Rowan Stevens GPS Marks for Wrecks

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lloyd_borrett
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Rowan Stevens GPS Marks for Wrecks

Postby lloyd_borrett » Thu, 21 Mar 2019 3:32 pm

G'day,

From a private source, plus some listings in various government publications, I've been able to take a look at GPS coords supplied by Rowan Stevens for various wreck sites. Usually Rowan's marks have been proven to be very good.

The Rowan Stevens marks published for the Batman, Beverwyk 19, Campana, Fawkner, Pioneer and VHB 53 all match what we have in the database.

The Rowan Stevens (RS) marks published for some sites are different to what we have (Sdoc). Some by just the last figure which probably isn't significant. But sometimes by a far bit.

Coogee - RS 38 18.429, 144 34.300 - Sdoc 38 18.421, 144 34.306

Courier - RS 38 19.488, 144 34.914 - Sdoc 38 19.488, 144 34.920

Dunloe - RS 38 21.360, 144 24.405 - Sdoc 38 21.360, 144 24.205

Euro - RS 38 20.651, 144 26.368 - Sdoc 38 20.744, 144 26.094

H.C. Piggot - RS 38 20.751, 144 26.095 - Sdoc 38 20.651, 144 26.368

Milora - RS 38 21.103, 144 23.379 - Sdoc 38 21.102, 144 23.378

Rotomahama - RS 38 19.197, 144 32.175 - Sdoc 38 19.191, 144 32.167

White Pine - RS38 21.757, 144 26.354 - Sdoc 38 21.979, 144 25.047

So unless we have people verify which marks are able to be used to locate the above dive sites (which might be different ones again), the marks for all of the above sites are now under suspicion.

Best regards, Lloyd Borrett.



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packo
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Re: Rowan Stevens GPS Marks for Wrecks

Postby packo » Sun, 07 Apr 2019 1:46 am

Lloyd wrote (before re-editing):
The Rowan Stevens marks published for the Batman, Beverwyk 19, Campana, Fawkner, Pioneer and VHB 53 all match what we have in the database.

Perhaps that is not such a good thing! If GPS marks from two sources match exactly, then it is around 99% certain that one is copied from the other and it is really only one source. The second instance in no way verifies the first.

A set of GPS marks that are different, but not by too much, provides far better verification provided that multiple sources are giving "roughly the same position" for a particular point of interest.

The second group of coordinates Lloyd gives is:-
The Rowan Stevens marks published for the Coogee (38 18.429, 144 34.300), Courier (38 19.488, 144 34.914), Dunloe (38 21.360, 144 24.405), Euro (38 20.651, 144 26.368), Milora (38 21.103, 144 23.379), Piggot (38 20.751, 144 26.095), Rotomahama (38 19.197, 144 32.175) and White Pine (38 21.757, 144 26.354) are different. Some by just the last figure which probably isn't significant. But sometimes by a far bit.

These might actually be more cause for joy than the first group of identical numbers - provided the difference in Rowan's marks and those given by other sources is not too large.

There is really no such thing as a "GPS mark for a shipwreck", but rather only "GPS marks for particular parts of a shipwreck". If two or or more dive groups mark different parts, then we may still be taking about the same wreck even if they are 50 to 100m apart (ie. a difference of of up to 4 or 5 hundredths of a minute).

To illustrate the point I have reproduced my rough "wreck outline" for the Coogee. I am not claiming the red outline is "gospel", but surface conditions were good that day. The main purpose is to give a frame of reference for a bunch of other GPS marks (all to WGS84 datum) that were scraped together from various internet sources:-
Image
This illustrates the typical problem of "mark spread" and how having a rough wreck outline might aid users of those marks to find the wreck, or their favourite part of it, more quickly. Plotting the spread of marks is also helpful even if the general outline is unknown or not clear.

It is really helpful to Lloyd and others if as well as handing over the mark, you also give some comment on which part of the wreck you think this mark applies to, and if possible the rough compass direction in which the bow points. (You will need to swim at least 7m clear to be sure "wreck iron" does not affect your compass too much.)

The image also shows there might be some small discrepancy over the exact whereabouts of the stern, with a number of sources claiming it extends down to around 18.429' S. The bow-stern distance of my outline also appears a little short at 61m compared to the known 68m overall length of the Coogee before scuttling. So my outline could be a little off.

