Sounding Reflections

An amateur hydrographer in Port Davey/Bathurst Harbour

Cruising Yacht Club of Tasmania

By Andrew Boonaboon@bigpond.com

2 Nov 2010

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

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.

Lead lineThe 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).

Handheld sounder

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

Chris is holding the handheld sounder and helming, I am entering waypoints on the handheld GPS (Mar 2008)

Battery cabling

NMEA data

Portable sounder for dinghy

Garmin GPS60CruzPro ATT120A

Prototype portable sounder

CruzPro sensor

Battery

GPS

Clamp to fasten totransom

Junction box,switch, fuse

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.

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

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.

Parker Bay 3 m contour

Using handheld sounder, row until sounder reads ‘3.0’ then press ‘Mark’ on GPS to record waypoint (March 2008).

Iola Bay 3 m contour

We (Chris Creese and Andrew Boon) were attempting to find the reported rock in Iola Bay.

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.)

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

> 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

Moulters Inlet

Tidal variation in Moulters Inlet

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

26/02/201012:00

26/02/201018:00

27/02/201000:00

27/02/201006:00

27/02/201012:00

27/02/201018:00

Time

Wat

er d

epth

Hannant Inlet

Reflections in Hannant Inlet

Soundings in Ila Bay

Melaleuca InletSoundings 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_2010Melaleuca Lagoon depths

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 Frog’s 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!

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

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

The End