Mystery GPS Marks

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lloyd_borrett
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Mystery GPS Marks

Postby lloyd_borrett » Fri, 02 Apr 2021 3:47 pm

G'day,

We have some mystery GPS Marks, often named for an existing dive site or wreck, but not located anywhere near the GPS marks we're currently using for the dive site.

18 Metre Reef - See https://www.scubadoctor.com.au/divesite.htm… (-38 17.663, 144 35.323)

Albert - Maybe https://vhd.heritagecouncil.vic.gov.au/shipwrecks/12 (-38 20.745, 144 26.327)

Australia RMS - See https://www.scubadoctor.com.au/divesite.htm… (-38 18.100, 144 38.620)

Bayonet - See https://www.scubadoctor.com.au/divesite ... te=Bayonet (-38 43.493, 144 35.249)

Casablanca - See https://www.scubadoctor.com.au/divesite ... Casablanca (-38 21.757, 144 26.354)

Fawkner - See https://www.scubadoctor.com.au/divesite ... te=Fawkner (-38 21.111, 144 25.023)

Gambier - See https://www.scubadoctor.com.au/divesite ... te=Gambier (-38 16.384, 144 41.357)

Gulf of Carpentaria - See https://www.scubadoctor.com.au/divesite.htm… (-39 08.640, 146 17.968)

Leeuwin - See https://www.scubadoctor.com.au/divesite ... te=Leeuwin (-38 21.267, 144 33.283)

The Pinnacles - See https://www.scubadoctor.com.au/divesite.htm… (-38 34.683, 145 20.216)

The Playground - See https://www.scubadoctor.com.au/divesite.htm… (-38 17.840, 144 37.752)

Torpedo Reef - See https://www.scubadoctor.com.au/divesite.htm… (-38 18.071, 144 37.312)

Twin Bommies - See https://www.scubadoctor.com.au/divesite.htm… (-38 17.902, 144 36.052)

Maybe you can help us to work out what, if anything, is there and resolve the mystery.

Best regards, Lloyd Borrett.



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packo
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Re: Mystery GPS Marks

Postby packo » Thu, 08 Apr 2021 1:03 am

Hello,

"Mystery Marks"? Who doesn't like a mystery? Lloyd's burst of Good Friday posts are in part concerned with weeding his dive site database of bogus or mystery GPS marks. That is indeed a worthwhile endeavour but is getting harder as the database is growing fast!

Packo's very long "two bob's worth" on some of these is given below. Note GPS Datum is always WGS84 except where stated.

---------------------------------------------------------------------
#1 THE PINNACLES (Off Cape Woolamai, Phillip Island)

"Mystery Mark" of: 38 34.683 S, 145 20.216 E versus "Accepted Mark" of: 38 34.363 S, 145 20.226 E

I first saw that mystery mark in the ScubaDoc list some time in 2017. In a Dive-Oz forum post Lloyd gave a list of seven (ALL DIFFERENT!) "Pinnacle Marks" he had been given by various people. By the time the list was narrowed down to locations actually anywhere near Cape Woolamai there were only two candidates left standing. One was about 1.2km off the cape, with the second about about 600m south of that.

In the end Lloyd chose the southern one as the "Accepted Mark" because it came from a charter operator. We argued about it over several months because although I didn't have any GPS numbers, my memory from many earlier dives was that mark was too far offshore and wouldn't fit my usual visual transits.

Eventually in May 2018, and with input from Peter Beaumount, the ScubaDoc Pinnacles mark was amended to the nearshore location given above. (Note: another forum member puts the summit of the Pinnacles a little further northeast at: 38 34.349 S, 145 20.246 E).

As to the original, and now listed as a "Mystery Mark", I make only the following points from some experience:-

a) Unwise to accept any GPS mark from the internet or any individual as "good", unless at least one "second source" is available that is not identical, but agrees with the primary to within say 50m.

b) Various digit "fumbles" including miscopies and digit swaps are not uncommon in human-to-human number transfers.

c) Note the longitudes of the two given marks are very close, with one only 15m east of the other. It seems probable that the last three "Mystery Mark" latitude digits have been jumbled somehow in the chain of transmission.