An alternate explanation is that the perceived wreck is now somewhat shorter than on scuttling day. Old photos show the Coogee had a fairly generous stern overhang of perhaps 5m to 6m beyond the waterline length. This would suggest perhaps it extended 7m to 8m beyond the rudder post and its steering quadrant. Much of this stern overhang collapsed at some time in the last 30 years. An old log book sketch of mine well before that time shows the stern far more intact than now:-
Image
Today there is not much left behind the steering quadrant and that may explain why older "stern marks" may extend further southward than the newer ones like my 2014 survey. (Or it might just be an error on my part - I now see the scribbled log book GPS points obtained that day did carry the heading "interim Coogee outline"!)

Although the stern is a great area, my usual "target point" is 38 18.411 S, 144 34.306 E because the boiler area is less vulnerable to anchor damage and also less prone to line entanglements. Typically using this mark I find the anchor either in the gap between the twin boilers, or draped over one of them, or occasionally just slipping off the outer edge.


***** Sounding around *****

The first image also shows that while many of the different marks shown fall outside the red outline, all of them would be good enough to find the Coogee with a bit of sensible echo sounding around each particular mark. (On my first visit with a GPS receiver the only target I had was the BSAC 2014 "green" mark, and so I had to sound around for a while before seeing the boiler area on the sounder about 25m away to the northwest.)

I note the original post was triggered by a VSAG crew claiming some SD marks (for other wrecks) were "duds". This also happened some time before with the Pioneer wreck, which they also failed to find. However when the Pioneer marks were updated with "verified marks", that position was only 28m away from the original mark.

Particularly for the deeper 50m+ wrecks, divers must be prepared to search around a mark for some distance in all directions before giving up. This is where knowing the rough alignment of the wreckage can be important in improving your search strategy.

There are many reasons why GPS marks may be off by quite some metres. These include:-


***** The GPS unit itself *****

A "retail level" GPS unit as most of us would have will, if left in the same position, will produce a series of "fixes" that will wander around over a few metres during the course of 15 minutes to half an hour. This is largely due to variations in the transmission delay through the ionosphere. The "fix" may also do little hops and skips of a few more metres as the GPS unit changes its selection of satellites that are used to compute the fix.

The unit may track all satellites that are in its sky-view, but then select the best sub-set to use to produce a fix based on their signal strength, bearing, and height above the horizon. This selection will change from time to time as the satellites slowly move across the sky. Changes in the selection often gives rise to small jumps and hiccups in the calculated fix.

As purchased, most retail GPS units will claim they are "WAAS enabled" to improve fix accuracy to around 2m. The "Wide Area Augmentation System", or "WAAS" uses additional (higher) satellites to transmit a correction signal which is determined by comparing the "fix" produced at a number of ground stations with the known coordinates of those stations.

These higher WAAS satellites are in geosynchronous orbits to give fixed coverage to various parts of the globe. Unfortunately since Oz is rather out on its lonesome, some cost-benefit analysis somewhere said: "Sorry Australia, you are going to miss out on WAAS."

So although your GPS retailer may have extolled the virtues of a "WAAS enabled" unit for better accuracy while he was taking your money, this "feature" is effectively disabled by the inability to pick up any correction signals in our part of the world. (very far northern Australia may just get some).

Without WAAS the accuracy claim for a retail GPS unit is typically "better than 15 to 20m for 95% of the time". We seem to do a little better at sea because generally the all-around-the-horizon view allows optimum satellite selection.


***** The GPS set-up *****

Many diving runabouts are set up with the GPS antenna (internal or external) 3 to 4 metres further forward than the transom mounted echo sounder transducer. Although convenient, this offset means that the coordinates of a sharp target may differ by up to 8 metres due solely to approaching it from different directions.

This offset can be made significantly worse because the faces of many transducers are set up to be near parallel to the slipstream when the hull is on the plane. This reduces cavitation and bubbles across the transducer face and gives best performance at speed. However when the boat drops off the plane to "survey speed", the bow drops by around 10 to 15 degrees. In deep water this tilt throws the sounder's echo detecting "sweet spot" some more metres further aft of the GPS antenna.

Although the sound beam is usually wide enough to still give some "straight down energy" and hence receive echoes from directly below the boat, these are easily swamped by stronger (and shallower) echoes from wreckage in the "sweet spot" but significantly offset behind the boat. This problem is accentuated in deeper areas, and also by swell and wave conditions at the surface.

