- Sounding crossed with bearing
Traditional navigation methods
- Charted object
The echo-sounder has of
late become simply a tool to
prevent you running aground,
but in conjunction with the
chart it can be a useful source
of position information.
To use the echo-sounder for
position, you need to know
the current height of tide.
For coastal passages at low
speeds it’s enough to choose
a port close to your planned
route and work out the height
of tide for each hour as a
quick reference. Armed with
this information, you can pick
up depth contours as you sail
towards the coast. To pick up
the 5m contour, for example,
add 5m to your predicted
height of tide, then sail in
until the echo-sounder
reaches the required depth.
Once you are on a depth
contour, take a bearing of
a charted object: your
approximate position will be
where the bearing and the
contour intersect. A depth
contour is unlikely to be
completely accurate, especially
if the chart is based on an
old survey, so treat positions
obtained this way with care. If
possible, another intersecting
bearing will add accuracy. A depth contour may be the fi rst useful postion line available when making landfallThis is the classic classroom
technique for fi xing your position.
Choose three charted objects,
maximising the angles between
them to minimise error. Lighthouses
and buildings are the ideal choice
as they don’t move and are easy
to sight through the compass.
Headlands can be good, but
shallow promontories are harder
to accurately sight than steep cliffs.
Buoys can be used, but remember
that they move on the tide.
Take your bearings as quickly
as possible, noting them down
as you go and leaving the one
most abeam until last – this
bearing changes the fastest.
Then plot them on the chart.
Unless you have a stationary boat
and extraordinarily steady hands- Transit and bearing
In the absence of another transit,
a bearing on a fi xed object
provides a good fi x. Again, try
to choose an object which lies
at roughly 90° to the transit –
shallow angles of intersection
create greater errors.
A river entrance transit crossed
with a bearing at roughly 90°
- Three-point fi x
the lines will not intersect perfectly,
instead coming together in a
triangle called a ‘cocked hat’. Yourtrue position could be anywhere
within this triangle, so the smaller
it is, the more accurate your fi x.235°T1025Eight traditional ways to get a fi x
- Transits 3. Crossed transits
One of the easiest ways to fi x your position is
to sail up alongside a charted object. Buoys
are the obvious choice, but remember that
they can be relocated, especially in areas of
shifting sand or mud banks. Keep your charts
up to date, and where possible choose an
object fi rmly fi xed to the seabed. Grab your
chart, make a cross next to the object, note
the time and log reading, and you’re done.
Transits are formed when two charted objects
are in line. They are often found marked on
charts, or in pilot books, to give a clearance
from a danger or a safe entrance into port.
A single transit provides an accurate line
of position and gets you halfway to an
accurate fi x – you just need to fi nd where
you are on the line.The best way to fi nd your position on a
transit is to fi nd another one which crosses
it, ideally at about 90°. This is a tremendously
accurate way to fi x your position, but you
generally need to plan the fi x beforehand,
so it’s often used to give an accurate fi x for
starting a passage.
The easiest way is to fi nd charted transits,
or objects on the chart which form suitable
transits. Then, to get to the planned position,
pick up one of the transits and sail along it
until the other transit marks come into line.283°T323°T 000°T1235
103nm1340
127nm➜
333°TChartlets not to be used for navigation. © Crown Copyright
and/or database rights. Reproduced by permission of the
Controller of Her Majesty’s Stationery Offi ce and the UK
Hydrographic Offi ce (www.ukho.gov.uk).