a chart who's horizontal datum cannot
be defined this illustration shows the
horizontal datum differences in meters
that exists between positions referred
to osg b-36 ed 50 wgs 72 and wgs84
datums in the Dover straight
thus for example a nun corrected gps
position referred to wgs 84 datum
plotted on a chart reference to osg b-36
datum would be 139 meters in error
satellite navigation has made the art of
position fixing extremely easy for
everybody
however position fixing is not the only
parameter to take into consideration
during a voyage as long as safe and
efficient navigation of a ship remains
the goal the author of this cbt believes
that professional and experienced
navigators will still be needed on ships
for many many years to come