Running back over the target on the reciprocal course and averaging the two fixes can reduce this problem but few would go to that bother. For the deeper wrecks this should really be done if you are trying to refine your mark. In deep water, trying to establish when your boat's GPS is directly above a target is a rather difficult problem. Bottom to surface buoy lines can be pretty good but must have thin lines and not too much buoyancy to counteract offsets due to water currents and/or wind. A low buoyancy "spar buoy" is good for this but be careful it is deployed on (just) enough line!


***** Format Grammar *****

The three most common GPS position formats are:-

a) Lat/Long in decimal degrees to four or five decimal places. Used mainly for computer plotting and not a very "human friendly" format as the eye and mind can't easily judge the distance between two points in roughly the same area. Very roughly, a change of one unit in the first decimal place corresponds to a position shift of about 5 nautical miles. A one unit change in the second decimal digit is roughly a 0.5nm (about 800 to 1100m) shift, 80 to 111m for the third etc. At least 4 and preferably 5 decimal places are needed for a useful diving mark.

b) Lat/Long in whole degrees, followed by the remainder expressed in minutes (1/60th of a degree) including a decimal point. It is probably the most common format and more human friendly. One minute of Lat is one nautical mile (~1852m) and one minute of Long is roughly 1453m at Victoria Coast latitudes. Minutes usually given to three decimal places but realistically rounding up or down to just two is quite common, with the max error in doing so no more than 10m. Also amenable to shortening by dropping the whole degrees if the general location is known.

c) Lat/Long in whole degrees, with remainder in two whole minute digits, followed by the remaining seconds as two whole seconds digits and usually one decimal digit. One second is 1/60th of a minute which for Lat is about a 30m distance and for Long about 20m.

Formats b) & c) need at least a space after each part but may additionally use a super script circle after the whole degrees and ' after the minutes part and " after the seconds part. However some boaties/divers/fishos run all the bloody digits together with either no "punctuation", or use dots . as separators eg:-
S3819349
E14437228
Is this 38 19.349' S 144 37.228' E ????, or 38 19' 34.9" S 144 37' 22.8" E ????, or -38.19349 144.37228 ???? They are all very different positions.

Sometimes a possible split up can be ruled out if the leading prospective "minute" or "seconds" digit exceeds "5".

eg. "Snapper 38.02.493 144.58.942 WGS84" is decipherable because the '9' in the last group of longitude digits rules out seconds, meaning the last 3 digits of both coordinates are decimal minutes. ie. Location is :- 38 02.493' S 144 58.942' E.

But: "Flathead 38.02.232 144.49.197" is ambiguous (not to mention no datum is specified).
ie. Could be:- 38 02.232' S 144 49.197' E or 38 02' 23.2" S 144 49' 19.7" E (= 38 02.387' S 144 49.328' E). But is it WGS84 or AGD66??

Best to use a dot only when you mean a decimal point, and use a space/hyphen/underscore as a general group separator.

Divers in general are much better at clearly separating the groups than the fishing fraternity. However as seen earlier a missing (or incorrect) datum spec can quickly cause some chaos.


***** What is a reasonable agreement between sources? *****

Although some very poorly bunched marks may be due to number handing problems, there is also a suggestion that some folks may not even record the mark until after the boat has laid right back on its anchor line! Excluding these sorts of "rough and ready" techniques, even on a tight target we have to expect that a group of marks for the site taken by different boats, and on different days, will have a spread of up to 10 - 20m. For a medium to large wreck it could be 3 to 8 times as much due to the extent of the wreckage.


***** Some futzing around, Mums, "Jaws", and dive site locations on AUS Charts *****

About a year back there was some uncertainty over SD's GPS mark for "The Pinnacles" off Cape Woolamai on Phillip Island. In what sounds like a typical occurrence, Lloyd was given around half a dozen sets of GPS numbers for this dive site. Many were wildly different yet all claimed by their sources to be a "good mark" for The Pinnacles.

My first effort to locate "The Pinnacles" in my own boat, then still quite new, was in the early 1970s. It was an ambitious enterprise as the "depth sounder" was a half-brick on the end of 40m of venetian blind cord. Nevertheless the water was very clear and I had some good visual transits from several previous charter dives.

We soon could see this "mountain" lurking in the depths and the brick and good vis guided us to the summit pretty quickly. However the still very new anchor line spilled its tight coils all over the floor of the boat. We were in danger of drifting away from the top of the reef before I could get it all sorted out.