For example simply moving the "3" in the decimal part of the "Mystery Mark" latitude from third place into first place, gives a location just 17m away from the new ScubaDoc "Accepted Mark". It seems unlikely that shifting just one digit around could bring two genuinely distant and independent marks so close together.

d) The depth around the "Mystery Mark" is 40m+ and I haven't seen any bathymetry of that area that shows any "pinnacle like" features. Yes although the original charting effort did miss the inshore "Pinnacles", they did detect several reefs in its neighbourhood covered by about 20m of water.

e) It seems unlikely, but not impossible that a similar reef would exist that much further offshore. However if some diver did locate such a structure at the "Mystery Mark", surely they would name it differently such as "Fred's Pinnacle", etc.

f) Given the "Mystery Mark" stood in the ScubaDoc list for at least one summer, it may have already been "checked out" dozens of times by folk looking for the real Pinnacles. Wouldn't news of any new discovery have leaked out by now?

Taking into account points a) --> f), I think it extremely unlikely anything special exists at the "Mystery Mark" and maybe it is time to finally put a stake through the heart of those numbers. They may have already caused enough trouble.

----------------------------------------------------------
#2 THE GAMBIER WRECK (between Queenscliff and Pope's Eye)

"Mystery Mark" of: 38 16.384 S, 144 41.357 E versus "Accepted Mark" of: 38 16.450 S, 144 40.500 E
(or in decimal degrees form: -38.273067, 144.689283 versus -38.274166, 144.675000)

Have the "Accepted" and "Mystery" marks been accidentally swapped? I believe this may be the case because in so many ways the "Accepted" doesn't fit the historical reports, while the "Mystery Mark" seems to be a good fit.

In rough terms these two positions are separated in an east-west direction by about 1500m. The "Mystery" site is on the
east side of the channel (the correct side for inbound vessels). The "Accepted" site is on the western side of the channel much closer to Queenscliff. Some pros & cons are listed below.

****** PROs for the "Mystery Mark" (from Dive Vic) being a good one *****

1) Other sources of Gambier marks that support this location:

Code: Select all

Getunder 2005: 38 16.386 S, 144 41.316 E
POMC document: 38 16.356 S, 144 41.352 E
eChart #1:     38 16.387 S, 144 41.393 E (from -38.273121, 144.689882)
eChart #2:     38 16.385 S, 144 41.387 E
AUS144 chart:  38 16.383 S, 144 41.386


2) Support from eyewitness accounts as reproduced in newspapers of the day.

a) Bearing from wreck to Popes Eye Annulus quoted as "east by south, half south magnetic". This is an "olde worlde" 32 point compass reference. In today's lingo it translates to a magnetic bearing of: 90 + 11.25 + 5.625 = 106.9 deg Magnetic. Taking the magnetic variation in 1891 as about 8 degrees East, the true bearing from wreck to Pope's Eye is about 115 deg True.

From the "POMC Mark" Popes Eye bears 114 True, with others being similar. From the "Accepted Mark" Popes Eye bears 96 True.

b) Eyewitness reports Geelong Advertiser 29/8/1891: "She is now lying with the tops of funnels and mast heads above water, close to the bank on the east side of the West Channel, in a line with with the Pope's Eye Annulus, and about a mile and a half (~2.4km) from the old Queenscliff jetty." The next report lowered the estimate to a mile and a quarter (~2.0km)

Measured distance: "Mystery Mark": ~1.9km from Jetty, and ~0.9km from Popes Eye ------ in fair agreement
Measured distance: "Accepted Mark": ~0.9km from Jetty, and ~2.0km from Popes Eye ------ way too far west!


3) "Mystery Mark" is near the path the car ferry takes from Queenscliff to Sorrento. Many yarns about the Gambier Wreck dive mention this complication. The ferry path goes nowhere near the "Accepted Mark".


4) Some personal experience.

