Where reliance is to be placed on electronic charts for this purpose, it is important
that an approved hydrographic office/national authority issues them, thus ensuring
data is accurate, and up to date. Particular care must be taken when using charts
based on older datums and plotting GNSS derived positions on them, as the
accuracy of the charts will probably not be equal to that of the GNSS position. It is
anticipated that VTS authorities will be able to broadcast local chart corrections to
suitably equipped (ECDIS/ECS) vessels and to issue navigational warnings
electronically using AIS.
In confined waters, it is likely that VTS operators in monitoring vessel manoeuvres
will occasionally have need to increase the scale of their displays. In such
circumstances, it will be important that the electronic chart acting as the background
to the traffic image, is capable of showing increasing levels of survey detail, as
operators reduce the scale on their displays. This will only be possible where the
electronic chart is compiled from source survey data, rather than from an existing
paper chart. In these circumstances, it will also be important that the charted location
of radar sites is accurate to a maximum of 10 metres, if errors between radar and AIS
generated tracks, which will be all the more obvious at reduced range scales, are to
be avoided.
IHO standard S52 defines the standards for symbols and colours on official electronic
charts. Four variations of the basic colour scheme are available. These colour
schemes, whilst optimised for navigation in varying light conditions on the bridge of a
vessel, may not be suitable for VTS purposes ashore, particularly where operators
are required to study a display constantly for long periods.
4.1.3.3 Choice of VTS Symbols
These symbols may be found to be unsuitable for VTS purposes, for two reasons.
Firstly, those selected to represent AIS tracks may need to be accommodated
logically within an existing framework of symbols. Secondly, VTS centres will often
have need to represent visually on the traffic image, a much wider range of
information than is necessary onboard a vessel. For example, traffic management
may necessitate the use of symbols that depict different types and sizes of vessels.
Alternatively, it may be necessary to show which vessels have pilots embarked, and
which do not.
Where it is required for a VTS to transmit an synthetic or virtual AIS target to an
AIS/ECDIS fitted vessel, it will be necessary for that information to be transmitted in
terms which will be recognised by the vessel, however it is represented internally
within a VTS centre.
4.1.4 BENEFITS OF AIS
4.1.4.1 Automatic Vessel Identification
Continuous operation of AIS brings many benefits to the mariner. Principal amongst
these, as the name implies, is the automatic and immediate provision of vessel
identity (MMSI, call sign etc) and position, thereby facilitating rapid radio
communication where necessary. This benefit is of equal, if not even greater, value
to VTS authorities.
VTS organisations require vessels to report to the VTS centre when approaching or
entering the VTS area. Without AIS, VTS centres have to rely on vessels reporting