My ever helpful Mum was also on board that day. She quickly offered to hop over the side, stand on the top of the reef, and hold the boat in place while I got untangled. I resisted the temptation to say "go ahead" and instead told her that despite appearances it was way over her head.

"Can't be", she replied, "I can see the periwinkles on the bottom as clear as a bell and I'm sure I could just about reach down and grab one!" (I suppose there is some logic here in that an 8 inch abalone at 8m down might look slightly similar to a 1 inch periwinkle in 1m of water.)

It was a nice dive in good conditions. I have been back quite a few times since then, but not since I acquired a GPS receiver. (A very late adopter!). So I have no GPS numbers of my own to refer to, just old memories of transits and how to get to roughly the right spot to then make sense of the visual transits.

Among the seven available marks Lloyd had to ponder, two were weeded out as being for a site off Pt Lonsdale variously called "Pinnacles Deep" or "Deep Pinnacles" by the old Dive Vic operation. Unfortunately I see that in the new owner's charter schedule, that site is now named as just "The Pinnacles" creating more ongoing confusion. Another "Pinnacles" site was off Rickett's Point, and another one off Pyramid Rock.

This left only three contenders actually anywhere near Cape Woolamai on Philip Island. One of these was way too far inshore and close under the Woolamai light. There is also a land based rock spire on the cape that is popular with land based photographers and in social media circles. That rock spire cluster goes by the name of (wait for it!) "The Pinnacles".

This is where "generic feature" dive site names would do well to refer to the general location in the name to avoid confusion, eg. "The Pinnacles - Cape Woolamai", or "Pinnacle Deep - Pt Lonsdale". At one time Lloyd was thinking of including a unique site number attached to the name to reduce site mixups due to similar sounding names.

Of all "The Pinnacles" marks at hand, Lloyd opted for the one provided by a charter operator. However I thought that position was a little too far offshore at around 1.7km out, whereas my memories were saying "around 1km off".

Another memory that popped up in the "charter operator" context was of a dive in the mid 1980s where our dive club chartered a San Remo fishing trawler to take the whole group of about 20 of us for a Pinnacles dive.

That skipper steamed over and around the Pinnacles site at quite a pace and when the depth sounder looked right he rushed out of the wheelhouse to deploy the "shot-line". This consisted of his spare admiralty style anchor with a large coil of rope and a yellow plastic barrel tied to the end.

The rope seem to really tear off the coil, in part due to the plummeting anchor and in part due to the vessel still steaming along at 3 to 4 knots. He warned us all to stay well clear of the line as it snaked off the deck and into the depths.

At the last coil it seemed to pause for a while and then resumed with gusto. The yellow barrel float stowed near the stern pulled out of its holding bracket and raced along the side-deck before leaping into the water. It seemed to pause for a split second before rushing off across the surface back towards "the drop zone". When it arrived at the drop point it showed no hesitation at all in plunging off into the depths in hot pursuit of its obviously too heavy anchor!

There was a stunned silence from all aboard until the club wag yelled "Jaws!". This broke everyone up into nervous but hearty laughter. All except the foolish looking skipper of course. (He was unaware that a few months prior, the club had held a cinema movie night with one of the Jaws movies showing, so that powerful imagery was still very fresh in mind!)

A more subdued second attempt got us hooked into the reef. Despite the offer of a handsome reward, none of the divers ever saw any trace of barrel, rope, or anchor. Still we all got several good lessons on how not to dive this great site.

Now back to the GPS marks. It really boiled down to just two possible marks, the SD chosen one about 1.7km offshore and another around 1.1km out. I emailed "AusHydro", who produce the Australian maritime charts, giving them the two positions and asking them that if they knew of this reef, then which was the better set of coordinates.

The reply was a bit startling. They weren't aware of "The Pinnacles" (it was never on the charts) and therefore said they must take my email as an official "report of an uncharted rock dangerous to shipping".

Furthermore they said if I couldn't nominate which of the two positions I gave was the correct one, under IMO rules they had no choice but to chart TWO dangerous reefs - one in each position!.

That silliness set me about gathering more information about its true GPS position. Finally with the help of Peter Beaumont and several more closely spaced marks I was able to confidently nominate the more inshore GPS mark as the correct one.

So from March 2018 "The Pinnacles" appears on charts as a "reported rock" with 9m of water over it. Presumably at some future time the Navy will send some assets out to do a proper survey, including accurate position and the correct minimum depth below chart datum.

