Sounding Reflections

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Sounding Reflections
Cruising Yacht Club of Tasmania

• An amateur hydrographer in Port Davey/Bathurst
  Harbour

By Andrew Boon aboon_at_bigpond.com 2 Nov 2010
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Why do I do it??

• Many good anchorages are in unsurveyed or
  inadequately surveyed areas
• I hit an uncharted rock
• Reflections had a sounder and GPS receiver which
  can easily log depths
• West Marine had a handheld sounder on special
• It becomes an addiction

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Reflections Sounders

• Lead line, marker every metre
• Fixed sounder, Raymarine ST40, offset to read
  depth below keel, set to read zero when touching
  bottom
• Handheld, built-in display
• Portable rig designed to be used in dinghy, made
  up of CruzPro intelligent sensor, GPS60
  handheld GPS receiver and battery.

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Lead line
The ultimate technology! The weight should have a
depression in the bottom filled with wax to
collect a sample of the sea floor (sand, mud,
etc).
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Handheld sounder

• Press button, hold end in water facing downwards,
  read depth.
• Switches off automatically when not in water.
• Resolution 0.1 metre.

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Chris is holding the handheld sounder and
helming, I am entering waypoints on the handheld
GPS (Mar 2008)
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Portable sounder for dinghy
Battery cabling
NMEA data
CruzPro ATT120A
Garmin GPS60
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Prototype portable sounder
With this arrangement, the pole had to be held in
place by hand when travelling at 2 knots or
more. The Mk 2 version will have a more rigid
frame to hold the sensor to the transom of the
dinghy. Depth readings appeared to be consistent
for speeds up to at least 6 knots.
Junction box, switch, fuse
GPS
Clamp to fasten to transom
Battery
CruzPro sensor
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What determines the published depth?

• The value displayed on your sounder
• The depth of the transducer
• Offsets (eg to convert to depth below keel)
• The time of the reading affects tidal height.
  Variation due to atmospheric pressure (10
  mm/hPa), wind, storm surges, diurnal and seasonal
  variations, etc.
• The accuracy (error) of the sounder
• The variation of depth in the near vicinity
  significant changes over the error range of GPS

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2008 attempt Kings Point
This is the first unsurveyed area we looked at,
recording the 2 metre depths. Later soundings
used 3 m as it is considered a more useful
minimum depth for keelboats.
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Parker Bay 3 m contour
Using handheld sounder, row until sounder reads
3.0 then press Mark on GPS to record waypoint
(March 2008).
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Iola Bay 3 m contour
We (Chris Creese and Andrew Boon) were attempting
to find the reported rock in Iola Bay.
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Casilda Cove 3 m track
In this one, we attempted to follow the 3 m
depth contour by watching the handheld sounder
display and recording the GPS Track (rather than
recording Waypoints). (The north-most end is the
rock at the entrance to the anchorage.)
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2010 Method

• Record depth every 10 seconds. At 2 kn, sample
  interval is 10 m.
• Use lower speed if more depths desired, eg if
  depth changes rapidly, to identify rocks etc.

Depth Display Interval Resolution
gt 10 m 200 m Integer, truncated
5 10 m 100 m Integer, truncated
3 5 m 50 m Integer, truncated
0 2.9 m 50 m Truncate to nearest 0.5 m
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Moulters Inlet
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Tidal variation in Moulters Inlet
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Hannant Inlet
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Reflections in Hannant Inlet
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Soundings in Ila Bay
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Melaleuca Inlet
Soundings taken with Reflections fixed sounder,
logged as part of GPS60 Track, edited in
OziExplorer. Soundings are legible on the PDF
image.
Melaleuca Inlet depths 3_2010 Melaleuca Lagoon
depths
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What is the best format?

• For a cruising guide, I think the 3 m contour is
  the best option.
• But you would never go near Frogs Hollow!
• It is safe, ie even with a 1 m error, most
  keelboats would still be OK.
• And it leaves the shallower areas undocumented,
  for intrepid explorers in dinghies!

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Future Procedure

• Estimate the height of the tide
• Sail and log the 2 m contour (actually 2 height
  of tide)
• Sail and log a grid at 100 m spacing, 10 second
  samples using the 2 m contour as a boundary
• Mark a waypoint at isolated rocks or shallows
• Log the tidal variation at anchor over at least
  12 hours, to establish a datum
• Use a bathiscope to see what is on the bottom

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Getting the numbers onto a chart

• Transfer track log from GPS to MapSource
• Save track to a text file
• Import into Excel, apply offset, tidal and
  atmospheric corrections and massage into
  OziExplorer waypoint format
• Save as a .wpt file
• Open relevant map in OziExplorer and load
  waypoints (.wpt file)
• Thin out the data for best presentation

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The End