In the late 1970s I had an interest in trying to find this wreck. After researching many old documents I established a search area off Popes Eye. Then it was "mowing the lawn" with eyes darting between depth sounder and the sextant angles I used to define the search grid. In the summer of 1978 I saw a few small bumps on the sounder and recorded the sextant angles. The bumps didn't look at all promising so I didn't dive it until the following summer. Not sure of what I found but I will let my dive log book commentary do the talking:

"Anchored on the sextant angles taken last summer where a 1 to 2 foot pimple had been seen on sounder. Viso was good over this sandy area and after 5 minutes we hit a low profile reef with a nice fish population. At the southern edge we encountered a long E-W cable about 1.5 inches thick which I initially took to be the undersea telephone cable running in this area.

Then it slowly dawned on me that the reef was the wreck site with all projections being pieces of iron sticking up through a heavy layer of sand. Coal chunks, copper pipe and decayed wood were ample evidence that we were on or near the spot. A nice pleasant dive site although I think the wreck is too wrecked to provide much interest."


Was I at or near the "Mystery Mark"? Well one of the visual transits I sketched back then was "Magic Eye over the southern part of Popes Eye". Transferring this to a chart recently did show this transit line passes quite close to the "Mystery Mark". I now see the 16.4 degrees angle measured between "Wedge Light" and "Popes Annulus Light" that I recorded way back then, does indeed put me at a point on the line quite close to the "Mystery Mark". I need to go back some day for another peek!


******* CONs against ScubaDocs "Accepted Mark" being a good one.

1) Mark location is not in accord with the 1891 news reports immediately following the sinking (as indicated above.) Neither is it anywhere near the ferry's path.

2) As Lloyd says in the dive site text for the Gambier, "we have one mark WAY east of the other". That distance is about 1500m. I then get suspicious because that distance is almost exactly 1 minute of longitude. Somewhere along the line was 144 40.500 E a fumbled version of 144 41.500 E? I think so, even though 144 40.500 E appears in so many places. That is part of the trust problem, numerous copies of the same bad mark appearing everywhere can create an "it must be right" mentality.

3) Delving further, the Australian Shipwreck Database link given on the dive site page gives low resolution WGS84 coordinates in degrees as: -38.27, 144.69. This converts to: 38 16.2 S, 144 41.4 E. Note a "1", rather than a "0", has now appeared in the units place of the longitude minutes.

4) Diving deeper again, another shipwreck mapping site ("beautifulaltona") gives an identical Gambier latitude to the "Accepted Mark" of -38.27417, but the longitude is given differently as 144.68806. Converting this location to "sensible speak" gives: 38 16.450 S, 144 41.284 E. Again we have 41 and not 40 minutes in the longitude.

5) Diving deeper still (hand me the Nitrox), that website map was established some time ago and many Lat/Long coordinates it gives are known to be tied to the older AGD66 Datum. A rough conversion to WGS84 is to shift the position north by 0.090 minutes, and eastward by 0.080 minutes. This gives that Gambier mark in WGS84 as: 38 16.360 S, 144 41.364 E.

This location is only 10m east, and 40m north of the "Mystery Mark" and is in reasonable accord of the alternative cluster of marks given earlier. (Remember she was 85m long and 10m wide - and later blown to smithereens!)


******** GAMBIER WRECK "MYSTERY MARK" CONCLUSIONS: *********

I am 99% sure the "Mystery Mark" is the real thing! The widely appearing "Accepted Mark" seems to be a version of the real location but corrupted by a typo of one minute in longitude, and with the wrong datum given.

Any other mark in the cluster given earlier, or the "Mystery Mark" itself from Dive Vic, might better lead you to finding some "iron among the sand".

cheers,
packo



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lloyd_borrett
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Re: Mystery GPS Marks

Postby lloyd_borrett » Mon, 12 Apr 2021 11:11 am

Thanks Packo,

After reading through your extensive research I've made your suggested changes, plus added in the details of the other marks for The Pinnacles and Gambier.

The problem of lots of GPS marks and not knowing which ones are correct is insidious. I found a research paper by someone at Heritage Victoria from a few years back. They have as many as 15 marks for some locations and no idea as to which, if any, are good!

It's why I'm now trying to list all of the GPS marks I have, plus provide the distance and bearing from the one being used.

Best regards, Lloyd Borrett.




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