I don't/can't use official electronic charts with my DIY chart plotter. It was only by using a free on-line electronic chart from some American website that I finally saw (several months later) "The Pinnacles" site appearing on this updated eChart. AusHydro did however forward me a freebie copy of the first AUS150 paper chart to show "The Pinnacles" dive site.


***** Movements in Charted Wreck Positions ******

During the chit chat with AusHydro over this issue, they asked if I ever dived wrecks in Victoria. They said their "wreck unit" was always interested in updated GPS positions from divers. I explained that I wasn't really that much into wrecks, but could offer up GPS positions and notes on at most perhaps half a dozen.

Rather than stuff around with paper charts I used the on-line eChart at maximum zoom to extract the GPS coordinates of some wrecks plotted on those charts. I then compared them with my own coordinates. The match was pretty good for the few southern Bay wrecks I visit.

However moving outside the Heads things were a bit different. For the Coogee, J4 Sub, and J5 Sub, their charted positions seemed about 200m off (to the southwest). I replied to AusHydro that if the positions I extracted from the American eChart matched current AusHydro knowledge, then I wondered if someone had accidentally taken old AGD66 numbers and used them as WGS84 positions. I didn't hear back which I thought was a little bit rude, but later thought maybe the American eChart info was off.

Lloyd's original post above did rekindle my interest to revisit the eChart issue now that almost a year had lapsed since the AusHydro interaction. Well blow me down! Those three wrecks had now all been moved to pretty close to the GPS numbers that I and many others use for them. This is shown in this WGS84 datum table:-

Code: Select all

WRECK     eCHART POSN March_2018     eCHART POSN April_2019   SHIFT DISTANCE and DIRECTION   
Coogee    38 18.516s 144 34.268e     38 18.422s 144 34.307e   183m, 18T
J4 Sub    38 18.069s 144 33.736e     38 17.980s 144 33.819e   204m, 36T
J5 Sub    38 18.753s 144 34.035e     38 18.648s 144 34.118e   228m, 32T

Unfortunately I don't know an exact conversion method from AGD66 to WGS84 but my rough rule of thumb is a 0.09' north shift in Lat and a 0.08' east shift in Long. On that basis if a site to AGD66 datum is plotted as if the numbers were WGS84, then that chart point would need to be corrected by moving it 203m in the direction of 35T.

The numbers in the last column of the table above will be a little rubbery due to eChart reading inaccuracies, but they do indicate that such a correction was probably done for the J4 & J5 positions. In the case of the Coogee it seems probable that an old and slightly off AGD66 mark was replaced by a better WGS84 mark.

I do wish AusHydro had responded and told me exactly what they did, why, and when. I suppose there might have been some level of embarrassment and so they choose the "quite path". Because these sea floor object positions have no consequence for surface navigation (other than anchoring), AusHydro is able to move them around without generating a "Notice to Mariners". A down side of this to us divers is that we never know what was done (or when) unless some nerd notices it! I guess it pays to renew your eChart licenses and renew any paper ones from time to time.


Have other graveyard wrecks also been "moved about"? *****

I don't go much further out into the Graveyard, so wasn't particularly interested in other wrecks. So unfortunately I didn't take any more eChart wreck positions in March 2018. Nevertheless I recently got out my 2012 paper AUS158 chart to see if more of the "Historic Wreck" circle indicators may have had their charted positions shifted recently.

The situation all got a bit confusing and incomplete due to the following:-
1) The paper chart only covers wrecks east of Lng 144 32.000e
2) The eChart does not show wrecks within Commonwealth Dumping Area #3
3) Many eCharted wreck markers were not present on the older paper chart.

-------- eChart vs Scuba Doctor locations:-
a) The following eChart wrecks were found to be within 2m of the SD locations. These positions are therefore considered most likely to have been copies of the SAME original source and so do not represent a 2nd source confirmation:-
Milora, Buninyong, J1 Deep Sub, J4 Shallow Sub, J5 Yellow Sub, J2 Broken Sub, John Nimmo, Werfra, Malaita, Hygera, Leewin, Campana, Auriga.

b) The following eChart wrecks were found to be within 30m of the SD positions and so these probably have come from 2 independent sources that roughly confirm each other:-
Coogee (16m), Rotomahana (15m), Sir-William McPherson (8m), South Milton (29m). While the eChart & SD positions of the Courier are identical, they differ from the Rowan Stevens mark by 8m and by around 6m from the larger scale AUS144 paper chart. The SD mark here is therefore considered "confirmed".

