This chapter will give you an
introduction to the principle used for
creating electronic charts help you
understand the limitations involved with
the different methods used to create
electronic charts and how to minimize
the effect of these limitations charting
and mapping organizations as well as
private companies were early users of
computer graphics to automate their
chart and map production
the aim was to increase production
efficiency and to reduce costs until a
few years ago the final product however
was the same printed paper charts
position related information about the
landscape is referred to as geographical
data the main element of geographical
data is geometric data IE description of
position size and shape of the objects
located in the landscape light houses
houses boys etc geometric data can be
stored digitally in two different ways
as raster data or vector data an
electronic navigational chart
what is it the understanding of this
expression very very much and the need
for better definitions resulted in some
standard terms accepted by I'm oh I most
standard terms used in connection with
electronic chart production and they use
are found in I'm Oh resolution a 8 17 19
the definition of an electronic chart
database abbreviated ecd be is a nice
EDB is a master database of chart
information held in digital form by a
national
hydrographic authority the definition of
an electronic navigational chart
abbreviated DNC is an e NC is a subset
of the ecd be held on a vessel it
contains useful information for
navigation such as coastlines
lighthouses beacons etc an electronic
chart is constructed using either raster
data or vector data scanning the master
components used in the production of the
paper chart produces raster data the
resultant image is made up of a number
of lines which in turn are made up of
large number of colored dots or pixels
and the chart is simply a digital
facsimile of a paper chart and so it is
not possible to interrogate the raster
data to obtain information about any
object on the chart rusty data is
therefore considered to be an
intelligent data giving digital values
to each and every object on the chart
produces vector data these values enable
a computer system to identify the object
to portray it in the required form and
color to place it in its correct
position and to provide information
about its attributes
it is therefore possible to interrogate
a vector chart in order to obtain
information about charted objects and to
enable arms to be raised when
potentially unsafe situations occur
vector charts can be customized and are
therefore considered to represent
intelligence data that
availability of official approved
electronic charts is very limited and
this situation is likely to continue for
a long time
this is a fact that must be fully taken
into consideration when using electronic
chart and electronic chart systems the
accuracy of data on a rusty chart is
exactly the same as that of an official
paper chart since the data depicted is
identical electronic chart systems used
to display rested charts converted to
the wgs 84 coordinates obtained from a
gps receiver into coordinates reference
to the datum of the relevant rested
chart wgs84 shift data for significant
areas of the world will not be available
for some time
currently geodetic survey data is
available to compute the relationship
between wgs84 and local datum for about
sixty percent of Admiralty charts the
worst known difference between a local
datum and the wgs 84 is of the order of
seven nautical miles and differences of
several hundred meters are not uncommon
navigators requiring using a chart which
no wgs84 shift information is available
should use GPS positions with extreme
caution
they should first establish the
relationship between the chart datum
wgs84 by taking visual and or radar
fixes of sure features this will provide
a means of adjusting a gps position to
fit locally on the chart datum
transformation parameters such as those
built into a number of gps receivers
should be used with caution
these transformation parameters may be
based on limited or provisional data or
they may represent
and an average across a wide area errors
in the range of hundreds of meters could
occur when using the gps in the
following areas areas which are at the
extremity of dayton coverage areas where
geometric control is weak areas where
survey data is too old
navigators should take particular care
when using the scale or zoom facility in
the electronic chart system it is all
too easy to zoom in and effectively
select a larger scale than the one used
for the compilation of the data
this could create a false impression
about the reliability of the charted
data regular checks should be made to
ensure that the scale selected on the
electronic chart system is the same as
that on the equivalent official paper
chart the high-tech appearance of an
electronic arrested chart interface with
systems such as GPS can easily mislead
the user into the belief that the
charted data is more accurate than is
really the case it will be many years
before a significant number of raster
charts exist with accuracy similar to
that provided by dgps until then
navigators must take particular care to
assess the accuracy of the data
presented to them by their roster charts
the IMO performance standards practice
state that information to be used
enactus should be the latest edition of
information originated by a government
authorized hydrographic office and
conform to standards laid down by iho
this means that officially approved
vector charts must be used enactus since
few officially approved vector charts
exist navigators using commercially
produced vector charts must work on the
principle that the data on a corrected
official paper chart
is more reliable than that on the
corresponding vector chart non-official
approved electronic charts are available
from a variety of sources for example
the ltk hydrographic offices Admiralty
rested chart service arcs provides
worldwide coverage with a series of
about 2,500 rusty charts commercial
chart providers such as trans ass marine
and see map claim to provide near
worldwide coverage with a series of
vector charts