-------- Wrecks whose chart position has moved:-
It seems many of the wrecks charted on older AUS158 charts were plotted as if the coordinates were WGS84 when in fact they appear to be AGD66 datum points. These points have been moved in the eChart versions that have been updated to beyond March 2018. In some cases the move amount & direction seems to be an arithmetic AGD66 to WGS84 correction, but in other cases a better WGS84 point from some other source was used. Those wrecks having be so shifted are:-
Coogee, J1, J2, J4, J5, Rotomahama. The Courier position has not been moved so it appears the correct WGS84 mark was used in the first instance.

-------- Unidentified old paper chart wreck markings from AUS158:-
I could not match some paper chart wreck markers to any SD locations, even when I assumed they could be either AGD66 or WGS84 points. In case others might like to try to find a close match I have listed these below:-

Code: Select all

Wreck   [direct from paper Chart]  (0.09' North shift, 0.08' East shift)
???#1    38 20.347s, 144 34.597e     38 20.257s, 144 34.677e
???#2    38 20.584s, 144 34.326e     38 20.494s, 144 34.406e
???#3    38 20.954s, 144 33.738e     38 20.864s, 144 33.818e
???#4    38 21.485s, 144 33.836e     38 21.395s, 144 33.916e


***** General Despair! ******

With muck ups in datums, formats, and "whatever" happening quite frequently, it so disappointing that so many GPS marks on the web can be unreliable. I've seen some fishing maps (where divers also contribute) where you would struggle to run a 1km long rope around the mark cluster for the same site. There are also the "outsiders" with marks many miles out of their named position. It is quite bewildering how much crap is out there. Please be careful and recheck everything before sending any GPS numbers out onto the web!

I am also sad that I can't really offer anything more constructive in reply to Lloyd's original post. It is a really awkward position for Lloyd in trying to pull together good information when sources that provide that info seem to be all over the place.

Of course there are tons of divers out there who do have an accurate set of marks, but find they aren't quite ready to share yet. Perhaps if you have a high profile and know for sure an SD mark is wrong, then even an "It ain't there!" comment could be helpful.

Once in the Oz-dive forum days, a member using the handle "peter69_56" (also on this forum) offered to scout out some of these graveyard locations that were in doubt and get some solid numbers that can be relied on (with his updated boat electronics). Are you still out there Peter?


****** And just one more thing! ******

Since this post is all about uncertain SD wreck marks, I'll just toss one more into the mix. The SD list has the wreck of the William Salthouse located at the centre of the Protected Zone. This isn't true and the remains are located about 100m to the northeast. (Any of you that are long in the tooth or grey in the bead will remember for some years in the 1980s a large yellow buoy was moored at the very centre of the zone with a large mooring block and heavy chain - something they just wouldn't put in the middle of a fragile wreck site.)

Again the "marks spread" of the different sources is perhaps best illustrated by a GPS coordinate plot:-
Image
The points shown are taken from these WGS84 numbers:-

Code: Select all

(Gazetted Protection Zone is to a 250m radius around 38 16.3766s, 144 42.3296e)
SD list 2018     38 16.377s, 144 42.330e (dark blue)
BSAC list 2014   38 16.316s, 144 42.427e (green)
Getunder 2005    38 16.322s, 144 42.404e (brown)
Packo  GPS 2013  38 16.334s, 144 42.387e (dark red)
HeritageVic 2016 38 16.342s, 144 42.392e (pink) 
PoMC Map 2004    38 16.340s, 144 42.383e (light blue)
eChart   2018    38 16.337s, 144 42.396e (orange)

Note 9/5/2019: Today I saw that the original coordinate plot I posted had errors in the horizontal positioning (longitude) of the "green" and "pink" plot-points. The numerical values in the table are all ok but somehow I fluffed converting two longitudes into the proper horizontal image pixel position. My apologies for this slip-up. All has now been corrected and "tightens" the marks cluster somewhat. Apologies to Heritage Vic if they were made to look a little off.

The Heritage Vic info was obtained from a March 2016 field trip report. The intention of that trip was to do inspection dives on a number of south bay wrecks. They copped some rather bad weather and several sites, including the Salthouse, weren't actually dived. The pink mark is for numbers stored in their vessel's depth sounder. The wreck site was clearly visible on their side scan sonar even though no confirmation dive was done.

The document reporting on their field work during that trip also seems to suggest that maybe the main Victorian Heritage Database may have at some stage unintentionally listed the protection zone centre point as the actual wreck coordinates. This is possibly where the SD mark (dark blue) may have originated from.

cheers,
packo
Last edited by packo on Sun, 28 Jul 2019 12:32 pm, edited 5 times in total.



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lloyd_borrett
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Re: Rowan Stevens GPS Marks for Wrecks

Postby lloyd_borrett » Sun, 07 Apr 2019 11:16 am

G'day,

I've used the details provided by Packo above to update the entry for the William Salthouse. See https://www.scubadoctor.com.au/divesite ... -Salthouse

It would be nice to do something similar for other dive site, especially shipwrecks, where there are lots of GPS marks in circulation. It would give people a better idea as to the history of the marks.

Best regards, Lloyd Borrett.



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packo
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Re: Rowan Stevens GPS Marks for Wrecks

Postby packo » Tue, 09 Apr 2019 3:56 pm

In the original post, after an edit, Lloyd wrote:
The Rowan Stevens (RS) marks published for some sites are different to what we have (Sdoc). Some by just the last figure which probably isn't significant. But sometimes by a far bit.

Coogee - RS 38 18.429, 144 34.300 - Sdoc 38 18.421, 144 34.306

Courier - RS 38 19.488, 144 34.914 - Sdoc 38 19.488, 144 34.920

Dunloe - RS 38 21.360, 144 24.405 - Sdoc 38 21.360, 144 24.205

Euro - RS 38 20.651, 144 26.368 - Sdoc 38 20.744, 144 26.094

H.C. Piggot - RS 38 20.751, 144 26.095 - Sdoc 38 20.651, 144 26.368

Milora - RS 38 21.103, 144 23.379 - Sdoc 38 21.102, 144 23.378

Rotomahama - RS 38 19.197, 144 32.175 - Sdoc 38 19.191, 144 32.167

White Pine - RS38 21.757, 144 26.354 - Sdoc 38 21.979, 144 25.047

So unless we have people verify which marks are able to be used to locate the above dive sites (which might be different ones again), the marks for all of the above sites are now under suspicion.

I only dive one of these wrecks so can't offer much other than some armchair observations:-

Coogee - Plenty of other marks from multiple sources confirm these are near enough (but search slightly northwards).

Courier - Have an old independent AGD66/AUS66 mark (http://phucinc.tripod.com/divesites.htm) that when converted to WGS84 gives: 38 19.489s 144 34.912. Getunder 2005 mark is 19.510s 144 34.950, which is 60m away to the southeast of the Sdoc mark. This is close to the length of the vessel and maybe Sdoc/Getunder marks are bow/stern marks? (midsection mostly collapsed).

Dunloe - Recheck both sources, first decimal longitude digit is the only difference and a high chance it is a typo - but in which mark? Still leaves only one source though. Why not ask Ian Scholey what mark he used when he didn't find the Dunloe (approx. 17/3/2019)?

Euro & H. C. Piggot - A cross-pollination mix-up here! Scuba Doc's Piggot wreck is lying on top of Rowan's Euro wreck, and vice-versa. When untangled it just leaves 2 copies of one source for one of the wrecks, but two different sources just 14m apart for the other wreck. This is some sort of confirmation of the 2nd one's position. Ian S might also be able to provide the failed Piggot mark from his recent diving which could be helpful.

Milora - Positions possibly copies of a single source. However an independent AUS66 source which when converted to WGS84 gives: 38 21.074s 144 23.397e possibly close enough for confirmation. (she's a "biggie").

Rotomahama - Positions close enough (16m) to be from independent sources and confirm each other as valid marks. However a Getunder 2005 mark is 38 19.160s 144 32.200e, and an old converted AUS66 phucinc mark gives 38 19.017s 144 32.305e. Not sure what to make of all these.

White Pine - Unresolveable difference. Positions around 2km apart and must be either different wrecks or one or both marks are wrong.

cheers,
packo



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lloyd_borrett
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Re: Rowan Stevens GPS Marks for Wrecks

Postby lloyd_borrett » Wed, 08 May 2019 4:22 pm

G'day,

I've used the details provided by Packo above to update the entry for the Coogee. See https://www.scubadoctor.com.au/divesite.htm?site=Coogee

Best regards, Lloyd Borrett.




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